CVE-2025-40138 f2fs: fix to avoid NULL pointer dereference in f2fs_check_quota_consistency()
A local attacker with root privileges can cause a kernel crash via a NULL pointer dereference in the f2fs filesystem during remount operations.
- Exploited in the wild
- Not known
- Login needed
- An administrator
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-40138?
CVE-2025-40138 is a vulnerability in Linux, published on 12 Nov 2025. 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:
f2fs: fix to avoid NULL pointer dereference in f2fs_check_quota_consistency()
syzbot reported a f2fs bug as below:
Oops: gen[ 107.736417][ T5848] Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] SMP KASAN PTI KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007] CPU: 1 UID: 0 PID: 5848 Comm: syz-executor263 Tainted: G W 6.17.0-rc1-syzkaller-00014-g0e39a731820a #0 PREEMPT_{RT,(full)} RIP: 0010:strcmp+0x3c/0xc0 lib/string.c:284 Call Trace: <TASK> f2fs_check_quota_consistency fs/f2fs/super.c:1188 [inline] f2fs_check_opt_consistency+0x1378/0x2c10 fs/f2fs/super.c:1436 __f2fs_remount fs/f2fs/super.c:2653 [inline] f2fs_reconfigure+0x482/0x1770 fs/f2fs/super.c:5297 reconfigure_super+0x224/0x890 fs/super.c:1077 do_remount fs/namespace.c:3314 [inline] path_mount+0xd18/0xfe0 fs/namespace.c:4112 do_mount fs/namespace.c:4133 [inline] __do_sys_mount fs/namespace.c:4344 [inline] __se_sys_mount+0x317/0x410 fs/namespace.c:4321 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline] do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94 entry_SYSCALL_64_after_hwframe+0x77/0x7f
The direct reason is f2fs_check_quota_consistency() may suffer null-ptr-deref issue in strcmp().
The bug can be reproduced w/ below scripts: mkfs.f2fs -f /dev/vdb mount -t f2fs -o usrquota /dev/vdb /mnt/f2fs quotacheck -uc /mnt/f2fs/ umount /mnt/f2fs mount -t f2fs -o usrjquota=aquota.user,jqfmt=vfsold /dev/vdb /mnt/f2fs mount -t f2fs -o remount,usrjquota=,jqfmt=vfsold /dev/vdb /mnt/f2fs umount /mnt/f2fs
So, before old_qname and new_qname comparison, we need to check whether they are all valid pointers, fix it.
Which versions are affected by CVE-2025-40138?
How do I fix CVE-2025-40138?
Apply the vendor's patch.
Is CVE-2025-40138 being exploited?
There is no known exploitation of CVE-2025-40138 in the wild and no public exploit code.
Sources
Also known as BELL-CVE-2025-40138, GHSA-p3hr-r37g-j63r, UBUNTU-CVE-2025-40138.