Subject: CVE-2024-35956: btrfs: qgroup: fix qgroup prealloc rsv leak in subvolume operations

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

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

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

btrfs: qgroup: fix qgroup prealloc rsv leak in subvolume operations

Create subvolume, create snapshot and delete subvolume all use
btrfs_subvolume_reserve_metadata() to reserve metadata for the changes
done to the parent subvolume's fs tree, which cannot be mediated in the
normal way via start_transaction. When quota groups (squota or qgroups)
are enabled, this reserves qgroup metadata of type PREALLOC. Once the
operation is associated to a transaction, we convert PREALLOC to
PERTRANS, which gets cleared in bulk at the end of the transaction.

However, the error paths of these three operations were not implementing
this lifecycle correctly. They unconditionally converted the PREALLOC to
PERTRANS in a generic cleanup step regardless of errors or whether the
operation was fully associated to a transaction or not. This resulted in
error paths occasionally converting this rsv to PERTRANS without calling
record_root_in_trans successfully, which meant that unless that root got
recorded in the transaction by some other thread, the end of the
transaction would not free that root's PERTRANS, leaking it. Ultimately,
this resulted in hitting a WARN in CONFIG_BTRFS_DEBUG builds at unmount
for the leaked reservation.

The fix is to ensure that every qgroup PREALLOC reservation observes the
following properties:

1. any failure before record_root_in_trans is called successfully
   results in freeing the PREALLOC reservation.
2. after record_root_in_trans, we convert to PERTRANS, and now the
   transaction owns freeing the reservation.

This patch enforces those properties on the three operations. Without
it, generic/269 with squotas enabled at mkfs time would fail in ~5-10
runs on my system. With this patch, it ran successfully 1000 times in a
row.

The Linux kernel CVE team has assigned CVE-2024-35956 to this issue.

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

	Issue introduced in 5.10 with commit e85fde5162bf and fixed in 6.6.28 with commit 14431815a4ae
	Issue introduced in 5.10 with commit e85fde5162bf and fixed in 6.8.7 with commit 6c95336f5d8e
	Issue introduced in 5.10 with commit e85fde5162bf and fixed in 6.9 with commit 74e97958121a
	Issue introduced in 5.9.5 with commit 2978cb474745

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-2024-35956
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/inode.c
	fs/btrfs/ioctl.c
	fs/btrfs/root-tree.c
	fs/btrfs/root-tree.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/14431815a4ae4bcd7c7a68b6a64c66c7712d27c9
	https://git.kernel.org/stable/c/6c95336f5d8eb9ab79cd7306d71b6d0477363f8c
	https://git.kernel.org/stable/c/74e97958121aa1f5854da6effba70143f051b0cd