btrfs: fix transaction abort on set received ioctl due to item overflow

commit 87f2c46003fce4d739138aab4af1942b1afdadac upstream.

If the set received ioctl fails due to an item overflow when attempting to
add the BTRFS_UUID_KEY_RECEIVED_SUBVOL we have to abort the transaction
since we did some metadata updates before.

This means that if a user calls this ioctl with the same received UUID
field for a lot of subvolumes, we will hit the overflow, trigger the
transaction abort and turn the filesystem into RO mode. A malicious user
could exploit this, and this ioctl does not even requires that a user
has admin privileges (CAP_SYS_ADMIN), only that he/she owns the subvolume.

Fix this by doing an early check for item overflow before starting a
transaction. This is also race safe because we are holding the subvol_sem
semaphore in exclusive (write) mode.

A test case for fstests will follow soon.

Fixes: dd5f9615fc5c ("Btrfs: maintain subvolume items in the UUID tree")
CC: stable@vger.kernel.org # 3.12+
Reviewed-by: Anand Jain <asj@kernel.org>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 19 insertions, 2 deletions

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 736a1b317070..f3d293297678 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -3984,6 +3984,25 @@ static long _btrfs_ioctl_set_received_subvol(struct file *file,
 		goto out;
 	}
 
+	received_uuid_changed = memcmp(root_item->received_uuid, sa->uuid,
+				       BTRFS_UUID_SIZE);
+
+	/*
+	 * Before we attempt to add the new received uuid, check if we have room
+	 * for it in case there's already an item. If the size of the existing
+	 * item plus this root's ID (u64) exceeds the maximum item size, we can
+	 * return here without the need to abort a transaction. If we don't do
+	 * this check, the btrfs_uuid_tree_add() call below would fail with
+	 * -EOVERFLOW and result in a transaction abort. Malicious users could
+	 * exploit this to turn the fs into RO mode.
+	 */
+	if (received_uuid_changed && !btrfs_is_empty_uuid(sa->uuid)) {
+		ret = btrfs_uuid_tree_check_overflow(fs_info, sa->uuid,
+						     BTRFS_UUID_KEY_RECEIVED_SUBVOL);
+		if (ret < 0)
+			goto out;
+	}
+
 	/*
 	 * 1 - root item
 	 * 2 - uuid items (received uuid + subvol uuid)
@@ -3999,8 +4018,6 @@ static long _btrfs_ioctl_set_received_subvol(struct file *file,
 	sa->rtime.sec = ct.tv_sec;
 	sa->rtime.nsec = ct.tv_nsec;
 
-	received_uuid_changed = memcmp(root_item->received_uuid, sa->uuid,
-				       BTRFS_UUID_SIZE);
 	if (received_uuid_changed &&
 	    !btrfs_is_empty_uuid(root_item->received_uuid)) {
 		ret = btrfs_uuid_tree_remove(trans, root_item->received_uuid,