CVE-2024-39276 ext4: fix mb_cache_entry's e_refcnt leak in ext4_xattr_block_cache_find()
A reference count leak in the Linux kernel ext4 filesystem can cause a warning and potential denial of service when memory allocation fails during extended attribute block cache lookups.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2024-39276?
CVE-2024-39276 is a medium-severity denial of service vulnerability in Linux kernel, published on 25 Jun 2024. It can only be exploited from the same machine or local network, with no action from the victim.
Official 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.
Which versions are affected by CVE-2024-39276?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2024-39276?
Apply the vendor's patch.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2024-39276 being exploited?
There is no known exploitation of CVE-2024-39276 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as BELL-CVE-2024-39276, DEBIAN-CVE-2024-39276, GHSA-hf2g-j6wm-qrrx, UBUNTU-CVE-2024-39276.