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

ext4: fix mb_cache_entry's e_refcnt leak in ext4_xattr_block_cache_find()

Syzbot reports a warning as follows:

============================================
WARNING: CPU: 0 PID: 5075 at fs/mbcache.c:419 mb_cache_destroy+0x224/0x290
Modules linked in:
CPU: 0 PID: 5075 Comm: syz-executor199 Not tainted 6.9.0-rc6-gb947cc5bf6d7
RIP: 0010:mb_cache_destroy+0x224/0x290 fs/mbcache.c:419
Call Trace:
 <TASK>
 ext4_put_super+0x6d4/0xcd0 fs/ext4/super.c:1375
 generic_shutdown_super+0x136/0x2d0 fs/super.c:641
 kill_block_super+0x44/0x90 fs/super.c:1675
 ext4_kill_sb+0x68/0xa0 fs/ext4/super.c:7327
[...]
============================================

This is because when finding an entry in ext4_xattr_block_cache_find(), if
ext4_sb_bread() returns -ENOMEM, the ce's e_refcnt, which has already grown
in the __entry_find(), won't be put away, and eventually trigger the above
issue in mb_cache_destroy() due to reference count leakage.

So call mb_cache_entry_put() on the -ENOMEM error branch as a quick fix.

kernel.org https://git.kernel.org/stable/c/0c0b4a49d3e7f49690a6827a41faeffad5df7e21 [No types assigned]

kernel.org https://git.kernel.org/stable/c/681ff9a09accd8a4379f8bd30b7a1641ee19bb3e [No types assigned]

kernel.org https://git.kernel.org/stable/c/76dc776153a47372719d664e0fc50d6355791abb [No types assigned]

kernel.org https://git.kernel.org/stable/c/896a7e7d0d555ad8b2b46af0c2fa7de7467f9483 [No types assigned]

kernel.org https://git.kernel.org/stable/c/9ad75e78747b5a50dc5a52f0f8e92e920a653f16 [No types assigned]

kernel.org https://git.kernel.org/stable/c/a95df6f04f2c37291adf26a74205cde0314d4577 [No types assigned]

kernel.org https://git.kernel.org/stable/c/b37c0edef4e66fb21a2fbc211471195a383e5ab8 [No types assigned]

kernel.org https://git.kernel.org/stable/c/e941b712e758f615d311946bf98216e79145ccd9 [No types assigned]