CVE-2025-40237
fs/notify: call exportfs_encode_fid with s_umount
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/
INFO
Published Date :
Dec. 4, 2025, 4:16 p.m.
Last Modified :
Dec. 4, 2025, 5:15 p.m.
Remotely Exploit :
No
Source :
416baaa9-dc9f-4396-8d5f-8c081fb06d67
Solution
- Update the Linux kernel.
- Apply the patch for the race condition.
- Protect exportfs_encode_fid calls.
- Use s_umount lock during unmount.
References to Advisories, Solutions, and Tools
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information, practical solutions, and valuable tools related to
CVE-2025-40237.
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2025-40237 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2025-40237
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
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The following table lists the changes that have been made to the
CVE-2025-40237 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
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New CVE Received by 416baaa9-dc9f-4396-8d5f-8c081fb06d67
Dec. 04, 2025
Action Type Old Value New Value Added 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/ Added Reference https://git.kernel.org/stable/c/3f307a9f7a7a2822e38ac451b73e2244e7279496 Added Reference https://git.kernel.org/stable/c/a7c4bb43bfdc2b9f06ee9d036028ed13a83df42a Added Reference https://git.kernel.org/stable/c/bc1c6b803e14ea2b8f7e33b7164013f666ceb656 Added Reference https://git.kernel.org/stable/c/d1894bc542becb0fda61e7e513b09523cab44030