2022-08-14 06:45:09

by Nadav Amit

Subject: [PATCH] x86/kprobes: fix JNG/JNLE emulation

From: Nadav Amit <[email protected]>

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'.

Cc: Masami Hiramatsu <[email protected]>
Cc: Peter Zijlstra (Intel) <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: [email protected]
Fixes: 6256e668b7af ("x86/kprobes: Use int3 instead of debug trap for single-step")
Signed-off-by: Nadav Amit <[email protected]>
---
arch/x86/kernel/kprobes/core.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index 74167dc5f55e..4c3c27b6aea3 100644
--- 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));
}
--
2.25.1

2022-08-15 07:54:14

by Peter Zijlstra

Subject: Re: [PATCH] x86/kprobes: fix JNG/JNLE emulation

On Sat, Aug 13, 2022 at 03:59:43PM -0700, Nadav Amit wrote:
> From: Nadav Amit <[email protected]>
>
> 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'.
>
> Cc: Masami Hiramatsu <[email protected]>
> Cc: Peter Zijlstra (Intel) <[email protected]>
> Cc: Andy Lutomirski <[email protected]>
> Cc: [email protected]
> Fixes: 6256e668b7af ("x86/kprobes: Use int3 instead of debug trap for single-step")
> Signed-off-by: Nadav Amit <[email protected]>

Urgghh..

Acked-by: Peter Zijlstra (Intel) <[email protected]>

> ---
> arch/x86/kernel/kprobes/core.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> index 74167dc5f55e..4c3c27b6aea3 100644
> --- 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));
> }
> --
> 2.25.1
>

2022-08-15 09:03:23

by tip-bot2 for Jacob Pan

Subject: [tip: perf/urgent] x86/kprobes: Fix JNG/JNLE emulation

The following commit has been merged into the perf/urgent branch of tip:

Commit-ID: 8924779df820c53875abaeb10c648e9cb75b46d4
Gitweb: https://git.kernel.org/tip/8924779df820c53875abaeb10c648e9cb75b46d4
Author: Nadav Amit <[email protected]>
AuthorDate: Sat, 13 Aug 2022 15:59:43 -07:00
Committer: Ingo Molnar <[email protected]>
CommitterDate: Sun, 14 Aug 2022 11:27:17 +02:00

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]
---
arch/x86/kernel/kprobes/core.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index 74167dc..4c3c27b 100644
--- 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));
}