linux-ext4 - Re: [PATCH 2/2] ext4: fix string copying in parse_apply_sb_mount

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Message-ID: <20251028162613-b7674fe12860ef66accbc78b-pchelkin@ispras>
Date: Tue, 28 Oct 2025 16:32:19 +0300
From: Fedor Pchelkin <pchelkin@...ras.ru>
To: Theodore Ts'o <tytso@....edu>
Cc: Andreas Dilger <adilger.kernel@...ger.ca>, Jan Kara <jack@...e.cz>, 
	"Darrick J. Wong" <djwong@...nel.org>, linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Kees Cook <kees@...nel.org>, lvc-project@...uxtesting.org, stable@...r.kernel.org
Subject: Re: [PATCH 2/2] ext4: fix string copying in
 parse_apply_sb_mount_options()

On Tue, 28. Oct 16:09, Fedor Pchelkin wrote:
> strscpy_pad() can't be used to copy a possibly non-NUL-term string into a
> NUL-term string.  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 s_es->s_mount_opts might be non-NUL-term, annotate it with
> __nonstring and use the proper memtostr_pad() routine to get its NULL-term
> copy.
> 
> Found by Linux Verification Center (linuxtesting.org) with Syzkaller.
> 
> Fixes: 8ecb790ea8c3 ("ext4: avoid potential buffer over-read in parse_apply_sb_mount_options()")


Though giving it a second glance I wonder what overflow problem does the
commit in Fixes address?

Behavior actually stays the same: before the blamed patch in case of a
non-NUL-term string kstrndup() just allocates a buffer of `size + 1`
(which equals to 65 here) and no overflow is possible, as far as I can
see.

If it's actually true, maybe we'd better just revert 8ecb790ea8c3 ("ext4:
avoid potential buffer over-read in parse_apply_sb_mount_options()") ?


> Cc: stable@...r.kernel.org
> Signed-off-by: Fedor Pchelkin <pchelkin@...ras.ru>
> ---
>  fs/ext4/ext4.h  | 2 +-
>  fs/ext4/super.c | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 57087da6c7be..4c8698316457 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -1429,7 +1429,7 @@ struct ext4_super_block {
>  	__le64	s_last_error_block;	/* block involved of last error */
>  	__u8	s_last_error_func[32] __nonstring;	/* function where the error happened */
>  #define EXT4_S_ERR_END offsetof(struct ext4_super_block, s_mount_opts)
> -	__u8	s_mount_opts[64];
> +	__u8	s_mount_opts[64] __nonstring;
>  	__le32	s_usr_quota_inum;	/* inode for tracking user quota */
>  	__le32	s_grp_quota_inum;	/* inode for tracking group quota */
>  	__le32	s_overhead_clusters;	/* overhead blocks/clusters in fs */
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 33e7c08c9529..57df129873e3 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -2483,7 +2483,7 @@ 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);
> +	memtostr_pad(s_mount_opts, sbi->s_es->s_mount_opts);
>  
>  	fc = kzalloc(sizeof(struct fs_context), GFP_KERNEL);
>  	if (!fc)
> -- 
> 2.51.0
>