Re: [PATCH v14 13/19] unwind_user: Enable archs that pass RA in a register

On 5/5/2026 8:35 PM, [email protected] wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [High] Unprivileged user space can trigger a kernel warning (`WARN_ON_ONCE`) and potential panic by providing a maliciously crafted `.sframe` section.
> --
> 
> commit 20f03d41baa46baef137fb9a88081c8f92b5eb5f
> Author: Jens Remus <[email protected]>
> 
> unwind_user: Enable archs that pass RA in a register
> 
> This commit updates the user space stack unwinder to support architectures
> that pass the return address in a register instead of saving it on the
> stack. It also handles architectures where the stack pointer may be
> unchanged in the topmost frame.
> 
>> diff --git a/include/linux/unwind_user.h b/include/linux/unwind_user.h
>> index 64618618febde..bc2edae399556 100644
>> --- a/include/linux/unwind_user.h
>> +++ b/include/linux/unwind_user.h
>> @@ -23,6 +23,15 @@ static inline bool unwind_user_at_function_start(struct pt_regs *regs)
>>  #define unwind_user_at_function_start unwind_user_at_function_start
>>  #endif
>>  
>> +#ifndef unwind_user_get_ra_reg
>> +static inline int unwind_user_get_ra_reg(unsigned long *val)
>> +{
>> +	WARN_ON_ONCE(1);

Replace with the following to resolve below issue:

	dbg_once("unwind_user_get_ra_reg() not implemented\n");

> 
> Can a maliciously crafted .sframe section trigger this warning and panic
> the kernel if panic_on_warn is enabled?
> 
> The SFrame unwind data is parsed from user-provided ELF binaries, so the
> .sframe sections are entirely user-controlled.
> 
> If an unprivileged user on an architecture using the generic fallback
> provides an SFrame Frame Row Entry with dataword_count > 0 but an
> explicit ra_off dataword of 0, it appears this would bypass the
> frame->outermost check while keeping frame->ra_off == 0.

Valid.  On architectures that pass the RA on the stack and therefore do
not implement unwind_user_get_ra_reg(), such as x86-64, above
WARN_ON_ONCE(1) can be triggered using forged .sframe that maliciously
specifies the SFrame fixed RA offset (from CFA) as zero and has a SFrame
FRE with either no RA and FP offsets with a RA padding offset of zero.

> 
> When the kernel unwinds the stack, unwind_user_next_common() would see
> frame->ra_off == 0 and call unwind_user_get_ra_reg():
> 
> kernel/unwind/user.c:unwind_user_next_common() {
>     ...
> 	if (frame->ra_off) {
> 		if (get_user_word(&ra, cfa, frame->ra_off, state->ws))
> 			return -EINVAL;
> 	} else {
> 		if (!state->topmost || unwind_user_get_ra_reg(&ra))
> 			return -EINVAL;
> 	}
>     ...
> }
> 
> Would it be better to gracefully return an error code like -EINVAL here
> without raising a warning?
> 
>> +	return -EINVAL;
>> +}
> 
> [ ... ]
> 

Regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
[email protected] / [email protected]

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