Re: [PATCH 1/4] signals/sigaltstack: If SS_AUTODISARM, bypass on_sig_stack

[Stas Sergeev <stsp@list.ru>]

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?