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loop: Avoid circular locking dependency between loop_ctl_mutex and bd_mutex 

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>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Jan Kara 2018-11-08 14:01:15 +01:00 committed by Jens Axboe
parent 0da03cab87
commit 1dded9acf6
1 changed files with 15 additions and 11 deletions
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26
drivers/block/loop.c
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@ -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;
}