Re: [PATCH v2] arm64: syscall: Ensure saved x0 is kept in-sync with tracer updates

[Will Deacon <will@kernel.org>]

On Thu, Jul 16, 2026 at 05:48:01PM +0100, Will Deacon wrote:
> On Thu, 16 Jul 2026 13:06:39 +0100, Will Deacon wrote:
> > When seccomp support was originally added to arm64 in a1ae65b21941
> > ("arm64: add seccomp support"), seccomp was erroneously called _before_
> > the ptrace syscall-enter-stop and therefore the tracer could trivially
> > manipulate the syscall register state after the seccomp check had
> > passed. This was subsequently fixed in a5cd110cb836 ("arm64/ptrace: run
> > seccomp after ptrace") by moving the seccomp check after the tracer has
> > run. Unfortunately, a decade later, that fix has been reported to be
> > incomplete.
> > 
> > [...]
> 
> Applied to arm64 (for-next/fixes), thanks!
> 
> [1/1] arm64: syscall: Ensure saved x0 is kept in-sync with tracer updates
>       https://git.kernel.org/arm64/c/e057b9477232

Bah, I've had to revert this. I think Sashiko makes a good point here
that the seccomp interaction is still broken when the filter is
re-evaluated after the tracer stop, because that all happens inside
secure_computing() so we don't get a chance to update 'orig_x0':

https://sashiko.dev/#/patchset/20260716120640.6590-1-will@kernel.org

I've got a v3 that takes a different approach, so I'll send that out
shortly. Jinjie, thanks for sending the selftests, but maybe we can
extend them to cover the loophole above as wel?

Will