core/signals changes for v4.7

index 548ddf7d6fd2..3e84ef16f657 100644
@@ -248,18 +248,17 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
 	if (config_enabled(CONFIG_X86_64))
 		sp -= 128;
 
-	if (!onsigstack) {
-		/* This is the X/Open sanctioned signal stack switching.  */
-		if (ka->sa.sa_flags & SA_ONSTACK) {
-			if (current->sas_ss_size)
-				sp = current->sas_ss_sp + current->sas_ss_size;
-		} else if (config_enabled(CONFIG_X86_32) &&
-			   (regs->ss & 0xffff) != __USER_DS &&
-			   !(ka->sa.sa_flags & SA_RESTORER) &&
-			   ka->sa.sa_restorer) {
-				/* This is the legacy signal stack switching. */
-				sp = (unsigned long) ka->sa.sa_restorer;
-		}
+	/* This is the X/Open sanctioned signal stack switching.  */
+	if (ka->sa.sa_flags & SA_ONSTACK) {
+		if (sas_ss_flags(sp) == 0)
+			sp = current->sas_ss_sp + current->sas_ss_size;
+	} else if (config_enabled(CONFIG_X86_32) &&
+		   !onsigstack &&
+		   (regs->ss & 0xffff) != __USER_DS &&
+		   !(ka->sa.sa_flags & SA_RESTORER) &&
+		   ka->sa.sa_restorer) {
+		/* This is the legacy signal stack switching. */
+		sp = (unsigned long) ka->sa.sa_restorer;
 	}
 
 	if (fpu->fpstate_active) {

index 52c4847b05e2..77fd49f20c5f 100644
@@ -1596,6 +1596,7 @@ struct task_struct {
 
 	unsigned long sas_ss_sp;
 	size_t sas_ss_size;
+	unsigned sas_ss_flags;
 
 	struct callback_head *task_works;
 

@@ -2575,6 +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 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;

@@ -2592,6 +2605,13 @@ static inline int sas_ss_flags(unsigned long sp)
 	return on_sig_stack(sp) ? SS_ONSTACK : 0;
 }
 
+static inline void sas_ss_reset(struct task_struct *p)
+{
+	p->sas_ss_sp = 0;
+	p->sas_ss_size = 0;
+	p->sas_ss_flags = SS_DISABLE;
+}
+
 static inline unsigned long sigsp(unsigned long sp, struct ksignal *ksig)
 {
 	if (unlikely((ksig->ka.sa.sa_flags & SA_ONSTACK)) && ! sas_ss_flags(sp))

index 92557bbce7e7..3fbe81444d31 100644
@@ -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) \
+		sas_ss_reset(t); \
 } while (0);
 
 #ifdef CONFIG_PROC_FS

index e1bd50c29ded..cd0804b6bfa2 100644
@@ -7,4 +7,9 @@
 #define SS_ONSTACK	1
 #define SS_DISABLE	2
 
+/* bit-flags */
+#define SS_AUTODISARM	(1U << 31)	/* disable sas during sighandling */
+/* mask for all SS_xxx flags */
+#define SS_FLAG_BITS	SS_AUTODISARM
+
 #endif /* _UAPI_LINUX_SIGNAL_H */

index d277e83ed3e0..3e8451527cbe 100644
@@ -1494,7 +1494,7 @@ 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)
-		p->sas_ss_sp = p->sas_ss_size = 0;
+		sas_ss_reset(p);
 
 	/*
 	 * Syscall tracing and stepping should be turned off in the

index aa9bf00749c1..ab122a2cee41 100644
@@ -3099,12 +3099,14 @@ do_sigaltstack (const stack_t __user *uss, stack_t __user *uoss, unsigned long s
 
 	oss.ss_sp = (void __user *) current->sas_ss_sp;
 	oss.ss_size = current->sas_ss_size;
-	oss.ss_flags = sas_ss_flags(sp);
+	oss.ss_flags = sas_ss_flags(sp) |
+		(current->sas_ss_flags & SS_FLAG_BITS);
 
 	if (uss) {
 		void __user *ss_sp;
 		size_t ss_size;
-		int ss_flags;
+		unsigned ss_flags;
+		int ss_mode;
 
 		error = -EFAULT;
 		if (!access_ok(VERIFY_READ, uss, sizeof(*uss)))

@@ -3119,18 +3121,13 @@ do_sigaltstack (const stack_t __user *uss, stack_t __user *uoss, unsigned long s
 		if (on_sig_stack(sp))
 			goto out;
 
+		ss_mode = ss_flags & ~SS_FLAG_BITS;
 		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)
+		if (ss_mode != SS_DISABLE && ss_mode != SS_ONSTACK &&
+				ss_mode != 0)
 			goto out;
 
-		if (ss_flags == SS_DISABLE) {
+		if (ss_mode == SS_DISABLE) {
 			ss_size = 0;
 			ss_sp = NULL;
 		} else {

@@ -3141,6 +3138,7 @@ 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_flags;
 	}
 
 	error = 0;

@@ -3171,9 +3169,14 @@ 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)
+		sas_ss_reset(t);
+	return 0;
 }
 
 #ifdef CONFIG_COMPAT

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