[PATCH 6.18 289/333] btrfs: fix transaction abort on set received ioctl due to item overflow

Greg Kroah-Hartman
2026-03-17T17:25:46Z

From: Filipe Manana <fdmanana@suse.com>

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>
---
fs/btrfs/ioctl.c | 21 +++++++++++++++++++--
fs/btrfs/uuid-tree.c | 38 ++++++++++++++++++++++++++++++++++++++
fs/btrfs/uuid-tree.h | 2 ++
3 files changed, 59 insertions(+), 2 deletions(-)

--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -3984,6 +3984,25 @@ static long _btrfs_ioctl_set_received_su
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_su
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,
--- a/fs/btrfs/uuid-tree.c
+++ b/fs/btrfs/uuid-tree.c
@@ -227,6 +227,44 @@ out:
return ret;
}

+/*
+ * Check if we can add one root ID to a UUID key.
+ * If the key does not yet exists, we can, otherwise only if extended item does
+ * not exceeds the maximum item size permitted by the leaf size.
+ *
+ * Returns 0 on success, negative value on error.
+ */
+int btrfs_uuid_tree_check_overflow(struct btrfs_fs_info *fs_info,
+ const u8 *uuid, u8 type)
+{
+ BTRFS_PATH_AUTO_FREE(path);
+ int ret;
+ u32 item_size;
+ struct btrfs_key key;
+
+ if (WARN_ON_ONCE(!fs_info->uuid_root))
+ return -EINVAL;
+
+ path = btrfs_alloc_path();
+ if (!path)
+ return -ENOMEM;
+
+ btrfs_uuid_to_key(uuid, type, &key);
+ ret = btrfs_search_slot(NULL, fs_info->uuid_root, &key, path, 0, 0);
+ if (ret < 0)
+ return ret;
+ if (ret > 0)
+ return 0;
+
+ item_size = btrfs_item_size(path->nodes[0], path->slots[0]);
+
+ if (sizeof(struct btrfs_item) + item_size + sizeof(u64) >
+ BTRFS_LEAF_DATA_SIZE(fs_info))
+ return -EOVERFLOW;
+
+ return 0;
+}
+
static int btrfs_uuid_iter_rem(struct btrfs_root *uuid_root, u8 *uuid, u8 type,
u64 subid)
{
--- a/fs/btrfs/uuid-tree.h
+++ b/fs/btrfs/uuid-tree.h
@@ -12,6 +12,8 @@ int btrfs_uuid_tree_add(struct btrfs_tra
u64 subid);
int btrfs_uuid_tree_remove(struct btrfs_trans_handle *trans, const u8 *uuid, u8 type,
u64 subid);
+int btrfs_uuid_tree_check_overflow(struct btrfs_fs_info *fs_info,
+ const u8 *uuid, u8 type);
int btrfs_uuid_tree_iterate(struct btrfs_fs_info *fs_info);
int btrfs_create_uuid_tree(struct btrfs_fs_info *fs_info);
int btrfs_uuid_scan_kthread(void *data);