4.20-stable review patch. If anyone has any objections, please let me know.
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From: Jan Kara jack@suse.cz
commit 1dded9acf6dc9a34cd27fcf8815507e4e65b3c4f upstream.
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 Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- drivers/block/loop.c | 26 +++++++++++++++----------- 1 file changed, 15 insertions(+), 11 deletions(-)
--- a/drivers/block/loop.c +++ b/drivers/block/loop.c @@ -677,7 +677,7 @@ static int loop_validate_file(struct fil 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_de 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_de
/* 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_de 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; }