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: Andy Lutomirski <[e-mail]>
Cc: Al Viro <[e-mail]>
Cc: Aleksa Sarai <[e-mail]>
Cc: Amanieu d'Antras <[e-mail]>
Cc: Andrea Arcangeli <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Brian Gerst <[e-mail]>
Cc: Denys Vlasenko <[e-mail]>
Cc: Eric W. Biederman <[e-mail]>
Cc: Frederic Weisbecker <[e-mail]>
Cc: H. Peter Anvin <[e-mail]>
Cc: Heinrich Schuchardt <[e-mail]>
Cc: Jason Low <[e-mail]>
Cc: Josh Triplett <[e-mail]>
Cc: Konstantin Khlebnikov <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Oleg Nesterov <[e-mail]>
Cc: Palmer Dabbelt <[e-mail]>
Cc: Paul Moore <[e-mail]>
Cc: Pavel Emelyanov <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Richard Weinberger <[e-mail]>
Cc: Sasha Levin <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Cc: Stas Sergeev <[e-mail]>
Cc: Tejun Heo <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Vladimir Davydov <[e-mail]>
Cc: [e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

include/linux/sched.h

···
  */
 static inline int on_sig_stack(unsigned long sp)
 {
+	/*
+	 * If the signal stack is SS_AUTODISARM then, by construction, we
+	 * can't be on the signal stack unless user code deliberately set
+	 * SS_AUTODISARM when we were already on it.
+	 *
+	 * This improves 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;