Re: [PATCH RFC 0/3] arm64/gcs: Allow reuse of user managed shadow stacks

[Yury Khrustalev <yury.khrustalev@arm.com>]

Hi,

On Thu, Sep 25, 2025 at 08:40:56PM +0000, Edgecombe, Rick P wrote:
> On Sun, 2025-09-21 at 14:21 +0100, Mark Brown wrote:
> > During the discussion of the clone3() support for shadow stacks concerns
> > were raised from the glibc side that since it is not possible to reuse
> > the allocated shadow stack[1]. This means that the benefit of being able
> > ...
> >
> Security-wise, it seems reasonable that if you are leaving a shadow stack, that
> you could leave a token behind. But for the userspace scheme to back up the SSP
> by doing a longjmp() or similar I have some doubts. IIRC there were some cross
> stack edge cases that we never figured out how to handle.
> 
> As far as re-using allocated shadow stacks, there is always the option to enable
> WRSS (or similar) to write the shadow stack as well as longjmp at will.
> 
> I think we should see a fuller solution from the glibc side before adding new
> kernel features like this. (apologies if I missed it).

What do you mean by "a fuller solution from the glibc side"? A solution
for re-using shadow stacks? Right now Glibc cannot do anything about
shadow stacks for new threads because clone3 interface doesn't allow it.

Thanks,
Yury