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

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Message-Id: <c46bee4654ca9e68c498462fd11746e2bd0d98c8.1462296606.git.luto@kernel.org>
Date:	Tue,  3 May 2016 10:31:49 -0700
From:	Andy Lutomirski <luto@...nel.org>
To:	x86@...nel.org
Cc:	linux-kernel@...r.kernel.org, Borislav Petkov <bp@...en8.de>,
	Andy Lutomirski <luto@...nel.org>, Stas Sergeev <stsp@...t.ru>,
	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: [PATCH 1/4] signals/sigaltstack: If SS_AUTODISARM, bypass on_sig_stack

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.
+	 *
+	 * This improve reliability: if user state gets corrupted such that
+	 * the stack pointer points very close to the end of the signal stack,
+	 * then this check will enable the signal to be handled anyway.
+	 */
+	if (current->sas_ss_flags & SS_AUTODISARM)
+		return 0;
+
 #ifdef CONFIG_STACK_GROWSUP
 	return sp >= current->sas_ss_sp &&
 		sp - current->sas_ss_sp < current->sas_ss_size;
-- 
2.5.5