[PATCH] loop: fix LOOP_GET_STATUS lock imbalance

[Omar Sandoval <osandov@osandov.com>]

From: Omar Sandoval <osandov@fb.com>

Commit 2d1d4c1e591f made loop_get_status() drop lo_ctx_mutex before
returning, but the loop_get_status_old(), loop_get_status64(), and
loop_get_status_compat() wrappers don't call loop_get_status() if the
passed argument is NULL. The callers expect that the lock is dropped, so
make sure we drop it in that case, too.

Reported-by: syzbot+31e8daa8b3fc129e75f2@syzkaller.appspotmail.com
Fixes: 2d1d4c1e591f ("loop: don't call into filesystem while holding lo_ctl_mutex")
Signed-off-by: Omar Sandoval <osandov@fb.com>
---
Based on Linus' tree.

 drivers/block/loop.c | 33 ++++++++++++++++++---------------
 1 file changed, 18 insertions(+), 15 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 264abaaff662..9b476fd2bc41 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1283,12 +1283,13 @@ static int
 loop_get_status_old(struct loop_device *lo, struct loop_info __user *arg) {
 	struct loop_info info;
 	struct loop_info64 info64;
-	int err = 0;
+	int err;
 
-	if (!arg)
-		err = -EINVAL;
-	if (!err)
-		err = loop_get_status(lo, &info64);
+	if (!arg) {
+		mutex_unlock(&lo->lo_ctl_mutex);
+		return -EINVAL;
+	}
+	err = loop_get_status(lo, &info64);
 	if (!err)
 		err = loop_info64_to_old(&info64, &info);
 	if (!err && copy_to_user(arg, &info, sizeof(info)))
@@ -1300,12 +1301,13 @@ loop_get_status_old(struct loop_device *lo, struct loop_info __user *arg) {
 static int
 loop_get_status64(struct loop_device *lo, struct loop_info64 __user *arg) {
 	struct loop_info64 info64;
-	int err = 0;
+	int err;
 
-	if (!arg)
-		err = -EINVAL;
-	if (!err)
-		err = loop_get_status(lo, &info64);
+	if (!arg) {
+		mutex_unlock(&lo->lo_ctl_mutex);
+		return -EINVAL;
+	}
+	err = loop_get_status(lo, &info64);
 	if (!err && copy_to_user(arg, &info64, sizeof(info64)))
 		err = -EFAULT;
 
@@ -1529,12 +1531,13 @@ loop_get_status_compat(struct loop_device *lo,
 		       struct compat_loop_info __user *arg)
 {
 	struct loop_info64 info64;
-	int err = 0;
+	int err;
 
-	if (!arg)
-		err = -EINVAL;
-	if (!err)
-		err = loop_get_status(lo, &info64);
+	if (!arg) {
+		mutex_unlock(&lo->lo_ctl_mutex);
+		return -EINVAL;
+	}
+	err = loop_get_status(lo, &info64);
 	if (!err)
 		err = loop_info64_to_compat(&info64, arg);
 	return err;
-- 
2.17.0