1 files changed, 51 insertions, 7 deletions

diff --git a/kernel/futex.c b/kernel/futex.c
index 2836ba1aa213..d0a36cba905d 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -3475,11 +3475,16 @@ err_unlock:
 	return ret;
 }
 
+/* Constants for the pending_op argument of handle_futex_death */
+#define HANDLE_DEATH_PENDING	true
+#define HANDLE_DEATH_LIST	false
+
 /*
  * Process a futex-list entry, check whether it's owned by the
  * dying task, and do notification if so:
  */
-static int handle_futex_death(u32 __user *uaddr, struct task_struct *curr, int pi)
+static int handle_futex_death(u32 __user *uaddr, struct task_struct *curr,
+			      bool pi, bool pending_op)
 {
 	u32 uval, uninitialized_var(nval), mval;
 	int err;
@@ -3492,6 +3497,42 @@ retry:
 	if (get_user(uval, uaddr))
 		return -1;
 
+	/*
+	 * Special case for regular (non PI) futexes. The unlock path in
+	 * user space has two race scenarios:
+	 *
+	 * 1. The unlock path releases the user space futex value and
+	 *    before it can execute the futex() syscall to wake up
+	 *    waiters it is killed.
+	 *
+	 * 2. A woken up waiter is killed before it can acquire the
+	 *    futex in user space.
+	 *
+	 * In both cases the TID validation below prevents a wakeup of
+	 * potential waiters which can cause these waiters to block
+	 * forever.
+	 *
+	 * In both cases the following conditions are met:
+	 *
+	 *	1) task->robust_list->list_op_pending != NULL
+	 *	   @pending_op == true
+	 *	2) User space futex value == 0
+	 *	3) Regular futex: @pi == false
+	 *
+	 * If these conditions are met, it is safe to attempt waking up a
+	 * potential waiter without touching the user space futex value and
+	 * trying to set the OWNER_DIED bit. The user space futex value is
+	 * uncontended and the rest of the user space mutex state is
+	 * consistent, so a woken waiter will just take over the
+	 * uncontended futex. Setting the OWNER_DIED bit would create
+	 * inconsistent state and malfunction of the user space owner died
+	 * handling.
+	 */
+	if (pending_op && !pi && !uval) {
+		futex_wake(uaddr, 1, 1, FUTEX_BITSET_MATCH_ANY);
+		return 0;
+	}
+
 	if ((uval & FUTEX_TID_MASK) != task_pid_vnr(curr))
 		return 0;
 
@@ -3611,10 +3652,11 @@ void exit_robust_list(struct task_struct *curr)
 		 * A pending lock might already be on the list, so
 		 * don't process it twice:
 		 */
-		if (entry != pending)
+		if (entry != pending) {
 			if (handle_futex_death((void __user *)entry + futex_offset,
-						curr, pi))
+						curr, pi, HANDLE_DEATH_LIST))
 				return;
+		}
 		if (rc)
 			return;
 		entry = next_entry;
@@ -3628,9 +3670,10 @@ void exit_robust_list(struct task_struct *curr)
 		cond_resched();
 	}
 
-	if (pending)
+	if (pending) {
 		handle_futex_death((void __user *)pending + futex_offset,
-				   curr, pip);
+				   curr, pip, HANDLE_DEATH_PENDING);
+	}
 }
 
 long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
@@ -3805,7 +3848,8 @@ void compat_exit_robust_list(struct task_struct *curr)
 		if (entry != pending) {
 			void __user *uaddr = futex_uaddr(entry, futex_offset);
 
-			if (handle_futex_death(uaddr, curr, pi))
+			if (handle_futex_death(uaddr, curr, pi,
+					       HANDLE_DEATH_LIST))
 				return;
 		}
 		if (rc)
@@ -3824,7 +3868,7 @@ void compat_exit_robust_list(struct task_struct *curr)
 	if (pending) {
 		void __user *uaddr = futex_uaddr(pending, futex_offset);
 
-		handle_futex_death(uaddr, curr, pip);
+		handle_futex_death(uaddr, curr, pip, HANDLE_DEATH_PENDING);
 	}
 }