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

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

[Jinke Han]

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.

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.

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;
-	int3_emulate_call(regs, func);
+	int3_emulate_push(regs, ip);
+	int3_emulate_jmp(regs, func);
 }
 NOKPROBE_SYMBOL(kprobe_emulate_call);
 
-- 
2.34.1

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

[Yafang Shao]

On Thu, 27 Aug 2026 at 10:57 AM 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.
>
> 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.
>
> 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>

LGTM
Acked-by: Yafang Shao <laoar.shao@gmail.com>

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

[Masami Hiramatsu]

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>