x86: signal: don't do sas_ss_reset() until we are certain that sigframe won't be abandoned

Currently we handle SS_AUTODISARM as soon as we have stored the altstack
settings into sigframe - that's the point when we have set the things up
for eventual sigreturn to restore the old settings.  And if we manage to
set the sigframe up (we are not done with that yet), everything's fine.
However, in case of failure we end up with sigframe-to-be abandoned and
SIGSEGV force-delivered.  And in that case we end up with inconsistent
rules - late failures have altstack reset, early ones do not.

It's trivial to get consistent behaviour - just handle SS_AUTODISARM once
we have set the sigframe up and are committed to entering the handler,
i.e.  in signal_delivered().

Link: https://lore.kernel.org/lkml/20200404170604.GN23230@ZenIV.linux.org.uk/
Link: https://github.com/ClangBuiltLinux/linux/issues/876
Link: https://lkml.kernel.org/r/20210422230846.1756380-1-ndesaulniers@google.comSigned-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Tested-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/compat.h

@@ -532,8 +532,6 @@ int __compat_save_altstack(compat_stack_t __user *, unsigned long);
 			&__uss->ss_sp, label); \
 	unsafe_put_user(t->sas_ss_flags, &__uss->ss_flags, label); \
 	unsafe_put_user(t->sas_ss_size, &__uss->ss_size, label); \
-	if (t->sas_ss_flags & SS_AUTODISARM) \
-		sas_ss_reset(t); \
 } while (0);
 
 /*

include/linux/signal.h

@@ -462,8 +462,6 @@ int __save_altstack(stack_t __user *, unsigned long);
 	unsafe_put_user((void __user *)t->sas_ss_sp, &__uss->ss_sp, label); \
 	unsafe_put_user(t->sas_ss_flags, &__uss->ss_flags, label); \
 	unsafe_put_user(t->sas_ss_size, &__uss->ss_size, label); \
-	if (t->sas_ss_flags & SS_AUTODISARM) \
-		sas_ss_reset(t); \
 } while (0);
 
 #ifdef CONFIG_PROC_FS

kernel/signal.c

@@ -2829,6 +2829,8 @@ static void signal_delivered(struct ksignal *ksig, int stepping)
 	if (!(ksig->ka.sa.sa_flags & SA_NODEFER))
 		sigaddset(&blocked, ksig->sig);
 	set_current_blocked(&blocked);
+	if (current->sas_ss_flags & SS_AUTODISARM)
+		sas_ss_reset(current);
 	tracehook_signal_handler(stepping);
 }
 
@@ -4147,11 +4149,7 @@ int __save_altstack(stack_t __user *uss, unsigned long sp)
 	int err = __put_user((void __user *)t->sas_ss_sp, &uss->ss_sp) |
 		__put_user(t->sas_ss_flags, &uss->ss_flags) |
 		__put_user(t->sas_ss_size, &uss->ss_size);
-	if (err)
-		return err;
-	if (t->sas_ss_flags & SS_AUTODISARM)
-		sas_ss_reset(t);
-	return 0;
+	return err;
 }
 
 #ifdef CONFIG_COMPAT
@@ -4206,11 +4204,7 @@ int __compat_save_altstack(compat_stack_t __user *uss, unsigned long sp)
 			 &uss->ss_sp) |
 		__put_user(t->sas_ss_flags, &uss->ss_flags) |
 		__put_user(t->sas_ss_size, &uss->ss_size);
-	if (err)
-		return err;
-	if (t->sas_ss_flags & SS_AUTODISARM)
-		sas_ss_reset(t);
-	return 0;
+	return err;
 }
 #endif