CVE-2026-43361: btrfs: fix transaction abort when snapshotting received subvolumes

CVE-2026-43361: btrfs: fix transaction abort when snapshotting received subvolumes

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

btrfs: fix transaction abort when snapshotting received subvolumes

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.

The Linux kernel CVE team has assigned CVE-2026-43361 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 3.12 with commit dd5f9615fc5c5e8d3751aab3a17b92768fb1ce70 and fixed in 6.1.167 with commit 9a9227b488ffb7cdbb5d930a01fc6956c05ba61a
	Issue introduced in 3.12 with commit dd5f9615fc5c5e8d3751aab3a17b92768fb1ce70 and fixed in 6.6.130 with commit 6bce705b699cba9afccb996c77d194fe003dfa2a
	Issue introduced in 3.12 with commit dd5f9615fc5c5e8d3751aab3a17b92768fb1ce70 and fixed in 6.12.78 with commit e3d8efc157bc590457d3e31da403af1a221643d6
	Issue introduced in 3.12 with commit dd5f9615fc5c5e8d3751aab3a17b92768fb1ce70 and fixed in 6.18.19 with commit bac55dde8efa457e769c934fd88a63f2141ba238
	Issue introduced in 3.12 with commit dd5f9615fc5c5e8d3751aab3a17b92768fb1ce70 and fixed in 6.19.9 with commit 770af8e465c2c3de528f85e840eab462dd41542b
	Issue introduced in 3.12 with commit dd5f9615fc5c5e8d3751aab3a17b92768fb1ce70 and fixed in 7.0 with commit e1b18b959025e6b5dbad668f391f65d34b39595a

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-43361
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	fs/btrfs/transaction.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/9a9227b488ffb7cdbb5d930a01fc6956c05ba61a
	https://git.kernel.org/stable/c/6bce705b699cba9afccb996c77d194fe003dfa2a
	https://git.kernel.org/stable/c/e3d8efc157bc590457d3e31da403af1a221643d6
	https://git.kernel.org/stable/c/bac55dde8efa457e769c934fd88a63f2141ba238
	https://git.kernel.org/stable/c/770af8e465c2c3de528f85e840eab462dd41542b
	https://git.kernel.org/stable/c/e1b18b959025e6b5dbad668f391f65d34b39595a