CVE-2025-40237 fs/notify: call exportfs_encode_fid with s_umount

Not scored Published 4 Dec 2025 Updated 23 May 2026

A race condition in the Linux kernel's inotify subsystem can cause a null pointer dereference and system crash when reading file descriptor info during an overlayfs unmount.

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-2025-40237?

CVE-2025-40237 is a vulnerability in Linux, published on 4 Dec 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:

fs/notify: call exportfs_encode_fid with s_umount

Calling intotify_show_fdinfo() on fd watching an overlayfs inode, while the overlayfs is being unmounted, can lead to dereferencing NULL ptr.

This issue was found by syzkaller.

Race Condition Diagram:

Thread 1 Thread 2 -------- --------

generic_shutdown_super() shrink_dcache_for_umount sb->s_root = NULL

Which then leads to:

[ 32.133461] Oops: general protection fault, probably for non-canonical address 0xdffffc0000000006: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN NOPTI [ 32.134438] KASAN: null-ptr-deref in range [0x0000000000000030-0x0000000000000037] [ 32.135032] CPU: 1 UID: 0 PID: 4468 Comm: systemd-coredum Not tainted 6.17.0-rc6 #22 PREEMPT(none)

[ 32.143353] Call Trace: [ 32.143732] ovl_encode_fh+0xd5/0x170 [ 32.144031] exportfs_encode_inode_fh+0x12f/0x300 [ 32.144425] show_mark_fhandle+0xbe/0x1f0 [ 32.145805] inotify_fdinfo+0x226/0x2d0 [ 32.146442] inotify_show_fdinfo+0x1c5/0x350 [ 32.147168] seq_show+0x530/0x6f0 [ 32.147449] seq_read_iter+0x503/0x12a0 [ 32.148419] seq_read+0x31f/0x410 [ 32.150714] vfs_read+0x1f0/0x9e0 [ 32.152297] ksys_read+0x125/0x240

IOW ovl_check_encode_origin derefs inode->i_sb->s_root, after it was set to NULL in the unmount path.

Fix it by protecting calling exportfs_encode_fid() from show_mark_fhandle() with s_umount lock.

This form of fix was suggested by Amir in [1].

[1]: https://lore.kernel.org/all/CAOQ4uxhbDwhb+2Brs1UdkoF0a3NSdBAOQPNfEHjahrgoKJpLEw@mail.gmail.com/

Which versions are affected by CVE-2025-40237?

How do I fix CVE-2025-40237?

Apply the vendor's patch.

Is CVE-2025-40237 being exploited?

There is no known exploitation of CVE-2025-40237 in the wild and no public exploit code.

Sources

Also known as BELL-CVE-2025-40237, DEBIAN-CVE-2025-40237, GHSA-rg97-jhp7-4449, ROOT-OS-UBUNTU-2204-CVE-2025-40237, ROOT-OS-UBUNTU-2404-CVE-2025-40237, UBUNTU-CVE-2025-40237.