CVE-2025-40138: f2fs: fix to avoid NULL pointer dereference in f2fs_check_quota_consistency()

CVE-2025-40138: f2fs: fix to avoid NULL pointer dereference in f2fs_check_quota_consistency()

[Greg Kroah-Hartman]

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

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.

The Linux kernel CVE team has assigned CVE-2025-40138 to this issue.


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

	Issue introduced in 6.17 with commit d185351325237da688de006a2c579e82ea97bdfe and fixed in 6.17.3 with commit 3f3458852bbfe79c60f2412b8b04677b96688b6e
	Issue introduced in 6.17 with commit d185351325237da688de006a2c579e82ea97bdfe and fixed in 6.18-rc1 with commit 930a9a6ee8e7ffa20af4bffbfc2bbd21d83bf81c

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-2025-40138
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/f2fs/super.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/3f3458852bbfe79c60f2412b8b04677b96688b6e
	https://git.kernel.org/stable/c/930a9a6ee8e7ffa20af4bffbfc2bbd21d83bf81c