Messages in this thread | | | Subject | Re: [PATCH 1/4] signals/sigaltstack: If SS_AUTODISARM, bypass on_sig_stack | From | Stas Sergeev <> | Date | Sat, 14 May 2016 14:18:14 +0300 |
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14.05.2016 07:18, Andy Lutomirski пишет: > On May 8, 2016 7:05 PM, "Stas Sergeev" <stsp@list.ru> wrote: >> 09.05.2016 04:32, Andy Lutomirski пишет: >> >>> On May 7, 2016 7:38 AM, "Stas Sergeev" <stsp@list.ru> wrote: >>>> 03.05.2016 20:31, Andy Lutomirski пишет: >>>> >>>>> If a signal stack is set up with SS_AUTODISARM, then the kernel >>>>> inherently avoids incorrectly resetting the signal stack if signals >>>>> recurse: the signal stack will be reset on the first signal >>>>> delivery. This means that we don't need check the stack pointer >>>>> when delivering signals if SS_AUTODISARM is set. >>>>> >>>>> This will make segmented x86 programs more robust: currently there's >>>>> a hole that could be triggered if ESP/RSP appears to point to the >>>>> signal stack but actually doesn't due to a nonzero SS base. >>>>> >>>>> Signed-off-by: Stas Sergeev <stsp@list.ru> >>>>> Cc: Al Viro <viro@zeniv.linux.org.uk> >>>>> Cc: Aleksa Sarai <cyphar@cyphar.com> >>>>> Cc: Amanieu d'Antras <amanieu@gmail.com> >>>>> Cc: Andrea Arcangeli <aarcange@redhat.com> >>>>> Cc: Andrew Morton <akpm@linux-foundation.org> >>>>> Cc: Andy Lutomirski <luto@amacapital.net> >>>>> Cc: Borislav Petkov <bp@alien8.de> >>>>> Cc: Brian Gerst <brgerst@gmail.com> >>>>> Cc: Denys Vlasenko <dvlasenk@redhat.com> >>>>> Cc: Eric W. Biederman <ebiederm@xmission.com> >>>>> Cc: Frederic Weisbecker <fweisbec@gmail.com> >>>>> Cc: H. Peter Anvin <hpa@zytor.com> >>>>> Cc: Heinrich Schuchardt <xypron.glpk@gmx.de> >>>>> Cc: Jason Low <jason.low2@hp.com> >>>>> Cc: Josh Triplett <josh@joshtriplett.org> >>>>> Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru> >>>>> Cc: Linus Torvalds <torvalds@linux-foundation.org> >>>>> Cc: Oleg Nesterov <oleg@redhat.com> >>>>> Cc: Palmer Dabbelt <palmer@dabbelt.com> >>>>> Cc: Paul Moore <pmoore@redhat.com> >>>>> Cc: Pavel Emelyanov <xemul@parallels.com> >>>>> Cc: Peter Zijlstra <peterz@infradead.org> >>>>> Cc: Richard Weinberger <richard@nod.at> >>>>> Cc: Sasha Levin <sasha.levin@oracle.com> >>>>> Cc: Shuah Khan <shuahkh@osg.samsung.com> >>>>> Cc: Tejun Heo <tj@kernel.org> >>>>> Cc: Thomas Gleixner <tglx@linutronix.de> >>>>> Cc: Vladimir Davydov <vdavydov@parallels.com> >>>>> Cc: linux-api@vger.kernel.org >>>>> Cc: linux-kernel@vger.kernel.org >>>>> Signed-off-by: Andy Lutomirski <luto@kernel.org> >>>>> --- >>>>> include/linux/sched.h | 12 ++++++++++++ >>>>> 1 file changed, 12 insertions(+) >>>>> >>>>> diff --git a/include/linux/sched.h b/include/linux/sched.h >>>>> index 2950c5cd3005..8f03a93348b9 100644 >>>>> --- a/include/linux/sched.h >>>>> +++ b/include/linux/sched.h >>>>> @@ -2576,6 +2576,18 @@ static inline int kill_cad_pid(int sig, int priv) >>>>> */ >>>>> static inline int on_sig_stack(unsigned long sp) >>>>> { >>>>> + /* >>>>> + * If the signal stack is AUTODISARM then, by construction, we >>>>> + * can't be on the signal stack unless user code deliberately set >>>>> + * SS_AUTODISARM when we were already on the it. >>>> "on the it" -> "on it". >>>> >>>> Anyway, I am a bit puzzled with this patch. >>>> You say "unless user code deliberately set >>>> >>>> SS_AUTODISARM when we were already on the it" >>>> so what happens in case it actually does? >>>> >>> Stack corruption. Don't do that. >> Only after your change, I have to admit. :) >> >> >>>> Without your patch: if user sets up the same sas - no stack switch. >>>> if user sets up different sas - stack switch on nested signal. >>>> >>>> With your patch: stack switch in any case, so if user >>>> set up same sas - stack corruption by nested signal. >>>> >>>> Or am I missing the intention? >>> The intention is to make everything completely explicit. With >>> SS_AUTODISARM, the kernel knows directly whether you're on the signal >>> stack, and there should be no need to look at sp. If you set >>> SS_AUTODISARM and get a signal, the signal stack gets disarmed. If >>> you take a nested signal, it's delivered normally. When you return >>> all the way out, the signal stack is re-armed. >>> >>> For DOSEMU, this means that no 16-bit register state can possibly >>> cause a signal to be delivered wrong, because the register state when >>> a signal is raised won't affect delivery, which seems like a good >>> thing to me. >> Yes, but doesn't affect dosemu1 which doesn't use SS_AUTODISARM. >> So IMHO the SS check should still be added, even if not for dosemu2. >> >> >>> If this behavior would be problematic for you, can you explain why? >> Only theoretically: if someone sets SS_AUTODISARM inside a >> sighandler. Since this doesn't give EPERM, I wouldn't deliberately >> make it a broken scenario (esp if it wasn't before the particular change). >> Ideally it would give EPERM, but we can't, so doesn't matter much. >> I just wanted to warn about the possible regression. > I suppose we could return an error if you are on the sigstack when > setting SS_AUTODISARM, although I was hoping to avoid yet more special > cases. Hmm. How about extending the generic check then? Currently it is roughly: if (on_sig_stack(sp)) return -EPERM;
and we could do: if (on_sig_stack(sp) || on_new_sas(new_sas, sp)) return -EPERM;
Looks like it will close the potential hole opened by your commit without introducing the special case for SS_AUTODISARM. What do you think?
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