x86/kprobes: Fix JNG/JNLE emulation

When kprobes emulates JNG/JNLE instructions on x86 it uses the wrong
condition. For JNG (opcode: 0F 8E), according to Intel SDM, the jump is
performed if (ZF == 1 or SF != OF). However the kernel emulation
currently uses 'and' instead of 'or'.

As a result, setting a kprobe on JNG/JNLE might cause the kernel to
behave incorrectly whenever the kprobe is hit.

Fix by changing the 'and' to 'or'.

Fixes: 6256e668b7af ("x86/kprobes: Use int3 instead of debug trap for single-step")
Signed-off-by: Nadav Amit <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index 74167dc5f55ec4f6056626b25a6fbceb0903efa4..4c3c27b6aea3b56b2ffe9b8c2375c0353f9ce206 100644 (file)
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -505,7 +505,7 @@ static void kprobe_emulate_jcc(struct kprobe *p, struct pt_regs *regs)
                match = ((regs->flags & X86_EFLAGS_SF) >> X86_EFLAGS_SF_BIT) ^
                        ((regs->flags & X86_EFLAGS_OF) >> X86_EFLAGS_OF_BIT);
                if (p->ainsn.jcc.type >= 0xe)
-                       match = match && (regs->flags & X86_EFLAGS_ZF);
+                       match = match || (regs->flags & X86_EFLAGS_ZF);
        }
        __kprobe_emulate_jmp(p, regs, (match && !invert) || (!match && invert));
 }