Subject: CVE-2023-52897: btrfs: qgroup: do not warn on record without old_roots populated

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

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

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

btrfs: qgroup: do not warn on record without old_roots populated

[BUG]
There are some reports from the mailing list that since v6.1 kernel, the
WARN_ON() inside btrfs_qgroup_account_extent() gets triggered during
rescan:

  WARNING: CPU: 3 PID: 6424 at fs/btrfs/qgroup.c:2756 btrfs_qgroup_account_extents+0x1ae/0x260 [btrfs]
  CPU: 3 PID: 6424 Comm: snapperd Tainted: P           OE      6.1.2-1-default #1 openSUSE Tumbleweed 05c7a1b1b61d5627475528f71f50444637b5aad7
  RIP: 0010:btrfs_qgroup_account_extents+0x1ae/0x260 [btrfs]
  Call Trace:
   <TASK>
  btrfs_commit_transaction+0x30c/0xb40 [btrfs c39c9c546c241c593f03bd6d5f39ea1b676250f6]
   ? start_transaction+0xc3/0x5b0 [btrfs c39c9c546c241c593f03bd6d5f39ea1b676250f6]
  btrfs_qgroup_rescan+0x42/0xc0 [btrfs c39c9c546c241c593f03bd6d5f39ea1b676250f6]
   btrfs_ioctl+0x1ab9/0x25c0 [btrfs c39c9c546c241c593f03bd6d5f39ea1b676250f6]
   ? __rseq_handle_notify_resume+0xa9/0x4a0
   ? mntput_no_expire+0x4a/0x240
   ? __seccomp_filter+0x319/0x4d0
   __x64_sys_ioctl+0x90/0xd0
   do_syscall_64+0x5b/0x80
   ? syscall_exit_to_user_mode+0x17/0x40
   ? do_syscall_64+0x67/0x80
  entry_SYSCALL_64_after_hwframe+0x63/0xcd
  RIP: 0033:0x7fd9b790d9bf
   </TASK>

[CAUSE]
Since commit e15e9f43c7ca ("btrfs: introduce
BTRFS_QGROUP_RUNTIME_FLAG_NO_ACCOUNTING to skip qgroup accounting"), if
our qgroup is already in inconsistent state, we will no longer do the
time-consuming backref walk.

This can leave some qgroup records without a valid old_roots ulist.
Normally this is fine, as btrfs_qgroup_account_extents() would also skip
those records if we have NO_ACCOUNTING flag set.

But there is a small window, if we have NO_ACCOUNTING flag set, and
inserted some qgroup_record without a old_roots ulist, but then the user
triggered a qgroup rescan.

During btrfs_qgroup_rescan(), we firstly clear NO_ACCOUNTING flag, then
commit current transaction.

And since we have a qgroup_record with old_roots = NULL, we trigger the
WARN_ON() during btrfs_qgroup_account_extents().

[FIX]
Unfortunately due to the introduction of NO_ACCOUNTING flag, the
assumption that every qgroup_record would have its old_roots populated
is no longer correct.

Fix the false alerts and drop the WARN_ON().

The Linux kernel CVE team has assigned CVE-2023-52897 to this issue.

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

	Issue introduced in 6.1 with commit e15e9f43c7ca and fixed in 6.1.8 with commit bb2c2e62539f
	Issue introduced in 6.1 with commit e15e9f43c7ca and fixed in 6.2 with commit 75181406b4ea

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-2023-52897
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/qgroup.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/bb2c2e62539f2b63c5e0beb51501d328260c7595
	https://git.kernel.org/stable/c/75181406b4eafacc531ff2ee5fb032bd93317e2b