linux-ext4 - [PATCH 1/2] ext4: fix up copying of mount

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Message-ID: <20251028130949.599847-1-pchelkin@ispras.ru>
Date: Tue, 28 Oct 2025 16:09:47 +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
Subject: [PATCH 1/2] ext4: fix up copying of mount_opts in superblock tuning ioctls

Judging by commit 8ecb790ea8c3 ("ext4: avoid potential buffer over-read in
parse_apply_sb_mount_options()"), the contents of s_mount_opts should be
treated as __nonstring, i.e. there might be no NUL-terminator in the
provided buffer.

Then the same holds for the corresponding mount_opts field of the struct
ext4_tune_sb_params exchanged with userspace via a recently implemented
superblock tuning ioctl.

The problem is that strscpy_pad() can't work properly with non-NUL-term
strings.  String fortifying infrastructure would complain if that happens.
Commit 0efc5990bca5 ("string.h: Introduce memtostr() and memtostr_pad()")
gives additional information in that regard.

Both buffers are just raw arrays of the similar fixed size, essentially
they should represent the same contents.  As they don't necessarily have
NUL-terminators, in both directions use plain memcpy() to copy their
contents.

Found by Linux Verification Center (linuxtesting.org).

Fixes: 04a91570ac67 ("ext4: implemet new ioctls to set and get superblock parameters")
Signed-off-by: Fedor Pchelkin <pchelkin@...ras.ru>
---

The 1/2 patch of the current series fixes an issue existing only in 6.18-rc
while 2/2 fixes the commit which was in turn backported to stable kernels.
That's the reasoning for separation.

 fs/ext4/ioctl.c           | 4 ++--
 include/uapi/linux/ext4.h | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
index a93a7baae990..c39b87d52cb0 100644
--- a/fs/ext4/ioctl.c
+++ b/fs/ext4/ioctl.c
@@ -1292,7 +1292,7 @@ static int ext4_ioctl_get_tune_sb(struct ext4_sb_info *sbi,
 	ret.raid_stripe_width = le32_to_cpu(es->s_raid_stripe_width);
 	ret.encoding = le16_to_cpu(es->s_encoding);
 	ret.encoding_flags = le16_to_cpu(es->s_encoding_flags);
-	strscpy_pad(ret.mount_opts, es->s_mount_opts);
+	memcpy(ret.mount_opts, es->s_mount_opts, sizeof(ret.mount_opts));
 	ret.feature_compat = le32_to_cpu(es->s_feature_compat);
 	ret.feature_incompat = le32_to_cpu(es->s_feature_incompat);
 	ret.feature_ro_compat = le32_to_cpu(es->s_feature_ro_compat);
@@ -1353,7 +1353,7 @@ static void ext4_sb_setparams(struct ext4_sb_info *sbi,
 		es->s_encoding = cpu_to_le16(params->encoding);
 	if (params->set_flags & EXT4_TUNE_FL_ENCODING_FLAGS)
 		es->s_encoding_flags = cpu_to_le16(params->encoding_flags);
-	strscpy_pad(es->s_mount_opts, params->mount_opts);
+	memcpy(es->s_mount_opts, params->mount_opts, sizeof(es->s_mount_opts));
 	if (params->set_flags & EXT4_TUNE_FL_EDIT_FEATURES) {
 		es->s_feature_compat |=
 			cpu_to_le32(params->set_feature_compat_mask);
diff --git a/include/uapi/linux/ext4.h b/include/uapi/linux/ext4.h
index 411dcc1e4a35..8ed9acbd0e03 100644
--- a/include/uapi/linux/ext4.h
+++ b/include/uapi/linux/ext4.h
@@ -138,7 +138,7 @@ struct ext4_tune_sb_params {
 	__u32 clear_feature_compat_mask;
 	__u32 clear_feature_incompat_mask;
 	__u32 clear_feature_ro_compat_mask;
-	__u8  mount_opts[64];
+	__u8  mount_opts[64] __nonstring;
 	__u8  pad[64];
 };

-- 
2.51.0