Re: [PATCH] ARM: ptrace: Restore syscall skipping and restart while tracing

[Kees Cook]

On Thu, Aug 10, 2023 at 10:10:08PM +0200, Arnd Bergmann wrote:
> On Thu, Aug 10, 2023, at 21:32, Kees Cook wrote:
> > On Wed, Aug 09, 2023 at 09:47:24PM +0200, Arnd Bergmann wrote:
> >
> >> If the local_restart code has to store the syscall number
> >> for an EABI-only kernel, wouldn't it have to also do this
> >> for a kernel with OABI-only or OABI_COMPAT support?
> >
> > This is the part I wasn't sure about. Initially I was thinking it didn't
> > matter because it's only a problem for a seccomp tracer, but I realize
> > it might be exposed to a PTRACE tracer too. I was only able to test with
> > EABI since seccomp is disabled for OABI_COMPAT.
> >
> > Anyway, syscall restart is done this way:
> >
> >         movlt   scno, #(__NR_restart_syscall - __NR_SYSCALL_BASE)
> >
> > Can a EABI call restart an OABI syscall? I think so?
> 
> There are very few differences between oabi and eabi syscalls, I
> think it basically comes down to 
> 
>  - the syscall number, and register in which it is passed to the kernel
>  - a few syscalls that exist for OABI backward compatibility and were
>    deprecated before EABI was added
>  - a few syscalls that pass a struct with different alignment rules
>  - epoll_wait() uses a runtime check for the output format
> 
> It also seems like the __NR_restart_syscall path is only relevant
> for syscalls using restart_block for restarting, and that means
> it's only poll(), futex(), nanosleep(), clock_nanosleep() and their
> time64 counterparts. All of these are handled by the same entry

Right -- it's a tiny corner case I tripped over years ago while building
seccomp filters, so it got added to the selftests. :)

> points for OABI and EABI, i.e. there is no overlap with the
> exceptions above. Crucially, epoll does not use restart_block,
> unlike poll().
> 
> > So maybe we just need to add:
> >
> > 	str     scno, [tsk, #TI_ABI_SYSCALL]    @ store scno for syscall restart
> >
> > after that instead of moving it like I did originally?
> 
> Yes, I think that works!
> 
> For pure EABI and pure OABI kernels, this just does the right thing,
> storing a plain __NR_restart_syscall in the field without an ABI
> marker. For an OABI compat task running on an EABI kernel, it will
> call the EABI version of restart_syscall(), but that is exactly
> the same as the OABI version, as shown above.

Okay, excellent. I came to the same conclusion. Patch 1 in the v2
addresses this and tested okay for me.

Thanks for looking at this!

-Kees

-- 
Kees Cook