[PATCH 1/2] sigaltstack: implement SS_AUTODISARM flag

[Stas Sergeev]

This patch implements the SS_AUTODISARM flag that can be ORed with
SS_ONSTACK when forming ss_flags.
When this flag is set, sigaltstack will be disabled when entering
the signal handler; more precisely, after saving sas to uc_stack.
When leaving the signal handler, the sigaltstack is restored by
uc_stack.
When this flag is used, it is safe to switch from sighandler with
swapcontext(). Without this flag, the subsequent signal will corrupt
the state of the switched-away sighandler.

CC: Ingo Molnar <mingo@xxxxxxxxxx>
CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CC: Richard Weinberger <richard@xxxxxx>
CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
CC: Oleg Nesterov <oleg@xxxxxxxxxx>
CC: Tejun Heo <tj@xxxxxxxxxx>
CC: Heinrich Schuchardt <xypron.glpk@xxxxxx>
CC: Jason Low <jason.low2@xxxxxx>
CC: Andrea Arcangeli <aarcange@xxxxxxxxxx>
CC: Frederic Weisbecker <fweisbec@xxxxxxxxx>
CC: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
CC: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
CC: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
CC: Aleksa Sarai <cyphar@xxxxxxxxxx>
CC: "Amanieu d'Antras" <amanieu@xxxxxxxxx>
CC: Paul Moore <pmoore@xxxxxxxxxx>
CC: Sasha Levin <sasha.levin@xxxxxxxxxx>
CC: Palmer Dabbelt <palmer@xxxxxxxxxxx>
CC: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
CC: linux-kernel@xxxxxxxxxxxxxxx
CC: linux-api@xxxxxxxxxxxxxxx
CC: Andy Lutomirski <luto@xxxxxxxxxxxxxx>

Signed-off-by: Stas Sergeev <stsp@xxxxxxxxxxxxxxxxxxxxx>
---
 include/linux/sched.h       |  1 +
 include/linux/signal.h      |  4 +++-
 include/uapi/linux/signal.h |  3 +++
 kernel/fork.c               |  4 +++-
 kernel/signal.c             | 23 ++++++++++++-----------
 5 files changed, 22 insertions(+), 13 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index a10494a..f561d34 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1587,6 +1587,7 @@ struct task_struct {
 
 	unsigned long sas_ss_sp;
 	size_t sas_ss_size;
+	unsigned sas_ss_flags;
 
 	struct callback_head *task_works;
 
diff --git a/include/linux/signal.h b/include/linux/signal.h
index 92557bb..be3ebe0 100644
--- a/include/linux/signal.h
+++ b/include/linux/signal.h
@@ -432,8 +432,10 @@ int __save_altstack(stack_t __user *, unsigned long);
 	stack_t __user *__uss = uss; \
 	struct task_struct *t = current; \
 	put_user_ex((void __user *)t->sas_ss_sp, &__uss->ss_sp); \
-	put_user_ex(sas_ss_flags(sp), &__uss->ss_flags); \
+	put_user_ex(t->sas_ss_flags, &__uss->ss_flags); \
 	put_user_ex(t->sas_ss_size, &__uss->ss_size); \
+	if (t->sas_ss_flags & SS_AUTODISARM) \
+		t->sas_ss_size = 0; \
 } while (0);
 
 #ifdef CONFIG_PROC_FS
diff --git a/include/uapi/linux/signal.h b/include/uapi/linux/signal.h
index e1bd50c2..4691bc5 100644
--- a/include/uapi/linux/signal.h
+++ b/include/uapi/linux/signal.h
@@ -6,5 +6,8 @@
 
 #define SS_ONSTACK	1
 #define SS_DISABLE	2
+#define SS_VALMASK	0xf
+/* bit-flags */
+#define SS_AUTODISARM	(1 << 4)	/* disable sas during sighandling */
 
 #endif /* _UAPI_LINUX_SIGNAL_H */
diff --git a/kernel/fork.c b/kernel/fork.c
index 2e391c7..ce840a9 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1482,8 +1482,10 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 	/*
 	 * sigaltstack should be cleared when sharing the same VM
 	 */
-	if ((clone_flags & (CLONE_VM|CLONE_VFORK)) == CLONE_VM)
+	if ((clone_flags & (CLONE_VM|CLONE_VFORK)) == CLONE_VM) {
 		p->sas_ss_sp = p->sas_ss_size = 0;
+		p->sas_ss_flags = SS_DISABLE;
+	}
 
 	/*
 	 * Syscall tracing and stepping should be turned off in the
diff --git a/kernel/signal.c b/kernel/signal.c
index 0508544..25199d4 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -3100,13 +3100,14 @@ do_sigaltstack (const stack_t __user *uss, stack_t __user *uoss, unsigned long s
 	if (uss) {
 		void __user *ss_sp;
 		size_t ss_size;
-		int ss_flags;
+		unsigned ss_flags;
+		unsigned ss_xflags;
 
 		error = -EFAULT;
 		if (!access_ok(VERIFY_READ, uss, sizeof(*uss)))
 			goto out;
 		error = __get_user(ss_sp, &uss->ss_sp) |
-			__get_user(ss_flags, &uss->ss_flags) |
+			__get_user(ss_xflags, &uss->ss_flags) |
 			__get_user(ss_size, &uss->ss_size);
 		if (error)
 			goto out;
@@ -3115,14 +3116,8 @@ do_sigaltstack (const stack_t __user *uss, stack_t __user *uoss, unsigned long s
 		if (on_sig_stack(sp))
 			goto out;
 
+		ss_flags = ss_xflags & SS_VALMASK;
 		error = -EINVAL;
-		/*
-		 * Note - this code used to test ss_flags incorrectly:
-		 *  	  old code may have been written using ss_flags==0
-		 *	  to mean ss_flags==SS_ONSTACK (as this was the only
-		 *	  way that worked) - this fix preserves that older
-		 *	  mechanism.
-		 */
 		if (ss_flags != SS_DISABLE && ss_flags != SS_ONSTACK && ss_flags != 0)
 			goto out;
 
@@ -3137,6 +3132,7 @@ do_sigaltstack (const stack_t __user *uss, stack_t __user *uoss, unsigned long s
 
 		current->sas_ss_sp = (unsigned long) ss_sp;
 		current->sas_ss_size = ss_size;
+		current->sas_ss_flags = ss_xflags;
 	}
 
 	error = 0;
@@ -3167,9 +3163,14 @@ int restore_altstack(const stack_t __user *uss)
 int __save_altstack(stack_t __user *uss, unsigned long sp)
 {
 	struct task_struct *t = current;
-	return  __put_user((void __user *)t->sas_ss_sp, &uss->ss_sp) |
-		__put_user(sas_ss_flags(sp), &uss->ss_flags) |
+	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)
+		t->sas_ss_size = 0;
+	return 0;
 }
 
 #ifdef CONFIG_COMPAT
-- 
2.7.2