On May 7, 2016 7:38 AM, "Stas Sergeev" <s...@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 <s...@list.ru>
>> Cc: Al Viro <v...@zeniv.linux.org.uk>
>> Cc: Aleksa Sarai <cyp...@cyphar.com>
>> Cc: Amanieu d'Antras <aman...@gmail.com>
>> Cc: Andrea Arcangeli <aarca...@redhat.com>
>> Cc: Andrew Morton <a...@linux-foundation.org>
>> Cc: Andy Lutomirski <l...@amacapital.net>
>> Cc: Borislav Petkov <b...@alien8.de>
>> Cc: Brian Gerst <brge...@gmail.com>
>> Cc: Denys Vlasenko <dvlas...@redhat.com>
>> Cc: Eric W. Biederman <ebied...@xmission.com>
>> Cc: Frederic Weisbecker <fweis...@gmail.com>
>> Cc: H. Peter Anvin <h...@zytor.com>
>> Cc: Heinrich Schuchardt <xypron.g...@gmx.de>
>> Cc: Jason Low <jason.l...@hp.com>
>> Cc: Josh Triplett <j...@joshtriplett.org>
>> Cc: Konstantin Khlebnikov <khlebni...@yandex-team.ru>
>> Cc: Linus Torvalds <torva...@linux-foundation.org>
>> Cc: Oleg Nesterov <o...@redhat.com>
>> Cc: Palmer Dabbelt <pal...@dabbelt.com>
>> Cc: Paul Moore <pmo...@redhat.com>
>> Cc: Pavel Emelyanov <xe...@parallels.com>
>> Cc: Peter Zijlstra <pet...@infradead.org>
>> Cc: Richard Weinberger <rich...@nod.at>
>> Cc: Sasha Levin <sasha.le...@oracle.com>
>> Cc: Shuah Khan <shua...@osg.samsung.com>
>> Cc: Tejun Heo <t...@kernel.org>
>> Cc: Thomas Gleixner <t...@linutronix.de>
>> Cc: Vladimir Davydov <vdavy...@parallels.com>
>> Cc: linux-...@vger.kernel.org
>> Cc: linux-kernel@vger.kernel.org
>> Signed-off-by: Andy Lutomirski <l...@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.
> 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.
If this behavior would be problematic for you, can you explain why?
