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

Vulnerability Description

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.

References

• https://git.kernel.org/stable/c/9ad75e78747b5a50dc5a52f0f8e92e920a653f16
• https://git.kernel.org/stable/c/896a7e7d0d555ad8b2b46af0c2fa7de7467f9483
• https://git.kernel.org/stable/c/76dc776153a47372719d664e0fc50d6355791abb
• https://git.kernel.org/stable/c/681ff9a09accd8a4379f8bd30b7a1641ee19bb3e
• https://git.kernel.org/stable/c/e941b712e758f615d311946bf98216e79145ccd9
• https://git.kernel.org/stable/c/a95df6f04f2c37291adf26a74205cde0314d4577
• https://git.kernel.org/stable/c/b37c0edef4e66fb21a2fbc211471195a383e5ab8
• https://git.kernel.org/stable/c/0c0b4a49d3e7f49690a6827a41faeffad5df7e21