CVE: CVE-2024-42103
Published: 2024-07-30
Description: In the Linux kernel, the following vulnerability has been resolved:

btrfs: fix adding block group to a reclaim list and the unused list during reclaim

There is a potential parallel list adding for retrying in
btrfs_reclaim_bgs_work and adding to the unused list. Since the block
group is removed from the reclaim list and it is on a relocation work,
it can be added into the unused list in parallel. When that happens,
adding it to the reclaim list will corrupt the list head and trigger
list corruption like below.

Fix it by taking fs_info->unused_bgs_lock.

  [177.504][T2585409] BTRFS error (device nullb1): error relocating ch= unk[PII_REDACTED]
  [177.514][T2585409] list_del corruption. next->prev should be ff1100= 0344b119c0, but was ff11000377e87c70. (next=3Dff110002390cd9c0)
  [177.529][T2585409] ------------[ cut here ]------------
  [177.537][T2585409] kernel BUG at lib/list_debug.c:65!
  [177.545][T2585409] Oops: invalid opcode: 0000 [#1] PREEMPT SMP KASAN NOPTI
  [177.555][T2585409] CPU: 9 PID: 2585409 Comm: kworker/u128:2 Tainted: G        W          6.10.0-rc5-kts #1
  [177.568][T2585409] Hardware name: Supermicro SYS-520P-WTR/X12SPW-TF, BIOS 1.2 02/14/2022
  [177.579][T2585409] Workqueue: events_unbound btrfs_reclaim_bgs_work[btrfs]
  [177.589][T2585409] RIP: 0010:__list_del_entry_valid_or_report.cold+0x70/0x72
  [177.624][T2585409] RSP: 0018:ff11000377e87a70 EFLAGS: 00010286
  [177.633][T2585409] RAX:[PII_REDACTED]00006d RBX: ff11000344b119c0 RCX:[PII_REDACTED]
  [177.644][T2585409] RDX:[PII_REDACTED]00006d RSI: [PII_REDACTED] RDI:ffe21c006efd0f40
  [177.655][T2585409] RBP: ff110002e0509f78 R08: [PII_REDACTED] R09:ffe21c006efd0f08
  [177.665][T2585409] R10: ff11000377e87847 R11: [PII_REDACTED] R12:ff110002390cd9c0
  [177.676][T2585409] R13: ff11000344b119c0 R14: ff110002e0508000 R15:dffffc[PII_REDACTED]
  [177.687][T2585409] FS:  [PII_REDACTED](0000) GS:ff11000fec880000(0000) knlGS:[PII_REDACTED]
  [177.700][T2585409] CS:  0010 DS: 0000 ES: 0000 CR0: [PII_REDACTED]
  [177.709][T2585409] CR2: 00007f06bc7b1978 CR3:[PII_REDACTED]e86005 CR4:[PII_REDACTED]771ef0
  [177.720][T2585409] DR0: [PII_REDACTED] DR1: [PII_REDACTED] DR2:[PII_REDACTED]
  [177.731][T2585409] DR3: [PII_REDACTED] DR6: 00000000fffe0ff0 DR7:[PII_REDACTED]
  [177.742][T2585409] PKRU: 55555554
  [177.748][T2585409] Call Trace:
  [177.753][T2585409]  <TASK>
  [177.759][T2585409]  ? __die_body.cold+0x19/0x27
  [177.766][T2585409]  ? die+0x2e/0x50
  [177.772][T2585409]  ? do_trap+0x1ea/0x2d0
  [177.779][T2585409]  ? __list_del_entry_valid_or_report.cold+0x70/0x72
  [177.788][T2585409]  ? do_error_trap+0xa3/0x160
  [177.795][T2585409]  ? __list_del_entry_valid_or_report.cold+0x70/0x72
  [177.805][T2585409]  ? handle_invalid_op+0x2c/0x40
  [177.812][T2585409]  ? __list_del_entry_valid_or_report.cold+0x70/0x72
  [177.820][T2585409]  ? exc_invalid_op+0x2d/0x40
  [177.827][T2585409]  ? asm_exc_invalid_op+0x1a/0x20
  [177.834][T2585409]  ? __list_del_entry_valid_or_report.cold+0x70/0x72
  [177.843][T2585409]  btrfs_delete_unused_bgs+0x3d9/0x14c0 [btrfs]

There is a similar retry_list code in btrfs_delete_unused_bgs(), but it is
safe, AFAICS. Since the block group was in the unused list, the used bytes
should be 0 when it was added to the unused list. Then, it checks
block_group->{used,reserved,pinned} are still 0 under the
block_group->lock. So, they should be still eligible for the unused list,
not the reclaim list.

The reason it is safe there it's because because we're holding
space_info->groups_sem in write mode.

That means no other task can allocate from the block group, so while we
are at deleted_unused_bgs() it's not possible for other tasks to
allocate and deallocate extents from the block group, so it can't be
added to the unused list or the reclaim list by anyone else.

The bug can be reproduced by btrfs/166 after a few rounds. In practice
this can be hit when relocation cannot find more chunk space and ends
with ENOSPC.
CVSS Score: 5.5 (MEDIUM)
References: https://git.kernel.org/stable/c/326fa14549d7969ef80d3f5beea5470cd1c8e67f; https://git.kernel.org/stable/c/48f091fd50b2eb33ae5eaea9ed3c4f81603acf38; https://git.kernel.org/stable/c/522b39bd7163e8dc49f8cf10b9b782218ac48746