lguest, x86/entry/32: Fix handling of guest syscalls using interrupt gates

In a798f091113e ("x86/entry/32: Change INT80 to be an interrupt gate")
Andy broke lguest.  This is because lguest had special code to allow
the 0x80 trap gate go straight into the guest itself; interrupts gates
(without more work, as mentioned in the file's comments) bounce via
the hypervisor.

His change made them go via the hypervisor, but as it's in the range of
normal hardware interrupts, they were not directed through to the guest
at all.  Turns out the guest userspace isn't very effective if syscalls
are all noops.

I haven't ripped out all the now-useless trap-direct-to-guest-kernel
code yet, since it will still be needed if someone decides to update
this optimization.

Signed-off-by: Rusty Russell <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Weisbecker <[email protected]>
Cc: x86\@kernel.org
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

diff --git a/drivers/lguest/interrupts_and_traps.c b/drivers/lguest/interrupts_and_traps.c
index eb934b0242e0e17652b7fc17c3255b5e70e40d15..67392b6ab845fbdec7d54798b591c24e544347a9 100644 (file)
--- a/drivers/lguest/interrupts_and_traps.c
+++ b/drivers/lguest/interrupts_and_traps.c
@@ -331,7 +331,7 @@ void set_interrupt(struct lg_cpu *cpu, unsigned int irq)
  * Actually now I think of it, it's possible that Ron *is* half the Plan 9
  * userbase.  Oh well.
  */
-static bool could_be_syscall(unsigned int num)
+bool could_be_syscall(unsigned int num)
 {
        /* Normal Linux IA32_SYSCALL_VECTOR or reserved vector? */
        return num == IA32_SYSCALL_VECTOR || num == syscall_vector;
@@ -416,6 +416,10 @@ bool deliver_trap(struct lg_cpu *cpu, unsigned int num)
  *
  * This routine indicates if a particular trap number could be delivered
  * directly.
+ *
+ * Unfortunately, Linux 4.6 started using an interrupt gate instead of a
+ * trap gate for syscalls, so this trick is ineffective.  See Mastery for
+ * how we could do this anyway...
  */
 static bool direct_trap(unsigned int num)
 {

diff --git a/drivers/lguest/lg.h b/drivers/lguest/lg.h
index ac8ad0461e809db526bfc1db1ca2290ac612a43d..69b3814afd2f63c7fabe82dde698fae44b2d1862 100644 (file)
--- a/drivers/lguest/lg.h
+++ b/drivers/lguest/lg.h
@@ -167,6 +167,7 @@ void guest_set_clockevent(struct lg_cpu *cpu, unsigned long delta);
 bool send_notify_to_eventfd(struct lg_cpu *cpu);
 void init_clockdev(struct lg_cpu *cpu);
 bool check_syscall_vector(struct lguest *lg);
+bool could_be_syscall(unsigned int num);
 int init_interrupts(void);
 void free_interrupts(void);
 

diff --git a/drivers/lguest/x86/core.c b/drivers/lguest/x86/core.c
index 6a4cd771a2be62b4172cc26a178ca85fbf7e6d27..adc162c7040d7ef0a2f8f738e7a21bab1d57ba09 100644 (file)
--- a/drivers/lguest/x86/core.c
+++ b/drivers/lguest/x86/core.c
@@ -429,8 +429,12 @@ void lguest_arch_handle_trap(struct lg_cpu *cpu)
                        return;
                break;
        case 32 ... 255:
+               /* This might be a syscall. */
+               if (could_be_syscall(cpu->regs->trapnum))
+                       break;
+
                /*
-                * These values mean a real interrupt occurred, in which case
+                * Other values mean a real interrupt occurred, in which case
                 * the Host handler has already been run. We just do a
                 * friendly check if another process should now be run, then
                 * return to run the Guest again.