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

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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@list.ru>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Amanieu d'Antras <amanieu@gmail.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Cc: Jason Low <jason.low2@hp.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Moore <pmoore@redhat.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Richard Weinberger <richard@nod.at>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vladimir Davydov <vdavydov@parallels.com>
Cc: linux-api@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Andy Lutomirski <luto@kernel.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

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[PATCH 0/4] SS_AUTODISARM fixes and an ABI change Andy Lutomirski <luto@kernel.org> - 2016-05-03 19:40 +0200
  [PATCH 1/4] signals/sigaltstack: If SS_AUTODISARM, bypass on_sig_stack Andy Lutomirski <luto@kernel.org> - 2016-05-03 19:40 +0200
    Re: [PATCH 1/4] signals/sigaltstack: If SS_AUTODISARM, bypass  on_sig_stack Ingo Molnar <mingo@kernel.org> - 2016-05-04 08:40 +0200
      Re: [PATCH 1/4] signals/sigaltstack: If SS_AUTODISARM, bypass on_sig_stack Andy Lutomirski <luto@amacapital.net> - 2016-05-05 01:10 +0200
    [tip:core/signals] signals/sigaltstack: If SS_AUTODISARM, bypass  on_sig_stack() tip-bot for Andy Lutomirski <tipbot@zytor.com> - 2016-05-04 09:20 +0200
    Re: [PATCH 1/4] signals/sigaltstack: If SS_AUTODISARM, bypass  on_sig_stack Stas Sergeev <stsp@list.ru> - 2016-05-07 16:40 +0200
      Re: [PATCH 1/4] signals/sigaltstack: If SS_AUTODISARM, bypass on_sig_stack Andy Lutomirski <luto@amacapital.net> - 2016-05-09 03:40 +0200
        Re: [PATCH 1/4] signals/sigaltstack: If SS_AUTODISARM, bypass  on_sig_stack Stas Sergeev <stsp@list.ru> - 2016-05-09 04:10 +0200
  [PATCH 4/4] signals/sigaltstack: Change SS_AUTODISARM to (1U << 31) Andy Lutomirski <luto@kernel.org> - 2016-05-03 19:40 +0200
    [tip:core/signals] signals/sigaltstack: Change SS_AUTODISARM to (1U  << 31) tip-bot for Andy Lutomirski <tipbot@zytor.com> - 2016-05-04 09:20 +0200
    Re: [PATCH 4/4] signals/sigaltstack: Change SS_AUTODISARM to (1U <<  31) Stas Sergeev <stsp@list.ru> - 2016-05-07 17:20 +0200
  Re: [PATCH 0/4] SS_AUTODISARM fixes and an ABI change Ingo Molnar <mingo@kernel.org> - 2016-05-04 08:30 +0200

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