On May 14, 2016 4:18 AM, "Stas Sergeev" <[email protected]> wrote: > > 14.05.2016 07:18, Andy Lutomirski пишет: > >> On May 8, 2016 7:05 PM, "Stas Sergeev" <[email protected]> wrote: >>> >>> 09.05.2016 04:32, Andy Lutomirski пишет: >>> >>>> On May 7, 2016 7:38 AM, "Stas Sergeev" <[email protected]> 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 <[email protected]> >>>>>> Cc: Al Viro <[email protected]> >>>>>> Cc: Aleksa Sarai <[email protected]> >>>>>> Cc: Amanieu d'Antras <[email protected]> >>>>>> Cc: Andrea Arcangeli <[email protected]> >>>>>> Cc: Andrew Morton <[email protected]> >>>>>> Cc: Andy Lutomirski <[email protected]> >>>>>> Cc: Borislav Petkov <[email protected]> >>>>>> Cc: Brian Gerst <[email protected]> >>>>>> Cc: Denys Vlasenko <[email protected]> >>>>>> Cc: Eric W. Biederman <[email protected]> >>>>>> Cc: Frederic Weisbecker <[email protected]> >>>>>> Cc: H. Peter Anvin <[email protected]> >>>>>> Cc: Heinrich Schuchardt <[email protected]> >>>>>> Cc: Jason Low <[email protected]> >>>>>> Cc: Josh Triplett <[email protected]> >>>>>> Cc: Konstantin Khlebnikov <[email protected]> >>>>>> Cc: Linus Torvalds <[email protected]> >>>>>> Cc: Oleg Nesterov <[email protected]> >>>>>> Cc: Palmer Dabbelt <[email protected]> >>>>>> Cc: Paul Moore <[email protected]> >>>>>> Cc: Pavel Emelyanov <[email protected]> >>>>>> Cc: Peter Zijlstra <[email protected]> >>>>>> Cc: Richard Weinberger <[email protected]> >>>>>> Cc: Sasha Levin <[email protected]> >>>>>> Cc: Shuah Khan <[email protected]> >>>>>> Cc: Tejun Heo <[email protected]> >>>>>> Cc: Thomas Gleixner <[email protected]> >>>>>> Cc: Vladimir Davydov <[email protected]> >>>>>> Cc: [email protected] >>>>>> Cc: [email protected] >>>>>> Signed-off-by: Andy Lutomirski <[email protected]> >>>>>> --- >>>>>> 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? >
It's still a wee bit ugly. Also, doesn't that change existing behavior for the existing non-AUTODISARM case? Also, we'd have to make sure that sigreturn doesn't trigger this check. My inclination would be leave it alone.
