[PATCH 15/16] loop: Avoid circular locking dependency between loop_ctl_mutex and bd_mutex

Jan Kara
2018-11-08T13:01:15Z

Code in loop_change_fd() drops reference to the old file (and also the
new file in a failure case) under loop_ctl_mutex. Similarly to a
situation in loop_set_fd() this can create a circular locking dependency
if this was the last reference holding the file open. Delay dropping of
the file reference until we have released loop_ctl_mutex.

Reported-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: Jan Kara <jack@suse.cz>
---
drivers/block/loop.c | 26 +++++++++++++++-----------
1 file changed, 15 insertions(+), 11 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index b3f981ac8ef1..112afc9bc604 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -677,7 +677,7 @@ static int loop_validate_file(struct file *file, struct block_device *bdev)
static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
unsigned int arg)
{
- struct file *file, *old_file;
+ struct file *file = NULL, *old_file;
int error;
bool partscan;

@@ -686,21 +686,21 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
return error;
error = -ENXIO;
if (lo->lo_state != Lo_bound)
- goto out_unlock;
+ goto out_err;

/* the loop device has to be read-only */
error = -EINVAL;
if (!(lo->lo_flags & LO_FLAGS_READ_ONLY))
- goto out_unlock;
+ goto out_err;

error = -EBADF;
file = fget(arg);
if (!file)
- goto out_unlock;
+ goto out_err;

error = loop_validate_file(file, bdev);
if (error)
- goto out_putf;
+ goto out_err;

old_file = lo->lo_backing_file;

@@ -708,7 +708,7 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,

/* size of the new backing store needs to be the same */
if (get_loop_size(lo, file) != get_loop_size(lo, old_file))
- goto out_putf;
+ goto out_err;

/* and ... switch */
blk_mq_freeze_queue(lo->lo_queue);
@@ -719,18 +719,22 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
lo->old_gfp_mask & ~(__GFP_IO|__GFP_FS));
loop_update_dio(lo);
blk_mq_unfreeze_queue(lo->lo_queue);
-
- fput(old_file);
partscan = lo->lo_flags & LO_FLAGS_PARTSCAN;
mutex_unlock(&loop_ctl_mutex);
+ /*
+ * We must drop file reference outside of loop_ctl_mutex as dropping
+ * the file ref can take bd_mutex which creates circular locking
+ * dependency.
+ */
+ fput(old_file);
if (partscan)
loop_reread_partitions(lo, bdev);
return 0;

-out_putf:
- fput(file);
-out_unlock:
+out_err:
mutex_unlock(&loop_ctl_mutex);
+ if (file)
+ fput(file);
return error;
}

--
2.16.4