Messages in this thread | | | Date | Thu, 10 Aug 2023 13:15:40 -0700 | From | Kees Cook <> | Subject | Re: [PATCH] ARM: ptrace: Restore syscall skipping and restart while tracing |
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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
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