Yikebaer reported an issue:
==================================================================
BUG: KASAN: slab-use-after-free in ext4_es_insert_extent+0xc68/0xcb0
fs/ext4/extents_status.c:894
Read of size 4 at addr ffff888112ecc1a4 by task syz-executor/8438

CPU: 1 PID: 8438 Comm: syz-executor Not tainted 6.5.0-rc5 #1
Call Trace:
 [...]
 kasan_report+0xba/0xf0 mm/kasan/report.c:588
 ext4_es_insert_extent+0xc68/0xcb0 fs/ext4/extents_status.c:894
 ext4_map_blocks+0x92a/0x16f0 fs/ext4/inode.c:680
 ext4_alloc_file_blocks.isra.0+0x2df/0xb70 fs/ext4/extents.c:4462
 ext4_zero_range fs/ext4/extents.c:4622 [inline]
 ext4_fallocate+0x251c/0x3ce0 fs/ext4/extents.c:4721
 [...]

Allocated by task 8438:
 [...]
 kmem_cache_zalloc include/linux/slab.h:693 [inline]
 __es_alloc_extent fs/ext4/extents_status.c:469 [inline]
 ext4_es_insert_extent+0x672/0xcb0 fs/ext4/extents_status.c:873
 ext4_map_blocks+0x92a/0x16f0 fs/ext4/inode.c:680
 ext4_alloc_file_blocks.isra.0+0x2df/0xb70 fs/ext4/extents.c:4462
 ext4_zero_range fs/ext4/extents.c:4622 [inline]
 ext4_fallocate+0x251c/0x3ce0 fs/ext4/extents.c:4721
 [...]

Freed by task 8438:
 [...]
 kmem_cache_free+0xec/0x490 mm/slub.c:3823
 ext4_es_try_to_merge_right fs/ext4/extents_status.c:593 [inline]
 __es_insert_extent+0x9f4/0x1440 fs/ext4/extents_status.c:802
 ext4_es_insert_extent+0x2ca/0xcb0 fs/ext4/extents_status.c:882
 ext4_map_blocks+0x92a/0x16f0 fs/ext4/inode.c:680
 ext4_alloc_file_blocks.isra.0+0x2df/0xb70 fs/ext4/extents.c:4462
 ext4_zero_range fs/ext4/extents.c:4622 [inline]
 ext4_fallocate+0x251c/0x3ce0 fs/ext4/extents.c:4721
 [...]
==================================================================

The flow of issue triggering is as follows:
1. remove es
      raw es               es  removed  es1
|-------------------| -> |----|.......|------|

2. insert es
  es   insert   es1      merge with es  es1     merge with es and free es1
|----|.......|------| -> |------------|------| -> |-------------------|

es merges with newes, then merges with es1, frees es1, then determines
if es1->es_len is 0 and triggers a UAF.

The code flow is as follows:
ext4_es_insert_extent
  es1 = __es_alloc_extent(true);
  es2 = __es_alloc_extent(true);
  __es_remove_extent(inode, lblk, end, NULL, es1)
    __es_insert_extent(inode, &newes, es1) ---> insert es1 to es tree
  __es_insert_extent(inode, &newes, es2)
    ext4_es_try_to_merge_right
      ext4_es_free_extent(inode, es1) --->  es1 is freed
  if (es1 && !es1->es_len)
    // Trigger UAF by determining if es1 is used.

We determine whether es1 or es2 is used immediately after calling
__es_remove_extent() or __es_insert_extent() to avoid triggering a
UAF if es1 or es2 is freed.
Reported-by: Yikebaer Aizezi <yikebaer61@gmail.com>
Closes: https://lore.kernel.org/lkml/CALcu4raD4h9coiyEBL4Bm0zjDwxC2CyPiTwsP3zFuhot6y9Beg@mail.gmail.com
Fixes: 2a69c450 ("ext4: using nofail preallocation in ext4_es_insert_extent()")
Cc: stable@kernel.org
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20230815070808.3377171-1-libaokun1@huawei.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>