Re: [PATCH v4 1/3] randomize_kstack: Maintain kstack_offset per task

[Ryan Roberts <ryan.roberts@arm.com>]

Thanks for the review!

On 19/01/2026 16:10, Dave Hansen wrote:
> On 1/19/26 05:01, Ryan Roberts wrote:
> ...
>> Cc: stable@vger.kernel.org
> 
> Since this doesn't fix any known functional issues, if it were me, I'd
> leave stable@ alone. It isn't clear that this is stable material.

I listed 2 issues in the commit log; I agree that issue 1 falls into the
category of "don't really care", but issue 2 means that kstack randomization is
currently trivial to defeat. That's the reason I thought it would valuable in
stable.

But if you're saying don't bother and others agree, then this whole patch can be
dropped; this is just intended to be the backportable fix. Patch 3 reimplements
this entirely for upstream.

I'll wait and see if others have opinions if that's ok?

> 
>> --- a/include/linux/sched.h
>> +++ b/include/linux/sched.h
>> @@ -1591,6 +1591,10 @@ struct task_struct {
>>  	unsigned long			prev_lowest_stack;
>>  #endif
>>  
>> +#ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
>> +	u32				kstack_offset;
>> +#endif
>> +
>>  #ifdef CONFIG_X86_MCE
>>  	void __user			*mce_vaddr;
> 
> Nit: This seems to be throwing a u32 potentially in between a couple of
> void*/ulong sized objects.

Yeah, I spent a bit of time with pahole but eventually concluded that it was
difficult to find somewhere to nestle it that would work reliably cross arch.
Eventually I just decided to group it with other stack meta data.

> 
> It probably doesn't matter with struct randomization and it's really
> hard to get right among the web of task_struct #ifdefs. But, it would be
> nice to at _least_ nestle this next to another int-sized thing.
> 
> Does it really even need to be 32 bits? x86 has this comment:
> 
>>         /*
>>          * This value will get limited by KSTACK_OFFSET_MAX(), which is 10
>>          * bits. The actual entropy will be further reduced by the compiler
>>          * when applying stack alignment constraints (see cc_stack_align4/8 in
>>          * arch/x86/Makefile), which will remove the 3 (x86_64) or 2 (ia32)
>>          * low bits from any entropy chosen here.
>>          *
>>          * Therefore, final stack offset entropy will be 7 (x86_64) or
>>          * 8 (ia32) bits.
>>          */

For more recent kernels it's 6 bits shifted by 4 for 64-bit kernels or 8 bits
shifted by 2 for 32-bit kernels regardless of arch. So could probably make it
work with 8 bits of storage. Although I was deliberately trying to keep the
change simple, since it was intended for backporting. Patch 3 rips it out.

Overall I'd prefer to leave it all as is. But if people don't think we should
backport, then let's just drop the whole patch.

Thanks,
Ryan