CVE-2025-40237
Unknown Unknown - Not Provided
BaseFortify

Publication date: 2025-12-04

Last updated on: 2025-12-04

Assigner: kernel.org

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 | | vfs_read() | inotify_fdinfo() | * inode get from mark * | show_mark_fhandle(m, inode) | exportfs_encode_fid(inode, ..) | ovl_encode_fh(inode, ..) | ovl_check_encode_origin(inode) | * deref i_sb->s_root * | | v fsnotify_sb_delete(sb) 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) <snip registers, unreliable trace> [ 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/
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Meta Information
Published
2025-12-04
Last Modified
2025-12-04
Generated
2026-05-06
AI Q&A
2025-12-04
EPSS Evaluated
2026-05-05
NVD
CVE-2025-40237
Affected Vendors & Products
Showing 1 associated CPE
Vendor Product Version / Range
linux linux_kernel 6.17.0-rc6
Helpful Resources
Exploitability
CWE
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KEV
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CWE ID Description
CWE-UNKNOWN
Attack-Flow Graph
AI Powered Q&A
Can you explain this vulnerability to me?

This vulnerability occurs in the Linux kernel's filesystem notification system (fs/notify) when calling intotify_show_fdinfo() on a file descriptor watching an overlayfs inode while the overlayfs is being unmounted. During this process, a race condition can cause a NULL pointer dereference because the inode's superblock root (i_sb->s_root) is set to NULL during unmount, but another thread may still try to access it. This leads to a general protection fault and kernel crash. The issue is fixed by protecting the call to exportfs_encode_fid() with a lock to prevent accessing the inode after it has been unmounted.


How can this vulnerability impact me? :

This vulnerability can cause a kernel crash (general protection fault) due to a NULL pointer dereference, potentially leading to system instability or denial of service. If an attacker can trigger this race condition, they might cause the system to become unresponsive or crash unexpectedly.


What immediate steps should I take to mitigate this vulnerability?

The vulnerability is fixed by protecting the call to exportfs_encode_fid() from show_mark_fhandle() with the s_umount lock to prevent dereferencing a NULL pointer during overlayfs unmount. Immediate mitigation steps include updating the Linux kernel to a version that includes this fix.


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