linux-cve-announce - CVE-2025-40119: ext4: fix potential null deref in ext4_mb

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Message-ID: <2025111252-CVE-2025-40119-41e0@gregkh>
Date: Wed, 12 Nov 2025 19:23:55 +0900
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2025-40119: ext4: fix potential null deref in ext4_mb_init()

From: Greg Kroah-Hartman <gregkh@...nel.org>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

ext4: fix potential null deref in ext4_mb_init()

In ext4_mb_init(), ext4_mb_avg_fragment_size_destroy() may be called
when sbi->s_mb_avg_fragment_size remains uninitialized (e.g., if groupinfo
slab cache allocation fails). Since ext4_mb_avg_fragment_size_destroy()
lacks null pointer checking, this leads to a null pointer dereference.

==================================================================
EXT4-fs: no memory for groupinfo slab cache
BUG: kernel NULL pointer dereference, address: 0000000000000000
PGD 0 P4D 0
Oops: Oops: 0002 [#1] SMP PTI
CPU:2 UID: 0 PID: 87 Comm:mount Not tainted 6.17.0-rc2 #1134 PREEMPT(none)
RIP: 0010:_raw_spin_lock_irqsave+0x1b/0x40
Call Trace:
 <TASK>
 xa_destroy+0x61/0x130
 ext4_mb_init+0x483/0x540
 __ext4_fill_super+0x116d/0x17b0
 ext4_fill_super+0xd3/0x280
 get_tree_bdev_flags+0x132/0x1d0
 vfs_get_tree+0x29/0xd0
 do_new_mount+0x197/0x300
 __x64_sys_mount+0x116/0x150
 do_syscall_64+0x50/0x1c0
 entry_SYSCALL_64_after_hwframe+0x76/0x7e
==================================================================

Therefore, add necessary null check to ext4_mb_avg_fragment_size_destroy()
to prevent this issue. The same fix is also applied to
ext4_mb_largest_free_orders_destroy().

The Linux kernel CVE team has assigned CVE-2025-40119 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 6.17 with commit f7eaacbb4e54f8a6c6674c16eff54f703ea63d5e and fixed in 6.17.3 with commit 00110f3cfc9b34b2dfee2a6c9e55a0ae6df125ae
	Issue introduced in 6.17 with commit f7eaacbb4e54f8a6c6674c16eff54f703ea63d5e and fixed in 6.18-rc1 with commit 3c3fac6bc0a9c00dbe65d8dc0d3a282afe4d3188

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2025-40119
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	fs/ext4/mballoc.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/00110f3cfc9b34b2dfee2a6c9e55a0ae6df125ae
	https://git.kernel.org/stable/c/3c3fac6bc0a9c00dbe65d8dc0d3a282afe4d3188