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

[Andy Lutomirski]

On May 8, 2016 7:05 PM, "Stas Sergeev" <stsp@xxxxxxx> wrote:
>
> 09.05.2016 04:32, Andy Lutomirski ÐÐÑÐÑ:
>
>> On May 7, 2016 7:38 AM, "Stas Sergeev" <stsp@xxxxxxx> wrote:
>>>
>>> 03.05.2016 20:31, Andy Lutomirski ÐÐÑÐÑ:
>>>
>>>> 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@xxxxxxx>
>>>> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
>>>> Cc: Aleksa Sarai <cyphar@xxxxxxxxxx>
>>>> Cc: Amanieu d'Antras <amanieu@xxxxxxxxx>
>>>> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
>>>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>>>> Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
>>>> Cc: Borislav Petkov <bp@xxxxxxxxx>
>>>> Cc: Brian Gerst <brgerst@xxxxxxxxx>
>>>> Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
>>>> Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
>>>> Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
>>>> Cc: H. Peter Anvin <hpa@xxxxxxxxx>
>>>> Cc: Heinrich Schuchardt <xypron.glpk@xxxxxx>
>>>> Cc: Jason Low <jason.low2@xxxxxx>
>>>> Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
>>>> Cc: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
>>>> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>>>> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
>>>> Cc: Palmer Dabbelt <palmer@xxxxxxxxxxx>
>>>> Cc: Paul Moore <pmoore@xxxxxxxxxx>
>>>> Cc: Pavel Emelyanov <xemul@xxxxxxxxxxxxx>
>>>> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>>>> Cc: Richard Weinberger <richard@xxxxxx>
>>>> Cc: Sasha Levin <sasha.levin@xxxxxxxxxx>
>>>> Cc: Shuah Khan <shuahkh@xxxxxxxxxxxxxxx>
>>>> Cc: Tejun Heo <tj@xxxxxxxxxx>
>>>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>>>> Cc: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
>>>> Cc: linux-api@xxxxxxxxxxxxxxx
>>>> Cc: linux-kernel@xxxxxxxxxxxxxxx
>>>> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
>>>> ---
>>>>    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.

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