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Date:	Sat, 14 May 2016 09:35:41 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Stas Sergeev <stsp@...t.ru>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Shuah Khan <shuahkh@....samsung.com>,
	Pavel Emelyanov <xemul@...allels.com>, X86 ML <x86@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux API <linux-api@...r.kernel.org>,
	Jason Low <jason.low2@...com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Josh Triplett <josh@...htriplett.org>,
	Aleksa Sarai <cyphar@...har.com>,
	Paul Moore <pmoore@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	"Amanieu d'Antras" <amanieu@...il.com>,
	Borislav Petkov <bp@...en8.de>,
	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	Heinrich Schuchardt <xypron.glpk@....de>,
	Tejun Heo <tj@...nel.org>, Brian Gerst <brgerst@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Palmer Dabbelt <palmer@...belt.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Richard Weinberger <richard@....at>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 1/4] signals/sigaltstack: If SS_AUTODISARM, bypass on_sig_stack

On May 14, 2016 4:18 AM, "Stas Sergeev" <stsp@...t.ru> wrote:
>
> 14.05.2016 07:18, Andy Lutomirski пишет:
>
>> On May 8, 2016 7:05 PM, "Stas Sergeev" <stsp@...t.ru> wrote:
>>>
>>> 09.05.2016 04:32, Andy Lutomirski пишет:
>>>
>>>> On May 7, 2016 7:38 AM, "Stas Sergeev" <stsp@...t.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@...t.ru>
>>>>>> Cc: Al Viro <viro@...iv.linux.org.uk>
>>>>>> Cc: Aleksa Sarai <cyphar@...har.com>
>>>>>> Cc: Amanieu d'Antras <amanieu@...il.com>
>>>>>> Cc: Andrea Arcangeli <aarcange@...hat.com>
>>>>>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>>>>>> Cc: Andy Lutomirski <luto@...capital.net>
>>>>>> Cc: Borislav Petkov <bp@...en8.de>
>>>>>> Cc: Brian Gerst <brgerst@...il.com>
>>>>>> Cc: Denys Vlasenko <dvlasenk@...hat.com>
>>>>>> Cc: Eric W. Biederman <ebiederm@...ssion.com>
>>>>>> Cc: Frederic Weisbecker <fweisbec@...il.com>
>>>>>> Cc: H. Peter Anvin <hpa@...or.com>
>>>>>> Cc: Heinrich Schuchardt <xypron.glpk@....de>
>>>>>> Cc: Jason Low <jason.low2@...com>
>>>>>> Cc: Josh Triplett <josh@...htriplett.org>
>>>>>> Cc: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
>>>>>> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
>>>>>> Cc: Oleg Nesterov <oleg@...hat.com>
>>>>>> Cc: Palmer Dabbelt <palmer@...belt.com>
>>>>>> Cc: Paul Moore <pmoore@...hat.com>
>>>>>> Cc: Pavel Emelyanov <xemul@...allels.com>
>>>>>> Cc: Peter Zijlstra <peterz@...radead.org>
>>>>>> Cc: Richard Weinberger <richard@....at>
>>>>>> Cc: Sasha Levin <sasha.levin@...cle.com>
>>>>>> Cc: Shuah Khan <shuahkh@....samsung.com>
>>>>>> Cc: Tejun Heo <tj@...nel.org>
>>>>>> Cc: Thomas Gleixner <tglx@...utronix.de>
>>>>>> Cc: Vladimir Davydov <vdavydov@...allels.com>
>>>>>> Cc: linux-api@...r.kernel.org
>>>>>> Cc: linux-kernel@...r.kernel.org
>>>>>> Signed-off-by: Andy Lutomirski <luto@...nel.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?
>

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.

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