Re: [PATCH v2 1/2] ext4: fix string copying in parse_apply_sb_mount_options()

[Baokun Li <libaokun1@huawei.com>]

On 2025-11-02 00:04, Fedor Pchelkin wrote:
> 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.
>
> Fixes: 8ecb790ea8c3 ("ext4: avoid potential buffer over-read in parse_apply_sb_mount_options()")
> Cc: stable@vger.kernel.org
> Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>

Looks good to me.

Reviewed-by: Baokun Li <libaokun1@huawei.com>

> ---
>
> v2: - treat non-NUL-term s_mount_opts as invalid case (Jan Kara)
>     - swap order of patches in series so the fixing-one goes first
>
> v1: https://lore.kernel.org/lkml/20251028130949.599847-1-pchelkin@ispras.ru/T/#u
>
>  fs/ext4/super.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 33e7c08c9529..15bef41f08bd 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -2475,7 +2475,7 @@ static int parse_apply_sb_mount_options(struct super_block *sb,
>  					struct ext4_fs_context *m_ctx)
>  {
>  	struct ext4_sb_info *sbi = EXT4_SB(sb);
> -	char s_mount_opts[65];
> +	char s_mount_opts[64];
>  	struct ext4_fs_context *s_ctx = NULL;
>  	struct fs_context *fc = NULL;
>  	int ret = -ENOMEM;
> @@ -2483,7 +2483,8 @@ static int parse_apply_sb_mount_options(struct super_block *sb,
>  	if (!sbi->s_es->s_mount_opts[0])
>  		return 0;
>  
> -	strscpy_pad(s_mount_opts, sbi->s_es->s_mount_opts);
> +	if (strscpy_pad(s_mount_opts, sbi->s_es->s_mount_opts) < 0)
> +		return -E2BIG;
>  
>  	fc = kzalloc(sizeof(struct fs_context), GFP_KERNEL);
>  	if (!fc)