loop: add recursion validation to LOOP_CHANGE_FD

BugLink: https://bugs.launchpad.net/bugs/1790620

commit d2ac838e upstream.

Refactor the validation code used in LOOP_SET_FD so it is also used in
LOOP_CHANGE_FD.  Otherwise it is possible to construct a set of loop
devices that all refer to each other.  This can lead to a infinite
loop in starting with "while (is_loop_device(f)) .." in loop_set_fd().

Fix this by refactoring out the validation code and using it for
LOOP_CHANGE_FD as well as LOOP_SET_FD.

Reported-by: syzbot+4349872271ece473a7c91190b68b4bac7c5dbc87@syzkaller.appspotmail.com
Reported-by: syzbot+40bd32c4d9a3cc12a339@syzkaller.appspotmail.com
Reported-by: syzbot+769c54e66f994b041be7@syzkaller.appspotmail.com
Reported-by: syzbot+0a89a9ce473936c57065@syzkaller.appspotmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/block/loop.c

@@ -663,6 +663,36 @@ static void loop_reread_partitions(struct loop_device *lo,
 			__func__, lo->lo_number, lo->lo_file_name, rc);
 }
 
+static inline int is_loop_device(struct file *file)
+{
+	struct inode *i = file->f_mapping->host;
+
+	return i && S_ISBLK(i->i_mode) && MAJOR(i->i_rdev) == LOOP_MAJOR;
+}
+
+static int loop_validate_file(struct file *file, struct block_device *bdev)
+{
+	struct inode    *inode = file->f_mapping->host;
+	struct file     *f = file;
+
+	/* Avoid recursion */
+	while (is_loop_device(f)) {
+		struct loop_device *l;
+
+		if (f->f_mapping->host->i_bdev == bdev)
+			return -EBADF;
+
+		l = f->f_mapping->host->i_bdev->bd_disk->private_data;
+		if (l->lo_state == Lo_unbound) {
+			return -EINVAL;
+		}
+		f = l->lo_backing_file;
+	}
+	if (!S_ISREG(inode->i_mode) && !S_ISBLK(inode->i_mode))
+		return -EINVAL;
+	return 0;
+}
+
 /*
  * loop_change_fd switched the backing store of a loopback device to
  * a new file. This is useful for operating system installers to free up
@@ -692,6 +722,11 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
 	file = fget(arg);
 	if (!file)
 		goto out;
+
+	error = loop_validate_file(file, bdev);
+	if (error)
+		goto out_putf;
+
 	f = loop_real_file(file);
 	if (f) {
 		virt_file = file;
@@ -705,9 +740,6 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
 
 	error = -EINVAL;
 
-	if (!S_ISREG(inode->i_mode) && !S_ISBLK(inode->i_mode))
-		goto out_putf;
-
 	/* 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;
@@ -732,13 +764,6 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
 	return error;
 }
 
-static inline int is_loop_device(struct file *file)
-{
-	struct inode *i = file->f_mapping->host;
-
-	return i && S_ISBLK(i->i_mode) && MAJOR(i->i_rdev) == LOOP_MAJOR;
-}
-
 /*
  * for AUFS
  * no get/put for file.
@@ -940,29 +965,13 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
 	if (lo->lo_state != Lo_unbound)
 		goto out_putf;
 
-	/* Avoid recursion */
-	f = file;
-	while (is_loop_device(f)) {
-		struct loop_device *l;
-
-		if (f->f_mapping->host->i_bdev == bdev)
-			goto out_putf;
-
-		l = f->f_mapping->host->i_bdev->bd_disk->private_data;
-		if (l->lo_state == Lo_unbound) {
-			error = -EINVAL;
-			goto out_putf;
-		}
-		f = l->lo_backing_file;
-	}
+	error = loop_validate_file(file, bdev);
+	if (error)
+		goto out_putf;
 
 	mapping = file->f_mapping;
 	inode = mapping->host;
 
-	error = -EINVAL;
-	if (!S_ISREG(inode->i_mode) && !S_ISBLK(inode->i_mode))
-		goto out_putf;
-
 	if (!(file->f_mode & FMODE_WRITE) || !(mode & FMODE_WRITE) ||
 	    !file->f_op->write_iter)
 		lo_flags |= LO_FLAGS_READ_ONLY;