CVE-2026-43299: btrfs: do not ASSERT() when the fs flips RO inside btrfs_repair_io_failure()

CVE-2026-43299: btrfs: do not ASSERT() when the fs flips RO inside btrfs_repair_io_failure()

[Greg Kroah-Hartman]

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

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

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

btrfs: do not ASSERT() when the fs flips RO inside btrfs_repair_io_failure()

[BUG]
There is a bug report that when btrfs hits ENOSPC error in a critical
path, btrfs flips RO (this part is expected, although the ENOSPC bug
still needs to be addressed).

The problem is after the RO flip, if there is a read repair pending, we
can hit the ASSERT() inside btrfs_repair_io_failure() like the following:

  BTRFS info (device vdc): relocating block group 30408704 flags metadata|raid1
  ------------[ cut here ]------------
  BTRFS: Transaction aborted (error -28)
  WARNING: fs/btrfs/extent-tree.c:3235 at __btrfs_free_extent.isra.0+0x453/0xfd0, CPU#1: btrfs/383844
  Modules linked in: kvm_intel kvm irqbypass
  [...]
  ---[ end trace 0000000000000000 ]---
  BTRFS info (device vdc state EA): 2 enospc errors during balance
  BTRFS info (device vdc state EA): balance: ended with status: -30
  BTRFS error (device vdc state EA): parent transid verify failed on logical 30556160 mirror 2 wanted 8 found 6
  BTRFS error (device vdc state EA): bdev /dev/nvme0n1 errs: wr 0, rd 0, flush 0, corrupt 10, gen 0
  [...]
  assertion failed: !(fs_info->sb->s_flags & SB_RDONLY) :: 0, in fs/btrfs/bio.c:938
  ------------[ cut here ]------------
  assertion failed: !(fs_info->sb->s_flags & SB_RDONLY) :: 0, in fs/btrfs/bio.c:938
  kernel BUG at fs/btrfs/bio.c:938!
  Oops: invalid opcode: 0000 [#1] SMP NOPTI
  CPU: 0 UID: 0 PID: 868 Comm: kworker/u8:13 Tainted: G        W        N  6.19.0-rc6+ #4788 PREEMPT(full)
  Tainted: [W]=WARN, [N]=TEST
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.17.0-0-gb52ca86e094d-prebuilt.qemu.org 04/01/2014
  Workqueue: btrfs-endio simple_end_io_work
  RIP: 0010:btrfs_repair_io_failure.cold+0xb2/0x120
  RSP: 0000:ffffc90001d2bcf0 EFLAGS: 00010246
  RAX: 0000000000000051 RBX: 0000000000001000 RCX: 0000000000000000
  RDX: 0000000000000000 RSI: ffffffff8305cf42 RDI: 00000000ffffffff
  RBP: 0000000000000002 R08: 00000000fffeffff R09: ffffffff837fa988
  R10: ffffffff8327a9e0 R11: 6f69747265737361 R12: ffff88813018d310
  R13: ffff888168b8a000 R14: ffffc90001d2bd90 R15: ffff88810a169000
  FS:  0000000000000000(0000) GS:ffff8885e752c000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  ------------[ cut here ]------------

[CAUSE]
The cause of -ENOSPC error during the test case btrfs/124 is still
unknown, although it's known that we still have cases where metadata can
be over-committed but can not be fulfilled correctly, thus if we hit
such ENOSPC error inside a critical path, we have no choice but abort
the current transaction.

This will mark the fs read-only.

The problem is inside the btrfs_repair_io_failure() path that we require
the fs not to be mount read-only. This is normally fine, but if we are
doing a read-repair meanwhile the fs flips RO due to a critical error,
we can enter btrfs_repair_io_failure() with super block set to
read-only, thus triggering the above crash.

[FIX]
Just replace the ASSERT() with a proper return if the fs is already
read-only.

The Linux kernel CVE team has assigned CVE-2026-43299 to this issue.


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

	Fixed in 6.19.6 with commit f6df18c001e3dcebc08482d0adeacd0cfea08593
	Fixed in 7.0 with commit 8ceaad6cd6e7fa5f73b0b2796a2e85d75d37e9f3

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-2026-43299
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/bio.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/f6df18c001e3dcebc08482d0adeacd0cfea08593
	https://git.kernel.org/stable/c/8ceaad6cd6e7fa5f73b0b2796a2e85d75d37e9f3