Re: [PATCH] fbdev: uvesafb: replace deprecated strncpy with strscpy_pad

[Kees Cook]

On Wed, Mar 20, 2024 at 11:04:44PM +, 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.
> 
> We expect v86d_path to be NUL-terminated based on its use with the
> C-string format specifier in printf-likes:
> | pr_err("failed to execute %s\n", v86d_path);
> and
> | return snprintf(buf, PAGE_SIZE, "%s\n", v86d_path);
> 
> Let's also opt to pad v86d_path since it may get used in and around
> userspace:
> | return call_usermodehelper(v86d_path, argv, envp, UMH_WAIT_PROC);
> 
> Considering the above, strscpy_pad() is the best replacement as it
> guarantees both NUL-termination and NUL-padding on the destination
> buffer.
> 
> Note that this patch relies on the _new_ 2-argument versions of
> strscpy() and strscpy_pad() introduced in Commit e6584c3964f2f ("string:
> Allow 2-argument strscpy()").
> 
> 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
> Link: https://github.com/KSPP/linux/issues/90
> Cc: linux-harden...@vger.kernel.org
> Signed-off-by: Justin Stitt 

Reviewed-by: Kees Cook 

-- 
Kees Cook

[PATCH] fbdev: uvesafb: replace deprecated strncpy with strscpy_pad

[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.

We expect v86d_path to be NUL-terminated based on its use with the
C-string format specifier in printf-likes:
|   pr_err("failed to execute %s\n", v86d_path);
and
|   return snprintf(buf, PAGE_SIZE, "%s\n", v86d_path);

Let's also opt to pad v86d_path since it may get used in and around
userspace:
|   return call_usermodehelper(v86d_path, argv, envp, UMH_WAIT_PROC);

Considering the above, strscpy_pad() is the best replacement as it
guarantees both NUL-termination and NUL-padding on the destination
buffer.

Note that this patch relies on the _new_ 2-argument versions of
strscpy() and strscpy_pad() introduced in Commit e6584c3964f2f ("string:
Allow 2-argument strscpy()").

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
Link: https://github.com/KSPP/linux/issues/90
Cc: linux-harden...@vger.kernel.org
Signed-off-by: Justin Stitt 
---
Note: build-tested only.

Found with: $ rg "strncpy\("
---
 drivers/video/fbdev/uvesafb.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/video/fbdev/uvesafb.c b/drivers/video/fbdev/uvesafb.c
index e1f421e91b4f..8b554fcf1bfb 100644
--- a/drivers/video/fbdev/uvesafb.c
+++ b/drivers/video/fbdev/uvesafb.c
@@ -1867,7 +1867,7 @@ static ssize_t v86d_show(struct device_driver *dev, char 
*buf)
 static ssize_t v86d_store(struct device_driver *dev, const char *buf,
size_t count)
 {
-   strncpy(v86d_path, buf, PATH_MAX - 1);
+   strscpy_pad(v86d_path, buf);
return count;
 }
 static DRIVER_ATTR_RW(v86d);

---
base-commit: a4145ce1e7bc247fd6f2846e8699473448717b37
change-id: 20240320-strncpy-drivers-video-fbdev-uvesafb-c-43668c4ef6c8

Best regards,
--
Justin Stitt