CVE-2025-71123

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

ext4: fix string copying in parse_apply_sb_mount_options()

strscpy_pad() can't be used to copy a non-NUL-term string into a NUL-term

string of possibly bigger size. Commit 0efc5990bca5 ("string.h: Introduce

memtostr() and memtostr_pad()") provides additional information in that

regard. So if this happens, the following warning is observed:

strnlen: detected buffer overflow: 65 byte read of buffer size 64

WARNING: CPU: 0 PID: 28655 at lib/string_helpers.c:1032 __fortify_report+0x96/0xc0 lib/string_helpers.c:1032

Modules linked in:

CPU: 0 UID: 0 PID: 28655 Comm: syz-executor.3 Not tainted 6.12.54-syzkaller-00144-g5f0270f1ba00 #0

Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014

RIP: 0010:__fortify_report+0x96/0xc0 lib/string_helpers.c:1032

Call Trace:

<TASK>

__fortify_panic+0x1f/0x30 lib/string_helpers.c:1039

strnlen include/linux/fortify-string.h:235 [inline]

sized_strscpy include/linux/fortify-string.h:309 [inline]

parse_apply_sb_mount_options fs/ext4/super.c:2504 [inline]

__ext4_fill_super fs/ext4/super.c:5261 [inline]

ext4_fill_super+0x3c35/0xad00 fs/ext4/super.c:5706

get_tree_bdev_flags+0x387/0x620 fs/super.c:1636

vfs_get_tree+0x93/0x380 fs/super.c:1814

do_new_mount fs/namespace.c:3553 [inline]

path_mount+0x6ae/0x1f70 fs/namespace.c:3880

do_mount fs/namespace.c:3893 [inline]

__do_sys_mount fs/namespace.c:4103 [inline]

__se_sys_mount fs/namespace.c:4080 [inline]

__x64_sys_mount+0x280/0x300 fs/namespace.c:4080

do_syscall_x64 arch/x86/entry/common.c:52 [inline]

do_syscall_64+0x64/0x140 arch/x86/entry/common.c:83

entry_SYSCALL_64_after_hwframe+0x76/0x7e

Since userspace is expected to provide s_mount_opts field to be at most 63

characters long with the ending byte being NUL-term, use a 64-byte buffer

which matches the size of s_mount_opts, so that strscpy_pad() does its job

properly. Return with error if the user still managed to provide a

non-NUL-term string here.

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.