Messages in this thread | | | Subject | Re: [PATCH 1/4] signals/sigaltstack: If SS_AUTODISARM, bypass on_sig_stack | From | Stas Sergeev <> | Date | Mon, 9 May 2016 05:04:45 +0300 |
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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.
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