Re: [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 <redacted>
Cc: Al Viro <viro-RmSDqhL/yNMiFSDQTTA3OLVCufUGDwFn@public.gmane.org>
Cc: Aleksa Sarai <cyphar-gVpy/LI/lHzQT0dZR+AlfA@public.gmane.org>
Cc: Amanieu d'Antras <redacted>
Cc: Andrea Arcangeli <redacted>
Cc: Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
Cc: Andy Lutomirski <redacted>
Cc: Borislav Petkov <redacted>
Cc: Brian Gerst <redacted>
Cc: Denys Vlasenko <redacted>
Cc: Eric W. Biederman <ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
Cc: Frederic Weisbecker <redacted>
Cc: H. Peter Anvin <redacted>
Cc: Heinrich Schuchardt <redacted>
Cc: Jason Low <redacted>
Cc: Josh Triplett <redacted>
Cc: Konstantin Khlebnikov <redacted>
Cc: Linus Torvalds <torvalds-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
Cc: Oleg Nesterov <redacted>
Cc: Palmer Dabbelt <redacted>
Cc: Paul Moore <redacted>
Cc: Pavel Emelyanov <redacted>
Cc: Peter Zijlstra <redacted>
Cc: Richard Weinberger <richard-/L3Ra7n9ekc@public.gmane.org>
Cc: Sasha Levin <redacted>
Cc: Shuah Khan <redacted>
Cc: Tejun Heo <redacted>
Cc: Thomas Gleixner <redacted>
Cc: Vladimir Davydov <redacted>
Cc: linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
Cc: linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
Signed-off-by: Andy Lutomirski <redacted>
---
  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.