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

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

[Fedor Pchelkin]

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>
---

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)
-- 
2.51.0

[PATCH v2 2/2] ext4: check if mount_opts is NUL-terminated in ext4_ioctl_set_tune_sb()

[Fedor Pchelkin]

params.mount_opts may come as potentially non-NUL-term string.  Userspace
is expected to pass a NUL-term string.  Add an extra check to ensure this
holds true.  Note that further code utilizes strscpy_pad() so this is just
for proper informing the user of incorrect data being provided.

Found by Linux Verification Center (linuxtesting.org).

Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
---

v2: check length of mount_opts in superblock tuning ioctl (Jan Kara)

    Can't plainly return error at strscpy_pad() call site in
    ext4_sb_setparams(), that's a void ext4_update_sb_callback.

v1: https://lore.kernel.org/lkml/20251028130949.599847-1-pchelkin@ispras.ru/T/#u

 fs/ext4/ioctl.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
index a93a7baae990..3dec26c939fd 100644
--- a/fs/ext4/ioctl.c
+++ b/fs/ext4/ioctl.c
@@ -1394,6 +1394,10 @@ static int ext4_ioctl_set_tune_sb(struct file *filp,
 	if (copy_from_user(&params, in, sizeof(params)))
 		return -EFAULT;
 
+	if (strnlen(params.mount_opts, sizeof(params.mount_opts)) ==
+	    sizeof(params.mount_opts))
+		return -E2BIG;
+
 	if ((params.set_flags & ~TUNE_OPS_SUPPORTED) != 0)
 		return -EOPNOTSUPP;
 
-- 
2.51.0

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

[Jan Kara]

On Sat 01-11-25 19:04:28, 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. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
> 
> 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)
> -- 
> 2.51.0
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH v2 2/2] ext4: check if mount_opts is NUL-terminated in ext4_ioctl_set_tune_sb()

[Jan Kara]

On Sat 01-11-25 19:04:29, Fedor Pchelkin wrote:
> params.mount_opts may come as potentially non-NUL-term string.  Userspace
> is expected to pass a NUL-term string.  Add an extra check to ensure this
> holds true.  Note that further code utilizes strscpy_pad() so this is just
> for proper informing the user of incorrect data being provided.
> 
> Found by Linux Verification Center (linuxtesting.org).
> 
> Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
> 
> v2: check length of mount_opts in superblock tuning ioctl (Jan Kara)
> 
>     Can't plainly return error at strscpy_pad() call site in
>     ext4_sb_setparams(), that's a void ext4_update_sb_callback.
> 
> v1: https://lore.kernel.org/lkml/20251028130949.599847-1-pchelkin@ispras.ru/T/#u
> 
>  fs/ext4/ioctl.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
> index a93a7baae990..3dec26c939fd 100644
> --- a/fs/ext4/ioctl.c
> +++ b/fs/ext4/ioctl.c
> @@ -1394,6 +1394,10 @@ static int ext4_ioctl_set_tune_sb(struct file *filp,
>  	if (copy_from_user(&params, in, sizeof(params)))
>  		return -EFAULT;
>  
> +	if (strnlen(params.mount_opts, sizeof(params.mount_opts)) ==
> +	    sizeof(params.mount_opts))
> +		return -E2BIG;
> +
>  	if ((params.set_flags & ~TUNE_OPS_SUPPORTED) != 0)
>  		return -EOPNOTSUPP;
>  
> -- 
> 2.51.0
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

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

[Baokun Li]

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)

Re: [PATCH v2 2/2] ext4: check if mount_opts is NUL-terminated in ext4_ioctl_set_tune_sb()

[Baokun Li]

On 2025-11-02 00:04, Fedor Pchelkin wrote:
> params.mount_opts may come as potentially non-NUL-term string.  Userspace
> is expected to pass a NUL-term string.  Add an extra check to ensure this
> holds true.  Note that further code utilizes strscpy_pad() so this is just
> for proper informing the user of incorrect data being provided.
>
> Found by Linux Verification Center (linuxtesting.org).
>
> Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>

Looks good to me.

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

> ---
>
> v2: check length of mount_opts in superblock tuning ioctl (Jan Kara)
>
>     Can't plainly return error at strscpy_pad() call site in
>     ext4_sb_setparams(), that's a void ext4_update_sb_callback.
>
> v1: https://lore.kernel.org/lkml/20251028130949.599847-1-pchelkin@ispras.ru/T/#u
>
>  fs/ext4/ioctl.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
> index a93a7baae990..3dec26c939fd 100644
> --- a/fs/ext4/ioctl.c
> +++ b/fs/ext4/ioctl.c
> @@ -1394,6 +1394,10 @@ static int ext4_ioctl_set_tune_sb(struct file *filp,
>  	if (copy_from_user(&params, in, sizeof(params)))
>  		return -EFAULT;
>  
> +	if (strnlen(params.mount_opts, sizeof(params.mount_opts)) ==
> +	    sizeof(params.mount_opts))
> +		return -E2BIG;
> +
>  	if ((params.set_flags & ~TUNE_OPS_SUPPORTED) != 0)
>  		return -EOPNOTSUPP;
>  

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

[Theodore Ts'o]

On Sat, 01 Nov 2025 19:04:28 +0300, 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
> 
> [...]

Applied, thanks!

[1/2] ext4: fix string copying in parse_apply_sb_mount_options()
      commit: c517b381de9490b910ab451c2177aa32064678ad
[2/2] ext4: check if mount_opts is NUL-terminated in ext4_ioctl_set_tune_sb()
      commit: 687524bafd4e86cfa783857f8045460f4caee921

Best regards,
-- 
Theodore Ts'o <tytso@mit.edu>