Date:	Sat, 14 May 2016 14:18:14 +0300
From:	Stas Sergeev <stsp@...t.ru>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Pavel Emelyanov <xemul@...allels.com>,
	Shuah Khan <shuahkh@....samsung.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>,
	Aleksa Sarai <cyphar@...har.com>,
	Josh Triplett <josh@...htriplett.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Paul Moore <pmoore@...hat.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Borislav Petkov <bp@...en8.de>,
	Amanieu d'Antras <amanieu@...il.com>,
	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	Heinrich Schuchardt <xypron.glpk@....de>,
	Brian Gerst <brgerst@...il.com>, Tejun Heo <tj@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Palmer Dabbelt <palmer@...belt.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

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?

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