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

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Message-ID: <20251028130949.599847-2-pchelkin@ispras.ru>
Date: Tue, 28 Oct 2025 16:09:48 +0300
From: Fedor Pchelkin <pchelkin@...ras.ru>
To: "Theodore Ts'o" <tytso@....edu>
Cc: Fedor Pchelkin <pchelkin@...ras.ru>,
	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: [PATCH 2/2] ext4: fix string copying in parse_apply_sb_mount_options()

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()")
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