[PATCH 6.18 287/333] btrfs: fix transaction abort when snapshotting received subvolumes

Greg Kroah-Hartman
2026-03-17T17:28:04Z

From: Filipe Manana <fdmanana@suse.com>

commit e1b18b959025e6b5dbad668f391f65d34b39595a upstream.

Currently a user can trigger a transaction abort by snapshotting a
previously received snapshot a bunch of times until we reach a
BTRFS_UUID_KEY_RECEIVED_SUBVOL item overflow (the maximum item size we
can store in a leaf). This is very likely not common in practice, but
if it happens, it turns the filesystem into RO mode. The snapshot, send
and set_received_subvol and subvol_setflags (used by receive) don't
require CAP_SYS_ADMIN, just inode_owner_or_capable(). A malicious user
could use this to turn a filesystem into RO mode and disrupt a system.

Reproducer script:

$ cat test.sh
#!/bin/bash

DEV=/dev/sdi
MNT=/mnt/sdi

# Use smallest node size to make the test faster.
mkfs.btrfs -f --nodesize 4K $DEV
mount $DEV $MNT

# Create a subvolume and set it to RO so that it can be used for send.
btrfs subvolume create $MNT/sv
touch $MNT/sv/foo
btrfs property set $MNT/sv ro true

# Send and receive the subvolume into snaps/sv.
mkdir $MNT/snaps
btrfs send $MNT/sv | btrfs receive $MNT/snaps

# Now snapshot the received subvolume, which has a received_uuid, a
# lot of times to trigger the leaf overflow.
total=500
for ((i = 1; i <= $total; i++)); do
echo -ne "\rCreating snapshot $i/$total"
btrfs subvolume snapshot -r $MNT/snaps/sv $MNT/snaps/sv_$i > /dev/null
done
echo

umount $MNT

When running the test:

$ ./test.sh
(...)
Create subvolume '/mnt/sdi/sv'
At subvol /mnt/sdi/sv
At subvol sv
Creating snapshot 496/500ERROR: Could not create subvolume: Value too large for defined data type
Creating snapshot 497/500ERROR: Could not create subvolume: Read-only file system
Creating snapshot 498/500ERROR: Could not create subvolume: Read-only file system
Creating snapshot 499/500ERROR: Could not create subvolume: Read-only file system
Creating snapshot 500/500ERROR: Could not create subvolume: Read-only file system

And in dmesg/syslog:

$ dmesg
(...)
[251067.627338] BTRFS warning (device sdi): insert uuid item failed -75 (0x4628b21c4ac8d898, 0x2598bee2b1515c91) type 252!
[251067.629212] ------------[ cut here ]------------
[251067.630033] BTRFS: Transaction aborted (error -75)
[251067.630871] WARNING: fs/btrfs/transaction.c:1907 at create_pending_snapshot.cold+0x52/0x465 [btrfs], CPU#10: btrfs/615235
[251067.632851] Modules linked in: btrfs dm_zero (...)
[251067.644071] CPU: 10 UID: 0 PID: 615235 Comm: btrfs Tainted: G W 6.19.0-rc8-btrfs-next-225+ #1 PREEMPT(full)
[251067.646165] Tainted: [W]=WARN
[251067.646733] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.2-0-gea1b7a073390-prebuilt.qemu.org 04/01/2014
[251067.648735] RIP: 0010:create_pending_snapshot.cold+0x55/0x465 [btrfs]
[251067.649984] Code: f0 48 0f (...)
[251067.653313] RSP: 0018:ffffce644908fae8 EFLAGS: 00010292
[251067.653987] RAX: 00000000ffffff01 RBX: ffff8e5639e63a80 RCX: 00000000ffffffd3
[251067.655042] RDX: ffff8e53faa76b00 RSI: 00000000ffffffb5 RDI: ffffffffc0919750
[251067.656077] RBP: ffffce644908fbd8 R08: 0000000000000000 R09: ffffce644908f820
[251067.657068] R10: ffff8e5adc1fffa8 R11: 0000000000000003 R12: ffff8e53c0431bd0
[251067.658050] R13: ffff8e5414593600 R14: ffff8e55efafd000 R15: 00000000ffffffb5
[251067.659019] FS: 00007f2a4944b3c0(0000) GS:ffff8e5b27dae000(0000) knlGS:0000000000000000
[251067.660115] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[251067.660943] CR2: 00007ffc5aa57898 CR3: 00000005813a2003 CR4: 0000000000370ef0
[251067.661972] Call Trace:
[251067.662292] <TASK>
[251067.662653] create_pending_snapshots+0x97/0xc0 [btrfs]
[251067.663413] btrfs_commit_transaction+0x26e/0xc00 [btrfs]
[251067.664257] ? btrfs_qgroup_convert_reserved_meta+0x35/0x390 [btrfs]
[251067.665238] ? _raw_spin_unlock+0x15/0x30
[251067.665837] ? record_root_in_trans+0xa2/0xd0 [btrfs]
[251067.666531] btrfs_mksubvol+0x330/0x580 [btrfs]
[251067.667145] btrfs_mksnapshot+0x74/0xa0 [btrfs]
[251067.667827] __btrfs_ioctl_snap_create+0x194/0x1d0 [btrfs]
[251067.668595] btrfs_ioctl_snap_create_v2+0x107/0x130 [btrfs]
[251067.669479] btrfs_ioctl+0x1580/0x2690 [btrfs]
[251067.670093] ? count_memcg_events+0x6d/0x180
[251067.670849] ? handle_mm_fault+0x1a0/0x2a0
[251067.671652] __x64_sys_ioctl+0x92/0xe0
[251067.672406] do_syscall_64+0x50/0xf20
[251067.673129] entry_SYSCALL_64_after_hwframe+0x76/0x7e
[251067.674096] RIP: 0033:0x7f2a495648db
[251067.674812] Code: 00 48 89 (...)
[251067.678227] RSP: 002b:00007ffc5aa57840 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
[251067.679691] RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007f2a495648db
[251067.681145] RDX: 00007ffc5aa588b0 RSI: 0000000050009417 RDI: 0000000000000004
[251067.682511] RBP: 0000000000000002 R08: 0000000000000000 R09: 0000000000000000
[251067.683842] R10: 000000000000000a R11: 0000000000000246 R12: 00007ffc5aa59910
[251067.685176] R13: 00007ffc5aa588b0 R14: 0000000000000004 R15: 0000000000000006
[251067.686524] </TASK>
[251067.686972] ---[ end trace 0000000000000000 ]---
[251067.687890] BTRFS: error (device sdi state A) in create_pending_snapshot:1907: errno=-75 unknown
[251067.689049] BTRFS info (device sdi state EA): forced readonly
[251067.689054] BTRFS warning (device sdi state EA): Skipping commit of aborted transaction.
[251067.690119] BTRFS: error (device sdi state EA) in cleanup_transaction:2043: errno=-75 unknown
[251067.702028] BTRFS info (device sdi state EA): last unmount of filesystem 46dc3975-30a2-4a69-a18f-418b859cccda

Fix this by ignoring -EOVERFLOW errors from btrfs_uuid_tree_add() in the
snapshot creation code when attempting to add the
BTRFS_UUID_KEY_RECEIVED_SUBVOL item. This is OK because it's not critical
and we are still able to delete the snapshot, as snapshot/subvolume
deletion ignores if a BTRFS_UUID_KEY_RECEIVED_SUBVOL is missing (see
inode.c:btrfs_delete_subvolume()). As for send/receive, we can still do
send/receive operations since it always peeks the first root ID in the
existing BTRFS_UUID_KEY_RECEIVED_SUBVOL (it could peek any since all
snapshots have the same content), and even if the key is missing, it
falls back to searching by BTRFS_UUID_KEY_SUBVOL key.

A test case for fstests will be sent soon.

Fixes: dd5f9615fc5c ("Btrfs: maintain subvolume items in the UUID tree")
CC: stable@vger.kernel.org # 3.12+
Reviewed-by: Boris Burkov <boris@bur.io>
Reviewed-by: Qu Wenruo <wqu@suse.com>
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/transaction.c | 16 ++++++++++++++++
1 file changed, 16 insertions(+)

--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -1890,6 +1890,22 @@ static noinline int create_pending_snaps
ret = btrfs_uuid_tree_add(trans, new_root_item->received_uuid,
BTRFS_UUID_KEY_RECEIVED_SUBVOL,
objectid);
+ /*
+ * We are creating of lot of snapshots of the same root that was
+ * received (has a received UUID) and reached a leaf's limit for
+ * an item. We can safely ignore this and avoid a transaction
+ * abort. A deletion of this snapshot will still work since we
+ * ignore if an item with a BTRFS_UUID_KEY_RECEIVED_SUBVOL key
+ * is missing (see btrfs_delete_subvolume()). Send/receive will
+ * work too since it peeks the first root id from the existing
+ * item (it could peek any), and in case it's missing it
+ * falls back to search by BTRFS_UUID_KEY_SUBVOL keys.
+ * Creation of a snapshot does not require CAP_SYS_ADMIN, so
+ * we don't want users triggering transaction aborts, either
+ * intentionally or not.
+ */
+ if (ret == -EOVERFLOW)
+ ret = 0;
if (unlikely(ret && ret != -EEXIST)) {
btrfs_abort_transaction(trans, ret);
goto fail;