Subject: CVE-2024-26905: btrfs: fix data races when accessing the reserved amount of block reserves

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

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

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

btrfs: fix data races when accessing the reserved amount of block reserves

At space_info.c we have several places where we access the ->reserved
field of a block reserve without taking the block reserve's spinlock
first, which makes KCSAN warn about a data race since that field is
always updated while holding the spinlock.

The reports from KCSAN are like the following:

  [117.193526] BUG: KCSAN: data-race in btrfs_block_rsv_release [btrfs] / need_preemptive_reclaim [btrfs]

  [117.195148] read to 0x000000017f587190 of 8 bytes by task 6303 on cpu 3:
  [117.195172]  need_preemptive_reclaim+0x222/0x2f0 [btrfs]
  [117.195992]  __reserve_bytes+0xbb0/0xdc8 [btrfs]
  [117.196807]  btrfs_reserve_metadata_bytes+0x4c/0x120 [btrfs]
  [117.197620]  btrfs_block_rsv_add+0x78/0xa8 [btrfs]
  [117.198434]  btrfs_delayed_update_inode+0x154/0x368 [btrfs]
  [117.199300]  btrfs_update_inode+0x108/0x1c8 [btrfs]
  [117.200122]  btrfs_dirty_inode+0xb4/0x140 [btrfs]
  [117.200937]  btrfs_update_time+0x8c/0xb0 [btrfs]
  [117.201754]  touch_atime+0x16c/0x1e0
  [117.201789]  filemap_read+0x674/0x728
  [117.201823]  btrfs_file_read_iter+0xf8/0x410 [btrfs]
  [117.202653]  vfs_read+0x2b6/0x498
  [117.203454]  ksys_read+0xa2/0x150
  [117.203473]  __s390x_sys_read+0x68/0x88
  [117.203495]  do_syscall+0x1c6/0x210
  [117.203517]  __do_syscall+0xc8/0xf0
  [117.203539]  system_call+0x70/0x98

  [117.203579] write to 0x000000017f587190 of 8 bytes by task 11 on cpu 0:
  [117.203604]  btrfs_block_rsv_release+0x2e8/0x578 [btrfs]
  [117.204432]  btrfs_delayed_inode_release_metadata+0x7c/0x1d0 [btrfs]
  [117.205259]  __btrfs_update_delayed_inode+0x37c/0x5e0 [btrfs]
  [117.206093]  btrfs_async_run_delayed_root+0x356/0x498 [btrfs]
  [117.206917]  btrfs_work_helper+0x160/0x7a0 [btrfs]
  [117.207738]  process_one_work+0x3b6/0x838
  [117.207768]  worker_thread+0x75e/0xb10
  [117.207797]  kthread+0x21a/0x230
  [117.207830]  __ret_from_fork+0x6c/0xb8
  [117.207861]  ret_from_fork+0xa/0x30

So add a helper to get the reserved amount of a block reserve while
holding the lock. The value may be not be up to date anymore when used by
need_preemptive_reclaim() and btrfs_preempt_reclaim_metadata_space(), but
that's ok since the worst it can do is cause more reclaim work do be done
sooner rather than later. Reading the field while holding the lock instead
of using the data_race() annotation is used in order to prevent load
tearing.

The Linux kernel CVE team has assigned CVE-2024-26905 to this issue.

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

	Fixed in 6.1.83 with commit 995e91c9556c
	Fixed in 6.6.23 with commit 82220b1835ba
	Fixed in 6.7.11 with commit c44542093525
	Fixed in 6.8 with commit e06cc89475ed

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-2024-26905
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.h
	fs/btrfs/space-info.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/995e91c9556c8fc6028b474145a36e947d1eb6b6
	https://git.kernel.org/stable/c/82220b1835baaebf4ae2e490f56353a341a09bd2
	https://git.kernel.org/stable/c/c44542093525699a30c307dae1ea5a1b03b3302f
	https://git.kernel.org/stable/c/e06cc89475eddc1f3a7a4d471524256152c68166