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Date: Tue, 31 Jan 2017 12:33:00 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Dmitry Vyukov <dvyukov@google.com>
cc: Al Viro <viro@zeniv.linux.org.uk>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        syzkaller <syzkaller@googlegroups.com>
Subject: Re: timerfd: use-after-free in timerfd_remove_cancel
In-Reply-To: <CACT4Y+apAff+hNa=q58=Z2TSh_av-GubQPSgFHc=f0gE0kuGGg@mail.gmail.com>
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On Mon, 30 Jan 2017, Dmitry Vyukov wrote:
> 
> Seems that ctx->might_cancel is racy.

Yes, it is. Fix below.

8<-------------------

--- a/fs/timerfd.c
+++ b/fs/timerfd.c
@@ -40,9 +40,12 @@ struct timerfd_ctx {
 	short unsigned settime_flags;	/* to show in fdinfo */
 	struct rcu_head rcu;
 	struct list_head clist;
-	bool might_cancel;
+	unsigned long flags;
 };
 
+/* Bit positions for ctx->flags */
+#define MIGHT_CANCEL	0
+
 static LIST_HEAD(cancel_list);
 static DEFINE_SPINLOCK(cancel_lock);
 
@@ -99,7 +102,7 @@ void timerfd_clock_was_set(void)
 
 	rcu_read_lock();
 	list_for_each_entry_rcu(ctx, &cancel_list, clist) {
-		if (!ctx->might_cancel)
+		if (!test_bit(MIGHT_CANCEL, &ctx->flags))
 			continue;
 		spin_lock_irqsave(&ctx->wqh.lock, flags);
 		if (ctx->moffs != moffs) {
@@ -114,8 +117,7 @@ void timerfd_clock_was_set(void)
 
 static void timerfd_remove_cancel(struct timerfd_ctx *ctx)
 {
-	if (ctx->might_cancel) {
-		ctx->might_cancel = false;
+	if (test_and_clear_bit(MIGHT_CANCEL, &ctx->flags)) {
 		spin_lock(&cancel_lock);
 		list_del_rcu(&ctx->clist);
 		spin_unlock(&cancel_lock);
@@ -124,7 +126,7 @@ static void timerfd_remove_cancel(struct
 
 static bool timerfd_canceled(struct timerfd_ctx *ctx)
 {
-	if (!ctx->might_cancel || ctx->moffs != KTIME_MAX)
+	if (!test_bit(MIGHT_CANCEL, &ctx->flags) || ctx->moffs != KTIME_MAX)
 		return false;
 	ctx->moffs = ktime_mono_to_real(0);
 	return true;
@@ -135,13 +137,12 @@ static void timerfd_setup_cancel(struct
 	if ((ctx->clockid == CLOCK_REALTIME ||
 	     ctx->clockid == CLOCK_REALTIME_ALARM) &&
 	    (flags & TFD_TIMER_ABSTIME) && (flags & TFD_TIMER_CANCEL_ON_SET)) {
-		if (!ctx->might_cancel) {
-			ctx->might_cancel = true;
+		if (test_and_set_bit(MIGHT_CANCEL, &ctx->flags)) {
 			spin_lock(&cancel_lock);
 			list_add_rcu(&ctx->clist, &cancel_list);
 			spin_unlock(&cancel_lock);
 		}
-	} else if (ctx->might_cancel) {
+	} else {
 		timerfd_remove_cancel(ctx);
 	}
 }