CVE-2024-57975

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

btrfs: do proper folio cleanup when run_delalloc_nocow() failed

[BUG]

With CONFIG_DEBUG_VM set, test case generic/476 has some chance to crash

with the following VM_BUG_ON_FOLIO():

BTRFS error (device dm-3): cow_file_range failed, start 1146880 end 1253375 len 106496 ret -28

BTRFS error (device dm-3): run_delalloc_nocow failed, start 1146880 end 1253375 len 106496 ret -28

page: refcount:4 mapcount:0 mapping:00000000592787cc index:0x12 pfn:0x10664

aops:btrfs_aops [btrfs] ino:101 dentry name(?):"f1774"

flags: 0x2fffff80004028(uptodate|lru|private|node=0|zone=2|lastcpupid=0xfffff)

page dumped because: VM_BUG_ON_FOLIO(!folio_test_locked(folio))

------------[ cut here ]------------

kernel BUG at mm/page-writeback.c:2992!

Internal error: Oops - BUG: 00000000f2000800 [#1] SMP

CPU: 2 UID: 0 PID: 3943513 Comm: kworker/u24:15 Tainted: G OE 6.12.0-rc7-custom+ #87

Tainted: [O]=OOT_MODULE, [E]=UNSIGNED_MODULE

Hardware name: QEMU KVM Virtual Machine, BIOS unknown 2/2/2022

Workqueue: events_unbound btrfs_async_reclaim_data_space [btrfs]

pc : folio_clear_dirty_for_io+0x128/0x258

lr : folio_clear_dirty_for_io+0x128/0x258

Call trace:

folio_clear_dirty_for_io+0x128/0x258

btrfs_folio_clamp_clear_dirty+0x80/0xd0 [btrfs]

__process_folios_contig+0x154/0x268 [btrfs]

extent_clear_unlock_delalloc+0x5c/0x80 [btrfs]

run_delalloc_nocow+0x5f8/0x760 [btrfs]

btrfs_run_delalloc_range+0xa8/0x220 [btrfs]

writepage_delalloc+0x230/0x4c8 [btrfs]

extent_writepage+0xb8/0x358 [btrfs]

extent_write_cache_pages+0x21c/0x4e8 [btrfs]

btrfs_writepages+0x94/0x150 [btrfs]

do_writepages+0x74/0x190

filemap_fdatawrite_wbc+0x88/0xc8

start_delalloc_inodes+0x178/0x3a8 [btrfs]

btrfs_start_delalloc_roots+0x174/0x280 [btrfs]

shrink_delalloc+0x114/0x280 [btrfs]

flush_space+0x250/0x2f8 [btrfs]

btrfs_async_reclaim_data_space+0x180/0x228 [btrfs]

process_one_work+0x164/0x408

worker_thread+0x25c/0x388

kthread+0x100/0x118

ret_from_fork+0x10/0x20

Code: 910a8021 a90363f7 a9046bf9 94012379 (d4210000)

---[ end trace 0000000000000000 ]---

[CAUSE]

The first two lines of extra debug messages show the problem is caused

by the error handling of run_delalloc_nocow().

E.g. we have the following dirtied range (4K blocksize 4K page size):

0 16K 32K

|//////////////////////////////////////|

| Pre-allocated |

And the range [0, 16K) has a preallocated extent.

- Enter run_delalloc_nocow() for range [0, 16K)

Which found range [0, 16K) is preallocated, can do the proper NOCOW

write.

- Enter fallback_to_fow() for range [16K, 32K)

Since the range [16K, 32K) is not backed by preallocated extent, we

have to go COW.

- cow_file_range() failed for range [16K, 32K)

So cow_file_range() will do the clean up by clearing folio dirty,

unlock the folios.

Now the folios in range [16K, 32K) is unlocked.

- Enter extent_clear_unlock_delalloc() from run_delalloc_nocow()

Which is called with PAGE_START_WRITEBACK to start page writeback.

But folios can only be marked writeback when it's properly locked,

thus this triggered the VM_BUG_ON_FOLIO().

Furthermore there is another hidden but common bug that

run_delalloc_nocow() is not clearing the folio dirty flags in its error

handling path.

This is the common bug shared between run_delalloc_nocow() and

cow_file_range().

[FIX]

- Clear folio dirty for range [@start, @cur_offset)

Introduce a helper, cleanup_dirty_folios(), which

will find and lock the folio in the range, clear the dirty flag and

start/end the writeback, with the extra handling for the

@locked_folio.

- Introduce a helper to clear folio dirty, start and end writeback

- Introduce a helper to record the last failed COW range end

This is to trace which range we should skip, to avoid double

unlocking.

- Skip the failed COW range for the e

---truncated---