linux-kernel - Re: [PATCH 1/4] signals/sigaltstack: If SS_AUTODISARM, bypass on_sig

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Message-ID: <0081b876-033b-4fb7-3daf-d38b2df1fda6@list.ru>
Date:	Sat, 7 May 2016 17:37:41 +0300
From:	Stas Sergeev <stsp@...t.ru>
To:	Andy Lutomirski <luto@...nel.org>, x86@...nel.org
Cc:	linux-kernel@...r.kernel.org, Borislav Petkov <bp@...en8.de>,
	Al Viro <viro@...iv.linux.org.uk>,
	Aleksa Sarai <cyphar@...har.com>,
	Amanieu d'Antras <amanieu@...il.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Brian Gerst <brgerst@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Heinrich Schuchardt <xypron.glpk@....de>,
	Jason Low <jason.low2@...com>,
	Josh Triplett <josh@...htriplett.org>,
	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Paul Moore <pmoore@...hat.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Richard Weinberger <richard@....at>,
	Sasha Levin <sasha.levin@...cle.com>,
	Shuah Khan <shuahkh@....samsung.com>,
	Tejun Heo <tj@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vladimir Davydov <vdavydov@...allels.com>,
	linux-api@...r.kernel.org
Subject: Re: [PATCH 1/4] signals/sigaltstack: If SS_AUTODISARM, bypass
 on_sig_stack

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?

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?