CVE-2026-74571: btrfs: skip global block reserve accounting for rescue mounts

From: Greg Kroah-Hartman <[email protected]>

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

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

btrfs: skip global block reserve accounting for rescue mounts

[BUG]
Mounting with rescue=ibadroots after corrupting the block group tree
root triggers a NULL pointer dereference:

  BUG: kernel NULL pointer dereference, address: 0000000000000100
  RIP: 0010:btrfs_update_global_block_rsv+0x9d/0x1c0 [btrfs]
  Call Trace:
   fill_dummy_bgs+0xd4/0x120 [btrfs]
   open_ctree+0xc6e/0x1ca0 [btrfs]
   btrfs_get_tree+0x50d/0xa40 [btrfs]

The same crash occurs with a corrupted raid stripe tree root, via
btrfs_read_block_groups() instead of fill_dummy_bgs().

[CAUSE]
With rescue=ibadroots, btrfs_read_roots() allows the mount to continue
when either root cannot be read, leaving the corresponding root pointer
NULL while its on-disk feature bit remains set.

btrfs_update_global_block_rsv() then dereferences the missing root based
on the feature bit alone.

[FIX]
Rescue mounts are fully read-only and cannot start transactions, so the
global reserve is never consumed. Under btrfs_is_full_ro(), mark the
reserve as full and return before performing the accounting.

And since we need to check if the fs is mount fully RO, export
fs_is_full_ro() as btrfs_is_full_ro(), and move it to fs.h.

[ Squash the fs_is_full_ro() export commit into this one. ]

The Linux kernel CVE team has assigned CVE-2026-74571 to this issue.


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

	Issue introduced in 6.5 with commit 8dbfc14fc736eb701089aff09645c3d4ad3decb1 and fixed in 7.1.8 with commit 076349e4c8d11f6b58c4549976a513b2b4dc6df2
	Issue introduced in 6.5 with commit 8dbfc14fc736eb701089aff09645c3d4ad3decb1 and fixed in 7.2-rc6 with commit 51a0e8399858621442807a26057bcd1cd3ced046
	Issue introduced in 6.1.43 with commit cbec34d3021d47007a0334c634f7053dbaf93d02
	Issue introduced in 6.4.8 with commit 1e8087589b5cf6fa17adaf57b64cf1656d77dfec

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-2026-74571
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/block-rsv.c
	fs/btrfs/disk-io.c
	fs/btrfs/fs.h


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/076349e4c8d11f6b58c4549976a513b2b4dc6df2
	https://git.kernel.org/stable/c/51a0e8399858621442807a26057bcd1cd3ced046