[PATCH] dm cache metadata: replace deprecated strncpy with strscpy

[PATCH] dm cache metadata: replace deprecated strncpy with strscpy

[Justin Stitt]

`strncpy` is deprecated for use on NUL-terminated destination strings
[1] and as such we should prefer more robust and less ambiguous string
interfaces.

It seems `cmd->policy_name` is intended to be NUL-terminated based on a
now changed line of code from Commit (c6b4fcbad044e6ff "dm: add cache
target"):
|       if (strcmp(cmd->policy_name, policy_name)) { // ...

However, now a length-bounded strncmp is used:
|       if (strncmp(cmd->policy_name, policy_name, sizeof(cmd->policy_name)))
... which means NUL-terminated may not strictly be required. However, I
believe the intent of the code is clear and we should maintain
NUL-termination of policy_names.

Moreover, __begin_transaction_flags() zero-allocates `cmd` before
calling read_superblock_fields():
|       cmd = kzalloc(sizeof(*cmd), GFP_KERNEL);

Also, `disk_super->policy_name` is zero-initialized
|       memset(disk_super->policy_name, 0, sizeof(disk_super->policy_name));
... therefore any NUL-padding is redundant.

Considering the above, a suitable replacement is `strscpy` [2] due to
the fact that it guarantees NUL-termination on the destination buffer
without unnecessarily NUL-padding.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
Link: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
---
Note: build-tested only.
---
 drivers/md/dm-cache-metadata.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/md/dm-cache-metadata.c b/drivers/md/dm-cache-metadata.c
index acffed750e3e..5a18b80d3666 100644
--- a/drivers/md/dm-cache-metadata.c
+++ b/drivers/md/dm-cache-metadata.c
@@ -597,7 +597,7 @@ static void read_superblock_fields(struct dm_cache_metadata *cmd,
 	cmd->discard_nr_blocks = to_dblock(le64_to_cpu(disk_super->discard_nr_blocks));
 	cmd->data_block_size = le32_to_cpu(disk_super->data_block_size);
 	cmd->cache_blocks = to_cblock(le32_to_cpu(disk_super->cache_blocks));
-	strncpy(cmd->policy_name, disk_super->policy_name, sizeof(cmd->policy_name));
+	strscpy(cmd->policy_name, disk_super->policy_name, sizeof(cmd->policy_name));
 	cmd->policy_version[0] = le32_to_cpu(disk_super->policy_version[0]);
 	cmd->policy_version[1] = le32_to_cpu(disk_super->policy_version[1]);
 	cmd->policy_version[2] = le32_to_cpu(disk_super->policy_version[2]);
@@ -707,7 +707,7 @@ static int __commit_transaction(struct dm_cache_metadata *cmd,
 	disk_super->discard_block_size = cpu_to_le64(cmd->discard_block_size);
 	disk_super->discard_nr_blocks = cpu_to_le64(from_dblock(cmd->discard_nr_blocks));
 	disk_super->cache_blocks = cpu_to_le32(from_cblock(cmd->cache_blocks));
-	strncpy(disk_super->policy_name, cmd->policy_name, sizeof(disk_super->policy_name));
+	strscpy(disk_super->policy_name, cmd->policy_name, sizeof(disk_super->policy_name));
 	disk_super->policy_version[0] = cpu_to_le32(cmd->policy_version[0]);
 	disk_super->policy_version[1] = cpu_to_le32(cmd->policy_version[1]);
 	disk_super->policy_version[2] = cpu_to_le32(cmd->policy_version[2]);
@@ -1726,7 +1726,7 @@ static int write_hints(struct dm_cache_metadata *cmd, struct dm_cache_policy *po
 	    (strlen(policy_name) > sizeof(cmd->policy_name) - 1))
 		return -EINVAL;
 
-	strncpy(cmd->policy_name, policy_name, sizeof(cmd->policy_name));
+	strscpy(cmd->policy_name, policy_name, sizeof(cmd->policy_name));
 	memcpy(cmd->policy_version, policy_version, sizeof(cmd->policy_version));
 
 	hint_size = dm_cache_policy_get_hint_size(policy);

---
base-commit: 6465e260f48790807eef06b583b38ca9789b6072
change-id: 20230925-strncpy-drivers-md-dm-cache-metadata-c-f708096d5d22

Best regards,
--
Justin Stitt <justinstitt@google.com>

Re: [PATCH] dm cache metadata: replace deprecated strncpy with strscpy

[Kees Cook]

On Mon, Sep 25, 2023 at 06:13:12AM +0000, Justin Stitt wrote:
> `strncpy` is deprecated for use on NUL-terminated destination strings
> [1] and as such we should prefer more robust and less ambiguous string
> interfaces.
> 
> It seems `cmd->policy_name` is intended to be NUL-terminated based on a
> now changed line of code from Commit (c6b4fcbad044e6ff "dm: add cache
> target"):
> |       if (strcmp(cmd->policy_name, policy_name)) { // ...
> 
> However, now a length-bounded strncmp is used:
> |       if (strncmp(cmd->policy_name, policy_name, sizeof(cmd->policy_name)))
> ... which means NUL-terminated may not strictly be required. However, I
> believe the intent of the code is clear and we should maintain
> NUL-termination of policy_names.
> 
> Moreover, __begin_transaction_flags() zero-allocates `cmd` before
> calling read_superblock_fields():
> |       cmd = kzalloc(sizeof(*cmd), GFP_KERNEL);
> 
> Also, `disk_super->policy_name` is zero-initialized
> |       memset(disk_super->policy_name, 0, sizeof(disk_super->policy_name));
> ... therefore any NUL-padding is redundant.
> 
> Considering the above, a suitable replacement is `strscpy` [2] due to
> the fact that it guarantees NUL-termination on the destination buffer
> without unnecessarily NUL-padding.
> 
> Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
> Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
> Link: https://github.com/KSPP/linux/issues/90
> Cc: linux-hardening@vger.kernel.org
> Signed-off-by: Justin Stitt <justinstitt@google.com>

Agreed about the %NUL termination and padding assessment.

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

-- 
Kees Cook

Re: [PATCH] dm cache metadata: replace deprecated strncpy with strscpy

[Kees Cook]

On Mon, 25 Sep 2023 06:13:12 +0000, Justin Stitt wrote:
> `strncpy` is deprecated for use on NUL-terminated destination strings
> [1] and as such we should prefer more robust and less ambiguous string
> interfaces.
> 
> It seems `cmd->policy_name` is intended to be NUL-terminated based on a
> now changed line of code from Commit (c6b4fcbad044e6ff "dm: add cache
> target"):
> |       if (strcmp(cmd->policy_name, policy_name)) { // ...
> 
> [...]

Applied to for-next/hardening, thanks!

[1/1] dm cache metadata: replace deprecated strncpy with strscpy
      https://git.kernel.org/kees/c/5d9bc443188f

Take care,

-- 
Kees Cook