Re: [PATCH] x86/kprobe: Fix crash when probe cs call

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Thu, 27 Aug 2026 10:57:56 +0800
Jinke Han <jinkehan@didiglobal.com> wrote:

> When I used eBPF to probe the call instructions within a function,
> we encountered a kernel crash.
> 
> The ebpf tool probes the 257 offset of the __hrtimer_run_queues
> function.
> 
> <__hrtimer_run_queues+249>:  nopl   0x0(%rax,%rax,1)
> <__hrtimer_run_queues+254>:  mov    %r14,%rdi
> <__hrtimer_run_queues+257>:  cs call <__x86_indirect_thunk_r12>
> <__hrtimer_run_queues+263>:  mov    %eax,%r12d
> <__hrtimer_run_queues+266>:  xchg   %ax,%ax
> <__hrtimer_run_queues+268>:  mov    %r13,%rdi
> 
> The scene of kernel crash is as follows:
> 
> [73665.737181] BUG: unable to handle page fault for address: 00000000000f41c9
> [73665.744253] #PF: supervisor write access in kernel mode
> [73665.749643] #PF: error_code(0x0002) - not-present page
> [73665.754843] PGD 0 P4D 0
> [73665.757390] Oops: 0002 [#1] SMP NOPTI
> [73665.761073] CPU: 1 PID: 0 Comm: swapper/1 Kdump: loaded Tainted: P
> [73665.782671] RIP: 0010:__hrtimer_run_queues+0x106/0x230
> 
> Note that __hrtimer_run_queues+0x106 is __hrtimer_run_queues+262, which is
> at the 6th byte of the above cs call instruction. Since the cs call
> instruction occupies 6 bytes, the exception occurred in the middle of that
> call instruction.

Ah, OK. So it adds a dummy CS prefix for padding the instruction.
Also, please Cc probe patch to linux-trace-kernel@vger.kernel.org too.

> 
> The root cause is that when using eBPF tools to probe in the middle of a
> function, kprobe with int3 is used as the underlying implementation.
> During single-step emulation of the original call instruction,
> int3_emulate_call assumes that the probed call instruction is 5 bytes
> long. However, the actual CS-prefixed call instruction occupies 6 bytes,
> so it constructs an incorrect exception return address. When the CPU
> returns from the kprobe handler, the next instruction to be executed is at
> the address of the last byte of that CS call instruction. Coincidentally,
> starting from that address, the CPU fetches and decodes a completely
> different instruction, which ultimately triggers a kernel crash.
> 
> Fix the issue by using the actual instruction length obtained from
> the instruction decoder when constructing the exception return
> address, rather than relying on the hardcoded CALL_INSN_SIZE macro.
> 

OK, this fixes kprobe local, but for cleanliness, I have a comment.

> Cc: stable@kernel.org
> Fixes: 6256e668b7af ("x86/kprobes: Use int3 instead of debug trap for single-step")
> Signed-off-by: Jinke Han <jinkehan@didiglobal.com>
> ---
>  arch/x86/kernel/kprobes/core.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> index 4e5f8c1736ec..b50e4a60bdfe 100644
> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -511,9 +511,11 @@ NOKPROBE_SYMBOL(kprobe_emulate_ret);
>  static void kprobe_emulate_call(struct kprobe *p, struct pt_regs *regs)
>  {
>  	unsigned long func = regs->ip - INT3_INSN_SIZE + p->ainsn.size;
> +	unsigned long ip = func;
>  
>  	func += p->ainsn.rel32;

This looks a bit less readability because func is once have a return address
and later it becomes call address. Moreover int3_emulate_call() is kept broken :(.

> -	int3_emulate_call(regs, func);
> +	int3_emulate_push(regs, ip);
> +	int3_emulate_jmp(regs, func);

What about updating int3_emulate_call() to get ip (=return address) and func?

void int3_emulate_call(struct pt_regs *regs, unsigned long ip, unsigned long func)
{
	int3_emulate_push(regs, ip);
	int3_emulate_jmp(regs, func);
}

Then in  smp_text_poke_int3_handler(struct pt_regs *regs),  we will call:

	int3_emulate_call(regs, (long)ip, (long)ip + tpl->disp);

And in this, we simply call;

	unsigned long ip = regs->ip - INT3_INSN_SIZE + p->ainsn.size;

	int3_emulate_call(regs, ip, ip + p->ainsn.rel32);

(Note: currently smp_text_poke_batch() does not handle non-5 byte nop.
it checks the size.)

Thanks,

>  }
>  NOKPROBE_SYMBOL(kprobe_emulate_call);
>  
> -- 
> 2.34.1
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>