[PATCH 3.18 31/52] block/loop: Use global lock for ioctl() operation.

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

3.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>

commit 310ca162d779efee8a2dc3731439680f3e9c1e86 upstream.

syzbot is reporting NULL pointer dereference [1] which is caused by
race condition between ioctl(loop_fd, LOOP_CLR_FD, 0) versus
ioctl(other_loop_fd, LOOP_SET_FD, loop_fd) due to traversing other
loop devices at loop_validate_file() without holding corresponding
lo->lo_ctl_mutex locks.

Since ioctl() request on loop devices is not frequent operation, we don't
need fine grained locking. Let's use global lock in order to allow safe
traversal at loop_validate_file().

Note that syzbot is also reporting circular locking dependency between
bdev->bd_mutex and lo->lo_ctl_mutex [2] which is caused by calling
blkdev_reread_part() with lock held. This patch does not address it.

[1] https://syzkaller.appspot.com/bug?id=f3cfe26e785d85f9ee259f385515291d21bd80a3
[2] https://syzkaller.appspot.com/bug?id=bf154052f0eea4bc7712499e4569505907d15889

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reported-by: syzbot <syzbot+bf89c128e05dd6c62523@syzkaller.appspotmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 drivers/block/loop.c |   47 ++++++++++++++++++++++++-----------------------
 drivers/block/loop.h |    1 -
 2 files changed, 24 insertions(+), 24 deletions(-)

--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -81,6 +81,7 @@
 
 static DEFINE_IDR(loop_index_idr);
 static DEFINE_MUTEX(loop_index_mutex);
+static DEFINE_MUTEX(loop_ctl_mutex);
 
 static int max_part;
 static int part_shift;
@@ -1012,7 +1013,7 @@ static int loop_clr_fd(struct loop_devic
 	 */
 	if (lo->lo_refcnt > 1) {
 		lo->lo_flags |= LO_FLAGS_AUTOCLEAR;
-		mutex_unlock(&lo->lo_ctl_mutex);
+		mutex_unlock(&loop_ctl_mutex);
 		return 0;
 	}
 
@@ -1061,12 +1062,12 @@ static int loop_clr_fd(struct loop_devic
 	lo->lo_flags = 0;
 	if (!part_shift)
 		lo->lo_disk->flags |= GENHD_FL_NO_PART_SCAN;
-	mutex_unlock(&lo->lo_ctl_mutex);
+	mutex_unlock(&loop_ctl_mutex);
 	/*
-	 * Need not hold lo_ctl_mutex to fput backing file.
-	 * Calling fput holding lo_ctl_mutex triggers a circular
+	 * Need not hold loop_ctl_mutex to fput backing file.
+	 * Calling fput holding loop_ctl_mutex triggers a circular
 	 * lock dependency possibility warning as fput can take
-	 * bd_mutex which is usually taken before lo_ctl_mutex.
+	 * bd_mutex which is usually taken before loop_ctl_mutex.
 	 */
 	fput(filp);
 	return 0;
@@ -1300,7 +1301,7 @@ static int lo_ioctl(struct block_device
 	struct loop_device *lo = bdev->bd_disk->private_data;
 	int err;
 
-	mutex_lock_nested(&lo->lo_ctl_mutex, 1);
+	mutex_lock_nested(&loop_ctl_mutex, 1);
 	switch (cmd) {
 	case LOOP_SET_FD:
 		err = loop_set_fd(lo, mode, bdev, arg);
@@ -1309,7 +1310,7 @@ static int lo_ioctl(struct block_device
 		err = loop_change_fd(lo, bdev, arg);
 		break;
 	case LOOP_CLR_FD:
-		/* loop_clr_fd would have unlocked lo_ctl_mutex on success */
+		/* loop_clr_fd would have unlocked loop_ctl_mutex on success */
 		err = loop_clr_fd(lo);
 		if (!err)
 			goto out_unlocked;
@@ -1340,7 +1341,7 @@ static int lo_ioctl(struct block_device
 	default:
 		err = lo->ioctl ? lo->ioctl(lo, cmd, arg) : -EINVAL;
 	}
-	mutex_unlock(&lo->lo_ctl_mutex);
+	mutex_unlock(&loop_ctl_mutex);
 
 out_unlocked:
 	return err;
@@ -1473,16 +1474,16 @@ static int lo_compat_ioctl(struct block_
 
 	switch(cmd) {
 	case LOOP_SET_STATUS:
-		mutex_lock(&lo->lo_ctl_mutex);
+		mutex_lock(&loop_ctl_mutex);
 		err = loop_set_status_compat(
 			lo, (const struct compat_loop_info __user *) arg);
-		mutex_unlock(&lo->lo_ctl_mutex);
+		mutex_unlock(&loop_ctl_mutex);
 		break;
 	case LOOP_GET_STATUS:
-		mutex_lock(&lo->lo_ctl_mutex);
+		mutex_lock(&loop_ctl_mutex);
 		err = loop_get_status_compat(
 			lo, (struct compat_loop_info __user *) arg);
-		mutex_unlock(&lo->lo_ctl_mutex);
+		mutex_unlock(&loop_ctl_mutex);
 		break;
 	case LOOP_SET_CAPACITY:
 	case LOOP_CLR_FD:
@@ -1513,9 +1514,9 @@ static int lo_open(struct block_device *
 		goto out;
 	}
 
-	mutex_lock(&lo->lo_ctl_mutex);
+	mutex_lock(&loop_ctl_mutex);
 	lo->lo_refcnt++;
-	mutex_unlock(&lo->lo_ctl_mutex);
+	mutex_unlock(&loop_ctl_mutex);
 out:
 	mutex_unlock(&loop_index_mutex);
 	return err;
@@ -1525,7 +1526,7 @@ static void __lo_release(struct loop_dev
 {
 	int err;
 
-	mutex_lock(&lo->lo_ctl_mutex);
+	mutex_lock(&loop_ctl_mutex);
 
 	if (--lo->lo_refcnt)
 		goto out;
@@ -1547,7 +1548,7 @@ static void __lo_release(struct loop_dev
 	}
 
 out:
-	mutex_unlock(&lo->lo_ctl_mutex);
+	mutex_unlock(&loop_ctl_mutex);
 }
 
 static void lo_release(struct gendisk *disk, fmode_t mode)
@@ -1593,10 +1594,10 @@ static int unregister_transfer_cb(int id
 	struct loop_device *lo = ptr;
 	struct loop_func_table *xfer = data;
 
-	mutex_lock(&lo->lo_ctl_mutex);
+	mutex_lock(&loop_ctl_mutex);
 	if (lo->lo_encryption == xfer)
 		loop_release_xfer(lo);
-	mutex_unlock(&lo->lo_ctl_mutex);
+	mutex_unlock(&loop_ctl_mutex);
 	return 0;
 }
 
@@ -1677,7 +1678,7 @@ static int loop_add(struct loop_device *
 	if (!part_shift)
 		disk->flags |= GENHD_FL_NO_PART_SCAN;
 	disk->flags |= GENHD_FL_EXT_DEVT;
-	mutex_init(&lo->lo_ctl_mutex);
+	mutex_init(&loop_ctl_mutex);
 	lo->lo_number		= i;
 	lo->lo_thread		= NULL;
 	init_waitqueue_head(&lo->lo_event);
@@ -1789,19 +1790,19 @@ static long loop_control_ioctl(struct fi
 		ret = loop_lookup(&lo, parm);
 		if (ret < 0)
 			break;
-		mutex_lock(&lo->lo_ctl_mutex);
+		mutex_lock(&loop_ctl_mutex);
 		if (lo->lo_state != Lo_unbound) {
 			ret = -EBUSY;
-			mutex_unlock(&lo->lo_ctl_mutex);
+			mutex_unlock(&loop_ctl_mutex);
 			break;
 		}
 		if (lo->lo_refcnt > 0) {
 			ret = -EBUSY;
-			mutex_unlock(&lo->lo_ctl_mutex);
+			mutex_unlock(&loop_ctl_mutex);
 			break;
 		}
 		lo->lo_disk->private_data = NULL;
-		mutex_unlock(&lo->lo_ctl_mutex);
+		mutex_unlock(&loop_ctl_mutex);
 		idr_remove(&loop_index_idr, lo->lo_number);
 		loop_remove(lo);
 		break;
--- a/drivers/block/loop.h
+++ b/drivers/block/loop.h
@@ -55,7 +55,6 @@ struct loop_device {
 	struct bio_list		lo_bio_list;
 	unsigned int		lo_bio_count;
 	int			lo_state;
-	struct mutex		lo_ctl_mutex;
 	struct task_struct	*lo_thread;
 	wait_queue_head_t	lo_event;
 	/* wait queue for incoming requests */