1 /* SPDX-License-Identifier: GPL-2.0 */
3 * linux/arch/x86_64/entry.S
5 * Copyright (C) 1991, 1992 Linus Torvalds
6 * Copyright (C) 2000, 2001, 2002 Andi Kleen SuSE Labs
9 * entry.S contains the system-call and fault low-level handling routines.
11 * Some of this is documented in Documentation/x86/entry_64.rst
13 * A note on terminology:
14 * - iret frame: Architecture defined interrupt frame from SS to RIP
15 * at the top of the kernel process stack.
18 * - SYM_FUNC_START/END:Define functions in the symbol table.
19 * - idtentry: Define exception entry points.
21 #include <linux/linkage.h>
22 #include <asm/segment.h>
23 #include <asm/cache.h>
24 #include <asm/errno.h>
25 #include <asm/asm-offsets.h>
27 #include <asm/unistd.h>
28 #include <asm/thread_info.h>
29 #include <asm/hw_irq.h>
30 #include <asm/page_types.h>
31 #include <asm/irqflags.h>
32 #include <asm/paravirt.h>
33 #include <asm/percpu.h>
36 #include <asm/pgtable_types.h>
37 #include <asm/export.h>
38 #include <asm/frame.h>
39 #include <asm/trapnr.h>
40 #include <asm/nospec-branch.h>
41 #include <asm/fsgsbase.h>
42 #include <linux/err.h>
47 .section .entry.text, "ax"
50 * 64-bit SYSCALL instruction entry. Up to 6 arguments in registers.
52 * This is the only entry point used for 64-bit system calls. The
53 * hardware interface is reasonably well designed and the register to
54 * argument mapping Linux uses fits well with the registers that are
55 * available when SYSCALL is used.
57 * SYSCALL instructions can be found inlined in libc implementations as
58 * well as some other programs and libraries. There are also a handful
59 * of SYSCALL instructions in the vDSO used, for example, as a
60 * clock_gettimeofday fallback.
62 * 64-bit SYSCALL saves rip to rcx, clears rflags.RF, then saves rflags to r11,
63 * then loads new ss, cs, and rip from previously programmed MSRs.
64 * rflags gets masked by a value from another MSR (so CLD and CLAC
65 * are not needed). SYSCALL does not save anything on the stack
66 * and does not change rsp.
69 * rax system call number
71 * r11 saved rflags (note: r11 is callee-clobbered register in C ABI)
75 * r10 arg3 (needs to be moved to rcx to conform to C ABI)
78 * (note: r12-r15, rbp, rbx are callee-preserved in C ABI)
80 * Only called from user space.
82 * When user can change pt_regs->foo always force IRET. That is because
83 * it deals with uncanonical addresses better. SYSRET has trouble
84 * with them due to bugs in both AMD and Intel CPUs.
87 SYM_CODE_START(entry_SYSCALL_64)
91 /* tss.sp2 is scratch space. */
92 movq %rsp, PER_CPU_VAR(cpu_tss_rw + TSS_sp2)
93 SWITCH_TO_KERNEL_CR3 scratch_reg=%rsp
94 movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp
96 SYM_INNER_LABEL(entry_SYSCALL_64_safe_stack, SYM_L_GLOBAL)
98 /* Construct struct pt_regs on stack */
99 pushq $__USER_DS /* pt_regs->ss */
100 pushq PER_CPU_VAR(cpu_tss_rw + TSS_sp2) /* pt_regs->sp */
101 pushq %r11 /* pt_regs->flags */
102 pushq $__USER_CS /* pt_regs->cs */
103 pushq %rcx /* pt_regs->ip */
104 SYM_INNER_LABEL(entry_SYSCALL_64_after_hwframe, SYM_L_GLOBAL)
105 pushq %rax /* pt_regs->orig_ax */
107 PUSH_AND_CLEAR_REGS rax=$-ENOSYS
111 /* Sign extend the lower 32bit as syscall numbers are treated as int */
113 call do_syscall_64 /* returns with IRQs disabled */
116 * Try to use SYSRET instead of IRET if we're returning to
117 * a completely clean 64-bit userspace context. If we're not,
118 * go to the slow exit path.
119 * In the Xen PV case we must use iret anyway.
122 ALTERNATIVE "", "jmp swapgs_restore_regs_and_return_to_usermode", \
128 cmpq %rcx, %r11 /* SYSRET requires RCX == RIP */
129 jne swapgs_restore_regs_and_return_to_usermode
132 * On Intel CPUs, SYSRET with non-canonical RCX/RIP will #GP
133 * in kernel space. This essentially lets the user take over
134 * the kernel, since userspace controls RSP.
136 * If width of "canonical tail" ever becomes variable, this will need
137 * to be updated to remain correct on both old and new CPUs.
139 * Change top bits to match most significant bit (47th or 56th bit
140 * depending on paging mode) in the address.
142 #ifdef CONFIG_X86_5LEVEL
143 ALTERNATIVE "shl $(64 - 48), %rcx; sar $(64 - 48), %rcx", \
144 "shl $(64 - 57), %rcx; sar $(64 - 57), %rcx", X86_FEATURE_LA57
146 shl $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
147 sar $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
150 /* If this changed %rcx, it was not canonical */
152 jne swapgs_restore_regs_and_return_to_usermode
154 cmpq $__USER_CS, CS(%rsp) /* CS must match SYSRET */
155 jne swapgs_restore_regs_and_return_to_usermode
158 cmpq %r11, EFLAGS(%rsp) /* R11 == RFLAGS */
159 jne swapgs_restore_regs_and_return_to_usermode
162 * SYSCALL clears RF when it saves RFLAGS in R11 and SYSRET cannot
163 * restore RF properly. If the slowpath sets it for whatever reason, we
164 * need to restore it correctly.
166 * SYSRET can restore TF, but unlike IRET, restoring TF results in a
167 * trap from userspace immediately after SYSRET. This would cause an
168 * infinite loop whenever #DB happens with register state that satisfies
169 * the opportunistic SYSRET conditions. For example, single-stepping
172 * movq $stuck_here, %rcx
177 * would never get past 'stuck_here'.
179 testq $(X86_EFLAGS_RF|X86_EFLAGS_TF), %r11
180 jnz swapgs_restore_regs_and_return_to_usermode
182 /* nothing to check for RSP */
184 cmpq $__USER_DS, SS(%rsp) /* SS must match SYSRET */
185 jne swapgs_restore_regs_and_return_to_usermode
188 * We win! This label is here just for ease of understanding
189 * perf profiles. Nothing jumps here.
191 syscall_return_via_sysret:
192 /* rcx and r11 are already restored (see code above) */
193 POP_REGS pop_rdi=0 skip_r11rcx=1
196 * Now all regs are restored except RSP and RDI.
197 * Save old stack pointer and switch to trampoline stack.
200 movq PER_CPU_VAR(cpu_tss_rw + TSS_sp0), %rsp
203 pushq RSP-RDI(%rdi) /* RSP */
204 pushq (%rdi) /* RDI */
207 * We are on the trampoline stack. All regs except RDI are live.
208 * We can do future final exit work right here.
210 STACKLEAK_ERASE_NOCLOBBER
212 SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi
218 SYM_CODE_END(entry_SYSCALL_64)
224 .pushsection .text, "ax"
225 SYM_FUNC_START(__switch_to_asm)
227 * Save callee-saved registers
228 * This must match the order in inactive_task_frame
238 movq %rsp, TASK_threadsp(%rdi)
239 movq TASK_threadsp(%rsi), %rsp
241 #ifdef CONFIG_STACKPROTECTOR
242 movq TASK_stack_canary(%rsi), %rbx
243 movq %rbx, PER_CPU_VAR(fixed_percpu_data) + stack_canary_offset
246 #ifdef CONFIG_RETPOLINE
248 * When switching from a shallower to a deeper call stack
249 * the RSB may either underflow or use entries populated
250 * with userspace addresses. On CPUs where those concerns
251 * exist, overwrite the RSB with entries which capture
252 * speculative execution to prevent attack.
254 FILL_RETURN_BUFFER %r12, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_CTXSW
257 /* restore callee-saved registers */
266 SYM_FUNC_END(__switch_to_asm)
270 * A newly forked process directly context switches into this address.
272 * rax: prev task we switched from
273 * rbx: kernel thread func (NULL for user thread)
274 * r12: kernel thread arg
276 .pushsection .text, "ax"
277 SYM_CODE_START(ret_from_fork)
280 call schedule_tail /* rdi: 'prev' task parameter */
282 testq %rbx, %rbx /* from kernel_thread? */
283 jnz 1f /* kernel threads are uncommon */
288 call syscall_exit_to_user_mode /* returns with IRQs disabled */
289 jmp swapgs_restore_regs_and_return_to_usermode
297 * A kernel thread is allowed to return here after successfully
298 * calling kernel_execve(). Exit to userspace to complete the execve()
303 SYM_CODE_END(ret_from_fork)
306 .macro DEBUG_ENTRY_ASSERT_IRQS_OFF
307 #ifdef CONFIG_DEBUG_ENTRY
310 testl $X86_EFLAGS_IF, %eax
319 * idtentry_body - Macro to emit code calling the C function
320 * @cfunc: C function to be called
321 * @has_error_code: Hardware pushed error code on stack
323 .macro idtentry_body cfunc has_error_code:req
328 movq %rsp, %rdi /* pt_regs pointer into 1st argument*/
330 .if \has_error_code == 1
331 movq ORIG_RAX(%rsp), %rsi /* get error code into 2nd argument*/
332 movq $-1, ORIG_RAX(%rsp) /* no syscall to restart */
341 * idtentry - Macro to generate entry stubs for simple IDT entries
342 * @vector: Vector number
343 * @asmsym: ASM symbol for the entry point
344 * @cfunc: C function to be called
345 * @has_error_code: Hardware pushed error code on stack
347 * The macro emits code to set up the kernel context for straight forward
348 * and simple IDT entries. No IST stack, no paranoid entry checks.
350 .macro idtentry vector asmsym cfunc has_error_code:req
351 SYM_CODE_START(\asmsym)
352 UNWIND_HINT_IRET_REGS offset=\has_error_code*8
355 .if \has_error_code == 0
356 pushq $-1 /* ORIG_RAX: no syscall to restart */
359 .if \vector == X86_TRAP_BP
361 * If coming from kernel space, create a 6-word gap to allow the
362 * int3 handler to emulate a call instruction.
364 testb $3, CS-ORIG_RAX(%rsp)
365 jnz .Lfrom_usermode_no_gap_\@
369 UNWIND_HINT_IRET_REGS offset=8
370 .Lfrom_usermode_no_gap_\@:
373 idtentry_body \cfunc \has_error_code
375 _ASM_NOKPROBE(\asmsym)
376 SYM_CODE_END(\asmsym)
380 * Interrupt entry/exit.
382 + The interrupt stubs push (vector) onto the stack, which is the error_code
383 * position of idtentry exceptions, and jump to one of the two idtentry points
386 * common_interrupt is a hotpath, align it to a cache line
388 .macro idtentry_irq vector cfunc
389 .p2align CONFIG_X86_L1_CACHE_SHIFT
390 idtentry \vector asm_\cfunc \cfunc has_error_code=1
394 * System vectors which invoke their handlers directly and are not
395 * going through the regular common device interrupt handling code.
397 .macro idtentry_sysvec vector cfunc
398 idtentry \vector asm_\cfunc \cfunc has_error_code=0
402 * idtentry_mce_db - Macro to generate entry stubs for #MC and #DB
403 * @vector: Vector number
404 * @asmsym: ASM symbol for the entry point
405 * @cfunc: C function to be called
407 * The macro emits code to set up the kernel context for #MC and #DB
409 * If the entry comes from user space it uses the normal entry path
410 * including the return to user space work and preemption checks on
413 * If hits in kernel mode then it needs to go through the paranoid
414 * entry as the exception can hit any random state. No preemption
415 * check on exit to keep the paranoid path simple.
417 .macro idtentry_mce_db vector asmsym cfunc
418 SYM_CODE_START(\asmsym)
419 UNWIND_HINT_IRET_REGS
422 pushq $-1 /* ORIG_RAX: no syscall to restart */
425 * If the entry is from userspace, switch stacks and treat it as
428 testb $3, CS-ORIG_RAX(%rsp)
429 jnz .Lfrom_usermode_switch_stack_\@
431 /* paranoid_entry returns GS information for paranoid_exit in EBX. */
436 movq %rsp, %rdi /* pt_regs pointer */
442 /* Switch to the regular task stack and use the noist entry point */
443 .Lfrom_usermode_switch_stack_\@:
444 idtentry_body noist_\cfunc, has_error_code=0
446 _ASM_NOKPROBE(\asmsym)
447 SYM_CODE_END(\asmsym)
450 #ifdef CONFIG_AMD_MEM_ENCRYPT
452 * idtentry_vc - Macro to generate entry stub for #VC
453 * @vector: Vector number
454 * @asmsym: ASM symbol for the entry point
455 * @cfunc: C function to be called
457 * The macro emits code to set up the kernel context for #VC. The #VC handler
458 * runs on an IST stack and needs to be able to cause nested #VC exceptions.
460 * To make this work the #VC entry code tries its best to pretend it doesn't use
461 * an IST stack by switching to the task stack if coming from user-space (which
462 * includes early SYSCALL entry path) or back to the stack in the IRET frame if
463 * entered from kernel-mode.
465 * If entered from kernel-mode the return stack is validated first, and if it is
466 * not safe to use (e.g. because it points to the entry stack) the #VC handler
467 * will switch to a fall-back stack (VC2) and call a special handler function.
469 * The macro is only used for one vector, but it is planned to be extended in
470 * the future for the #HV exception.
472 .macro idtentry_vc vector asmsym cfunc
473 SYM_CODE_START(\asmsym)
474 UNWIND_HINT_IRET_REGS
478 * If the entry is from userspace, switch stacks and treat it as
481 testb $3, CS-ORIG_RAX(%rsp)
482 jnz .Lfrom_usermode_switch_stack_\@
485 * paranoid_entry returns SWAPGS flag for paranoid_exit in EBX.
486 * EBX == 0 -> SWAPGS, EBX == 1 -> no SWAPGS
493 * Switch off the IST stack to make it free for nested exceptions. The
494 * vc_switch_off_ist() function will switch back to the interrupted
495 * stack if it is safe to do so. If not it switches to the VC fall-back
498 movq %rsp, %rdi /* pt_regs pointer */
499 call vc_switch_off_ist
500 movq %rax, %rsp /* Switch to new stack */
505 movq ORIG_RAX(%rsp), %rsi /* get error code into 2nd argument*/
506 movq $-1, ORIG_RAX(%rsp) /* no syscall to restart */
508 movq %rsp, %rdi /* pt_regs pointer */
513 * No need to switch back to the IST stack. The current stack is either
514 * identical to the stack in the IRET frame or the VC fall-back stack,
515 * so it is definitely mapped even with PTI enabled.
519 /* Switch to the regular task stack */
520 .Lfrom_usermode_switch_stack_\@:
521 idtentry_body user_\cfunc, has_error_code=1
523 _ASM_NOKPROBE(\asmsym)
524 SYM_CODE_END(\asmsym)
529 * Double fault entry. Straight paranoid. No checks from which context
530 * this comes because for the espfix induced #DF this would do the wrong
533 .macro idtentry_df vector asmsym cfunc
534 SYM_CODE_START(\asmsym)
535 UNWIND_HINT_IRET_REGS offset=8
538 /* paranoid_entry returns GS information for paranoid_exit in EBX. */
542 movq %rsp, %rdi /* pt_regs pointer into first argument */
543 movq ORIG_RAX(%rsp), %rsi /* get error code into 2nd argument*/
544 movq $-1, ORIG_RAX(%rsp) /* no syscall to restart */
549 _ASM_NOKPROBE(\asmsym)
550 SYM_CODE_END(\asmsym)
554 * Include the defines which emit the idt entries which are shared
555 * shared between 32 and 64 bit and emit the __irqentry_text_* markers
556 * so the stacktrace boundary checks work.
559 .globl __irqentry_text_start
560 __irqentry_text_start:
562 #include <asm/idtentry.h>
565 .globl __irqentry_text_end
568 SYM_CODE_START_LOCAL(common_interrupt_return)
569 SYM_INNER_LABEL(swapgs_restore_regs_and_return_to_usermode, SYM_L_GLOBAL)
570 #ifdef CONFIG_DEBUG_ENTRY
571 /* Assert that pt_regs indicates user mode. */
578 ALTERNATIVE "", "jmp xenpv_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
584 * The stack is now user RDI, orig_ax, RIP, CS, EFLAGS, RSP, SS.
585 * Save old stack pointer and switch to trampoline stack.
588 movq PER_CPU_VAR(cpu_tss_rw + TSS_sp0), %rsp
591 /* Copy the IRET frame to the trampoline stack. */
592 pushq 6*8(%rdi) /* SS */
593 pushq 5*8(%rdi) /* RSP */
594 pushq 4*8(%rdi) /* EFLAGS */
595 pushq 3*8(%rdi) /* CS */
596 pushq 2*8(%rdi) /* RIP */
598 /* Push user RDI on the trampoline stack. */
602 * We are on the trampoline stack. All regs except RDI are live.
603 * We can do future final exit work right here.
605 STACKLEAK_ERASE_NOCLOBBER
607 SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi
615 SYM_INNER_LABEL(restore_regs_and_return_to_kernel, SYM_L_GLOBAL)
616 #ifdef CONFIG_DEBUG_ENTRY
617 /* Assert that pt_regs indicates kernel mode. */
624 addq $8, %rsp /* skip regs->orig_ax */
626 * ARCH_HAS_MEMBARRIER_SYNC_CORE rely on IRET core serialization
627 * when returning from IPI handler.
631 SYM_INNER_LABEL_ALIGN(native_iret, SYM_L_GLOBAL)
632 UNWIND_HINT_IRET_REGS
634 * Are we returning to a stack segment from the LDT? Note: in
635 * 64-bit mode SS:RSP on the exception stack is always valid.
637 #ifdef CONFIG_X86_ESPFIX64
638 testb $4, (SS-RIP)(%rsp)
639 jnz native_irq_return_ldt
642 SYM_INNER_LABEL(native_irq_return_iret, SYM_L_GLOBAL)
644 * This may fault. Non-paranoid faults on return to userspace are
645 * handled by fixup_bad_iret. These include #SS, #GP, and #NP.
646 * Double-faults due to espfix64 are handled in exc_double_fault.
647 * Other faults here are fatal.
651 #ifdef CONFIG_X86_ESPFIX64
652 native_irq_return_ldt:
654 * We are running with user GSBASE. All GPRs contain their user
655 * values. We have a percpu ESPFIX stack that is eight slots
656 * long (see ESPFIX_STACK_SIZE). espfix_waddr points to the bottom
657 * of the ESPFIX stack.
659 * We clobber RAX and RDI in this code. We stash RDI on the
660 * normal stack and RAX on the ESPFIX stack.
662 * The ESPFIX stack layout we set up looks like this:
664 * --- top of ESPFIX stack ---
669 * RIP <-- RSP points here when we're done
670 * RAX <-- espfix_waddr points here
671 * --- bottom of ESPFIX stack ---
674 pushq %rdi /* Stash user RDI */
675 swapgs /* to kernel GS */
676 SWITCH_TO_KERNEL_CR3 scratch_reg=%rdi /* to kernel CR3 */
678 movq PER_CPU_VAR(espfix_waddr), %rdi
679 movq %rax, (0*8)(%rdi) /* user RAX */
680 movq (1*8)(%rsp), %rax /* user RIP */
681 movq %rax, (1*8)(%rdi)
682 movq (2*8)(%rsp), %rax /* user CS */
683 movq %rax, (2*8)(%rdi)
684 movq (3*8)(%rsp), %rax /* user RFLAGS */
685 movq %rax, (3*8)(%rdi)
686 movq (5*8)(%rsp), %rax /* user SS */
687 movq %rax, (5*8)(%rdi)
688 movq (4*8)(%rsp), %rax /* user RSP */
689 movq %rax, (4*8)(%rdi)
690 /* Now RAX == RSP. */
692 andl $0xffff0000, %eax /* RAX = (RSP & 0xffff0000) */
695 * espfix_stack[31:16] == 0. The page tables are set up such that
696 * (espfix_stack | (X & 0xffff0000)) points to a read-only alias of
697 * espfix_waddr for any X. That is, there are 65536 RO aliases of
698 * the same page. Set up RSP so that RSP[31:16] contains the
699 * respective 16 bits of the /userspace/ RSP and RSP nonetheless
700 * still points to an RO alias of the ESPFIX stack.
702 orq PER_CPU_VAR(espfix_stack), %rax
704 SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi
705 swapgs /* to user GS */
706 popq %rdi /* Restore user RDI */
709 UNWIND_HINT_IRET_REGS offset=8
712 * At this point, we cannot write to the stack any more, but we can
715 popq %rax /* Restore user RAX */
718 * RSP now points to an ordinary IRET frame, except that the page
719 * is read-only and RSP[31:16] are preloaded with the userspace
720 * values. We can now IRET back to userspace.
722 jmp native_irq_return_iret
724 SYM_CODE_END(common_interrupt_return)
725 _ASM_NOKPROBE(common_interrupt_return)
728 * Reload gs selector with exception handling
731 * Is in entry.text as it shouldn't be instrumented.
733 SYM_FUNC_START(asm_load_gs_index)
738 2: ALTERNATIVE "", "mfence", X86_BUG_SWAPGS_FENCE
742 SYM_FUNC_END(asm_load_gs_index)
743 EXPORT_SYMBOL(asm_load_gs_index)
745 _ASM_EXTABLE(.Lgs_change, .Lbad_gs)
746 .section .fixup, "ax"
747 /* running with kernelgs */
748 SYM_CODE_START_LOCAL_NOALIGN(.Lbad_gs)
749 swapgs /* switch back to user gs */
751 /* This can't be a string because the preprocessor needs to see it. */
752 movl $__USER_DS, %eax
755 ALTERNATIVE "", "ZAP_GS", X86_BUG_NULL_SEG
759 SYM_CODE_END(.Lbad_gs)
764 * A note on the "critical region" in our callback handler.
765 * We want to avoid stacking callback handlers due to events occurring
766 * during handling of the last event. To do this, we keep events disabled
767 * until we've done all processing. HOWEVER, we must enable events before
768 * popping the stack frame (can't be done atomically) and so it would still
769 * be possible to get enough handler activations to overflow the stack.
770 * Although unlikely, bugs of that kind are hard to track down, so we'd
771 * like to avoid the possibility.
772 * So, on entry to the handler we detect whether we interrupted an
773 * existing activation in its critical region -- if so, we pop the current
774 * activation and restart the handler using the previous one.
776 * C calling convention: exc_xen_hypervisor_callback(struct *pt_regs)
778 SYM_CODE_START_LOCAL(exc_xen_hypervisor_callback)
781 * Since we don't modify %rdi, evtchn_do_upall(struct *pt_regs) will
782 * see the correct pointer to the pt_regs
785 movq %rdi, %rsp /* we don't return, adjust the stack frame */
788 call xen_pv_evtchn_do_upcall
791 SYM_CODE_END(exc_xen_hypervisor_callback)
794 * Hypervisor uses this for application faults while it executes.
795 * We get here for two reasons:
796 * 1. Fault while reloading DS, ES, FS or GS
797 * 2. Fault while executing IRET
798 * Category 1 we do not need to fix up as Xen has already reloaded all segment
799 * registers that could be reloaded and zeroed the others.
800 * Category 2 we fix up by killing the current process. We cannot use the
801 * normal Linux return path in this case because if we use the IRET hypercall
802 * to pop the stack frame we end up in an infinite loop of failsafe callbacks.
803 * We distinguish between categories by comparing each saved segment register
804 * with its current contents: any discrepancy means we in category 1.
806 SYM_CODE_START(xen_failsafe_callback)
820 /* All segments match their saved values => Category 2 (Bad IRET). */
825 UNWIND_HINT_IRET_REGS offset=8
826 jmp asm_exc_general_protection
827 1: /* Segment mismatch => Category 1 (Bad segment). Retry the IRET. */
831 UNWIND_HINT_IRET_REGS
832 pushq $-1 /* orig_ax = -1 => not a system call */
836 SYM_CODE_END(xen_failsafe_callback)
837 #endif /* CONFIG_XEN_PV */
840 * Save all registers in pt_regs. Return GSBASE related information
841 * in EBX depending on the availability of the FSGSBASE instructions:
844 * N 0 -> SWAPGS on exit
845 * 1 -> no SWAPGS on exit
847 * Y GSBASE value at entry, must be restored in paranoid_exit
849 SYM_CODE_START_LOCAL(paranoid_entry)
852 PUSH_AND_CLEAR_REGS save_ret=1
853 ENCODE_FRAME_POINTER 8
856 * Always stash CR3 in %r14. This value will be restored,
857 * verbatim, at exit. Needed if paranoid_entry interrupted
858 * another entry that already switched to the user CR3 value
859 * but has not yet returned to userspace.
861 * This is also why CS (stashed in the "iret frame" by the
862 * hardware at entry) can not be used: this may be a return
863 * to kernel code, but with a user CR3 value.
865 * Switching CR3 does not depend on kernel GSBASE so it can
866 * be done before switching to the kernel GSBASE. This is
867 * required for FSGSBASE because the kernel GSBASE has to
868 * be retrieved from a kernel internal table.
870 SAVE_AND_SWITCH_TO_KERNEL_CR3 scratch_reg=%rax save_reg=%r14
873 * Handling GSBASE depends on the availability of FSGSBASE.
875 * Without FSGSBASE the kernel enforces that negative GSBASE
876 * values indicate kernel GSBASE. With FSGSBASE no assumptions
877 * can be made about the GSBASE value when entering from user
880 ALTERNATIVE "jmp .Lparanoid_entry_checkgs", "", X86_FEATURE_FSGSBASE
883 * Read the current GSBASE and store it in %rbx unconditionally,
884 * retrieve and set the current CPUs kernel GSBASE. The stored value
885 * has to be restored in paranoid_exit unconditionally.
887 * The unconditional write to GS base below ensures that no subsequent
888 * loads based on a mispredicted GS base can happen, therefore no LFENCE
891 SAVE_AND_SET_GSBASE scratch_reg=%rax save_reg=%rbx
894 .Lparanoid_entry_checkgs:
895 /* EBX = 1 -> kernel GSBASE active, no restore required */
899 * The kernel-enforced convention is a negative GSBASE indicates
900 * a kernel value. No SWAPGS needed on entry and exit.
902 movl $MSR_GS_BASE, %ecx
905 js .Lparanoid_kernel_gsbase
907 /* EBX = 0 -> SWAPGS required on exit */
910 .Lparanoid_kernel_gsbase:
912 FENCE_SWAPGS_KERNEL_ENTRY
914 SYM_CODE_END(paranoid_entry)
917 * "Paranoid" exit path from exception stack. This is invoked
918 * only on return from non-NMI IST interrupts that came
921 * We may be returning to very strange contexts (e.g. very early
922 * in syscall entry), so checking for preemption here would
923 * be complicated. Fortunately, there's no good reason to try
924 * to handle preemption here.
926 * R/EBX contains the GSBASE related information depending on the
927 * availability of the FSGSBASE instructions:
930 * N 0 -> SWAPGS on exit
931 * 1 -> no SWAPGS on exit
933 * Y User space GSBASE, must be restored unconditionally
935 SYM_CODE_START_LOCAL(paranoid_exit)
938 * The order of operations is important. RESTORE_CR3 requires
941 * NB to anyone to try to optimize this code: this code does
942 * not execute at all for exceptions from user mode. Those
943 * exceptions go through error_exit instead.
945 RESTORE_CR3 scratch_reg=%rax save_reg=%r14
947 /* Handle the three GSBASE cases */
948 ALTERNATIVE "jmp .Lparanoid_exit_checkgs", "", X86_FEATURE_FSGSBASE
950 /* With FSGSBASE enabled, unconditionally restore GSBASE */
952 jmp restore_regs_and_return_to_kernel
954 .Lparanoid_exit_checkgs:
955 /* On non-FSGSBASE systems, conditionally do SWAPGS */
957 jnz restore_regs_and_return_to_kernel
959 /* We are returning to a context with user GSBASE */
961 jmp restore_regs_and_return_to_kernel
962 SYM_CODE_END(paranoid_exit)
965 * Save all registers in pt_regs, and switch GS if needed.
967 SYM_CODE_START_LOCAL(error_entry)
970 PUSH_AND_CLEAR_REGS save_ret=1
971 ENCODE_FRAME_POINTER 8
973 jz .Lerror_kernelspace
976 * We entered from user mode or we're pretending to have entered
977 * from user mode due to an IRET fault.
980 FENCE_SWAPGS_USER_ENTRY
981 /* We have user CR3. Change to kernel CR3. */
982 SWITCH_TO_KERNEL_CR3 scratch_reg=%rax
984 .Lerror_entry_from_usermode_after_swapgs:
985 /* Put us onto the real thread stack. */
986 popq %r12 /* save return addr in %12 */
987 movq %rsp, %rdi /* arg0 = pt_regs pointer */
989 movq %rax, %rsp /* switch stack */
995 * There are two places in the kernel that can potentially fault with
996 * usergs. Handle them here. B stepping K8s sometimes report a
997 * truncated RIP for IRET exceptions returning to compat mode. Check
998 * for these here too.
1000 .Lerror_kernelspace:
1001 leaq native_irq_return_iret(%rip), %rcx
1002 cmpq %rcx, RIP+8(%rsp)
1004 movl %ecx, %eax /* zero extend */
1005 cmpq %rax, RIP+8(%rsp)
1007 cmpq $.Lgs_change, RIP+8(%rsp)
1008 jne .Lerror_entry_done_lfence
1011 * hack: .Lgs_change can fail with user gsbase. If this happens, fix up
1012 * gsbase and proceed. We'll fix up the exception and land in
1013 * .Lgs_change's error handler with kernel gsbase.
1018 * Issue an LFENCE to prevent GS speculation, regardless of whether it is a
1019 * kernel or user gsbase.
1021 .Lerror_entry_done_lfence:
1022 FENCE_SWAPGS_KERNEL_ENTRY
1026 /* Fix truncated RIP */
1027 movq %rcx, RIP+8(%rsp)
1032 * We came from an IRET to user mode, so we have user
1033 * gsbase and CR3. Switch to kernel gsbase and CR3:
1036 FENCE_SWAPGS_USER_ENTRY
1037 SWITCH_TO_KERNEL_CR3 scratch_reg=%rax
1040 * Pretend that the exception came from user mode: set up pt_regs
1041 * as if we faulted immediately after IRET.
1046 jmp .Lerror_entry_from_usermode_after_swapgs
1047 SYM_CODE_END(error_entry)
1049 SYM_CODE_START_LOCAL(error_return)
1051 DEBUG_ENTRY_ASSERT_IRQS_OFF
1053 jz restore_regs_and_return_to_kernel
1054 jmp swapgs_restore_regs_and_return_to_usermode
1055 SYM_CODE_END(error_return)
1058 * Runs on exception stack. Xen PV does not go through this path at all,
1059 * so we can use real assembly here.
1062 * %r14: Used to save/restore the CR3 of the interrupted context
1063 * when PAGE_TABLE_ISOLATION is in use. Do not clobber.
1065 SYM_CODE_START(asm_exc_nmi)
1066 UNWIND_HINT_IRET_REGS
1069 * We allow breakpoints in NMIs. If a breakpoint occurs, then
1070 * the iretq it performs will take us out of NMI context.
1071 * This means that we can have nested NMIs where the next
1072 * NMI is using the top of the stack of the previous NMI. We
1073 * can't let it execute because the nested NMI will corrupt the
1074 * stack of the previous NMI. NMI handlers are not re-entrant
1077 * To handle this case we do the following:
1078 * Check the a special location on the stack that contains
1079 * a variable that is set when NMIs are executing.
1080 * The interrupted task's stack is also checked to see if it
1082 * If the variable is not set and the stack is not the NMI
1084 * o Set the special variable on the stack
1085 * o Copy the interrupt frame into an "outermost" location on the
1087 * o Copy the interrupt frame into an "iret" location on the stack
1088 * o Continue processing the NMI
1089 * If the variable is set or the previous stack is the NMI stack:
1090 * o Modify the "iret" location to jump to the repeat_nmi
1091 * o return back to the first NMI
1093 * Now on exit of the first NMI, we first clear the stack variable
1094 * The NMI stack will tell any nested NMIs at that point that it is
1095 * nested. Then we pop the stack normally with iret, and if there was
1096 * a nested NMI that updated the copy interrupt stack frame, a
1097 * jump will be made to the repeat_nmi code that will handle the second
1100 * However, espfix prevents us from directly returning to userspace
1101 * with a single IRET instruction. Similarly, IRET to user mode
1102 * can fault. We therefore handle NMIs from user space like
1103 * other IST entries.
1108 /* Use %rdx as our temp variable throughout */
1111 testb $3, CS-RIP+8(%rsp)
1112 jz .Lnmi_from_kernel
1115 * NMI from user mode. We need to run on the thread stack, but we
1116 * can't go through the normal entry paths: NMIs are masked, and
1117 * we don't want to enable interrupts, because then we'll end
1118 * up in an awkward situation in which IRQs are on but NMIs
1121 * We also must not push anything to the stack before switching
1122 * stacks lest we corrupt the "NMI executing" variable.
1127 FENCE_SWAPGS_USER_ENTRY
1128 SWITCH_TO_KERNEL_CR3 scratch_reg=%rdx
1130 movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp
1131 UNWIND_HINT_IRET_REGS base=%rdx offset=8
1132 pushq 5*8(%rdx) /* pt_regs->ss */
1133 pushq 4*8(%rdx) /* pt_regs->rsp */
1134 pushq 3*8(%rdx) /* pt_regs->flags */
1135 pushq 2*8(%rdx) /* pt_regs->cs */
1136 pushq 1*8(%rdx) /* pt_regs->rip */
1137 UNWIND_HINT_IRET_REGS
1138 pushq $-1 /* pt_regs->orig_ax */
1139 PUSH_AND_CLEAR_REGS rdx=(%rdx)
1140 ENCODE_FRAME_POINTER
1143 * At this point we no longer need to worry about stack damage
1144 * due to nesting -- we're on the normal thread stack and we're
1145 * done with the NMI stack.
1153 * Return back to user mode. We must *not* do the normal exit
1154 * work, because we don't want to enable interrupts.
1156 jmp swapgs_restore_regs_and_return_to_usermode
1160 * Here's what our stack frame will look like:
1161 * +---------------------------------------------------------+
1163 * | original Return RSP |
1164 * | original RFLAGS |
1167 * +---------------------------------------------------------+
1168 * | temp storage for rdx |
1169 * +---------------------------------------------------------+
1170 * | "NMI executing" variable |
1171 * +---------------------------------------------------------+
1172 * | iret SS } Copied from "outermost" frame |
1173 * | iret Return RSP } on each loop iteration; overwritten |
1174 * | iret RFLAGS } by a nested NMI to force another |
1175 * | iret CS } iteration if needed. |
1177 * +---------------------------------------------------------+
1178 * | outermost SS } initialized in first_nmi; |
1179 * | outermost Return RSP } will not be changed before |
1180 * | outermost RFLAGS } NMI processing is done. |
1181 * | outermost CS } Copied to "iret" frame on each |
1182 * | outermost RIP } iteration. |
1183 * +---------------------------------------------------------+
1185 * +---------------------------------------------------------+
1187 * The "original" frame is used by hardware. Before re-enabling
1188 * NMIs, we need to be done with it, and we need to leave enough
1189 * space for the asm code here.
1191 * We return by executing IRET while RSP points to the "iret" frame.
1192 * That will either return for real or it will loop back into NMI
1195 * The "outermost" frame is copied to the "iret" frame on each
1196 * iteration of the loop, so each iteration starts with the "iret"
1197 * frame pointing to the final return target.
1201 * Determine whether we're a nested NMI.
1203 * If we interrupted kernel code between repeat_nmi and
1204 * end_repeat_nmi, then we are a nested NMI. We must not
1205 * modify the "iret" frame because it's being written by
1206 * the outer NMI. That's okay; the outer NMI handler is
1207 * about to about to call exc_nmi() anyway, so we can just
1208 * resume the outer NMI.
1211 movq $repeat_nmi, %rdx
1214 movq $end_repeat_nmi, %rdx
1220 * Now check "NMI executing". If it's set, then we're nested.
1221 * This will not detect if we interrupted an outer NMI just
1228 * Now test if the previous stack was an NMI stack. This covers
1229 * the case where we interrupt an outer NMI after it clears
1230 * "NMI executing" but before IRET. We need to be careful, though:
1231 * there is one case in which RSP could point to the NMI stack
1232 * despite there being no NMI active: naughty userspace controls
1233 * RSP at the very beginning of the SYSCALL targets. We can
1234 * pull a fast one on naughty userspace, though: we program
1235 * SYSCALL to mask DF, so userspace cannot cause DF to be set
1236 * if it controls the kernel's RSP. We set DF before we clear
1240 /* Compare the NMI stack (rdx) with the stack we came from (4*8(%rsp)) */
1241 cmpq %rdx, 4*8(%rsp)
1242 /* If the stack pointer is above the NMI stack, this is a normal NMI */
1245 subq $EXCEPTION_STKSZ, %rdx
1246 cmpq %rdx, 4*8(%rsp)
1247 /* If it is below the NMI stack, it is a normal NMI */
1250 /* Ah, it is within the NMI stack. */
1252 testb $(X86_EFLAGS_DF >> 8), (3*8 + 1)(%rsp)
1253 jz first_nmi /* RSP was user controlled. */
1255 /* This is a nested NMI. */
1259 * Modify the "iret" frame to point to repeat_nmi, forcing another
1260 * iteration of NMI handling.
1263 leaq -10*8(%rsp), %rdx
1270 /* Put stack back */
1276 /* We are returning to kernel mode, so this cannot result in a fault. */
1283 /* Make room for "NMI executing". */
1286 /* Leave room for the "iret" frame */
1289 /* Copy the "original" frame to the "outermost" frame */
1293 UNWIND_HINT_IRET_REGS
1295 /* Everything up to here is safe from nested NMIs */
1297 #ifdef CONFIG_DEBUG_ENTRY
1299 * For ease of testing, unmask NMIs right away. Disabled by
1300 * default because IRET is very expensive.
1303 pushq %rsp /* RSP (minus 8 because of the previous push) */
1304 addq $8, (%rsp) /* Fix up RSP */
1306 pushq $__KERNEL_CS /* CS */
1308 iretq /* continues at repeat_nmi below */
1309 UNWIND_HINT_IRET_REGS
1315 * If there was a nested NMI, the first NMI's iret will return
1316 * here. But NMIs are still enabled and we can take another
1317 * nested NMI. The nested NMI checks the interrupted RIP to see
1318 * if it is between repeat_nmi and end_repeat_nmi, and if so
1319 * it will just return, as we are about to repeat an NMI anyway.
1320 * This makes it safe to copy to the stack frame that a nested
1323 * RSP is pointing to "outermost RIP". gsbase is unknown, but, if
1324 * we're repeating an NMI, gsbase has the same value that it had on
1325 * the first iteration. paranoid_entry will load the kernel
1326 * gsbase if needed before we call exc_nmi(). "NMI executing"
1329 movq $1, 10*8(%rsp) /* Set "NMI executing". */
1332 * Copy the "outermost" frame to the "iret" frame. NMIs that nest
1333 * here must not modify the "iret" frame while we're writing to
1334 * it or it will end up containing garbage.
1344 * Everything below this point can be preempted by a nested NMI.
1345 * If this happens, then the inner NMI will change the "iret"
1346 * frame to point back to repeat_nmi.
1348 pushq $-1 /* ORIG_RAX: no syscall to restart */
1351 * Use paranoid_entry to handle SWAPGS, but no need to use paranoid_exit
1352 * as we should not be calling schedule in NMI context.
1353 * Even with normal interrupts enabled. An NMI should not be
1354 * setting NEED_RESCHED or anything that normal interrupts and
1355 * exceptions might do.
1364 /* Always restore stashed CR3 value (see paranoid_entry) */
1365 RESTORE_CR3 scratch_reg=%r15 save_reg=%r14
1368 * The above invocation of paranoid_entry stored the GSBASE
1369 * related information in R/EBX depending on the availability
1372 * If FSGSBASE is enabled, restore the saved GSBASE value
1373 * unconditionally, otherwise take the conditional SWAPGS path.
1375 ALTERNATIVE "jmp nmi_no_fsgsbase", "", X86_FEATURE_FSGSBASE
1381 /* EBX == 0 -> invoke SWAPGS */
1392 * Skip orig_ax and the "outermost" frame to point RSP at the "iret"
1393 * at the "iret" frame.
1398 * Clear "NMI executing". Set DF first so that we can easily
1399 * distinguish the remaining code between here and IRET from
1400 * the SYSCALL entry and exit paths.
1402 * We arguably should just inspect RIP instead, but I (Andy) wrote
1403 * this code when I had the misapprehension that Xen PV supported
1404 * NMIs, and Xen PV would break that approach.
1407 movq $0, 5*8(%rsp) /* clear "NMI executing" */
1410 * iretq reads the "iret" frame and exits the NMI stack in a
1411 * single instruction. We are returning to kernel mode, so this
1412 * cannot result in a fault. Similarly, we don't need to worry
1413 * about espfix64 on the way back to kernel mode.
1416 SYM_CODE_END(asm_exc_nmi)
1418 #ifndef CONFIG_IA32_EMULATION
1420 * This handles SYSCALL from 32-bit code. There is no way to program
1421 * MSRs to fully disable 32-bit SYSCALL.
1423 SYM_CODE_START(ignore_sysret)
1427 SYM_CODE_END(ignore_sysret)
1430 .pushsection .text, "ax"
1431 SYM_CODE_START(rewind_stack_do_exit)
1433 /* Prevent any naive code from trying to unwind to our caller. */
1436 movq PER_CPU_VAR(cpu_current_top_of_stack), %rax
1437 leaq -PTREGS_SIZE(%rax), %rsp
1441 SYM_CODE_END(rewind_stack_do_exit)
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