loop: fix backing file reference leak on validation error

JIRA: https://issues.redhat.com/browse/RHEL-118049

commit 98b7bf5
Author: Li Chen <me@linux.beauty>
Date:   Tue Sep 30 08:35:59 2025 +0800

    loop: fix backing file reference leak on validation error

    loop_change_fd() and loop_configure() call loop_check_backing_file()
    to validate the new backing file. If validation fails, the reference
    acquired by fget() was not dropped, leaking a file reference.

    Fix this by calling fput(file) before returning the error.

    Cc: stable@vger.kernel.org
    Cc: Markus Elfring <Markus.Elfring@web.de>
    CC: Yang Erkun <yangerkun@huawei.com>
    Cc: Ming Lei <ming.lei@redhat.com>
    Cc: Yu Kuai <yukuai1@huaweicloud.com>
    Fixes: f5c84ef ("loop: Add sanity check for read/write_iter")
    Signed-off-by: Li Chen <chenl311@chinatelecom.cn>
    Reviewed-by: Ming Lei <ming.lei@redhat.com>
    Reviewed-by: Yang Erkun <yangerkun@huawei.com>
    Signed-off-by: Jens Axboe <axboe@kernel.dk>

Signed-off-by: Ming Lei <ming.lei@redhat.com>

Author: Ming Lei

drivers/block/loop.c

@@ -541,8 +541,10 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
 		return -EBADF;
 
 	error = loop_check_backing_file(file);
-	if (error)
+	if (error) {
+		fput(file);
 		return error;
+	}
 
 	/* suppress uevents while reconfiguring the device */
 	dev_set_uevent_suppress(disk_to_dev(lo->lo_disk), 1);
@@ -983,8 +985,10 @@ static int loop_configure(struct loop_device *lo, blk_mode_t mode,
 		return -EBADF;
 
 	error = loop_check_backing_file(file);
-	if (error)
+	if (error) {
+		fput(file);
 		return error;
+	}
 
 	is_loop = is_loop_device(file);