Re: [PATCH] string: Adjust strtomem_pad() logic to allow for smaller sources

["Arnd Bergmann" <arnd@arndb.de>]

On Wed, Oct 18, 2023, at 20:07, Kees Cook wrote:
> Arnd noticed we have a case where a shorter source string is being copied
> into a destination byte array, but this results in a strnlen() call that
> exceeds the size of the source. This is seen with -Wstringop-overread:
>
> In file included from ../include/linux/uuid.h:11,
>                  from ../include/linux/mod_devicetable.h:14,
>                  from ../include/linux/cpufeature.h:12,
>                  from ../arch/x86/coco/tdx/tdx.c:7:
> ../arch/x86/coco/tdx/tdx.c: In function 'tdx_panic.constprop':
> ../include/linux/string.h:284:9: error: 'strnlen' specified bound 64 
> exceeds source size 60 [-Werror=stringop-overread]
>   284 |         memcpy_and_pad(dest, _dest_len, src, strnlen(src, 
> _dest_len), pad); \
>       |         
> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ../arch/x86/coco/tdx/tdx.c:124:9: note: in expansion of macro 
> 'strtomem_pad'
>   124 |         strtomem_pad(message.str, msg, '\0');
>       |         ^~~~~~~~~~~~
>
> Use the smaller of the two buffer sizes when calling strnlen(). When
> src length is unknown (SIZE_MAX), it will use dest length, which is what
> the original code did.
>
> Reported-by: Arnd Bergmann <arnd@arndb.de>
> Fixes: dfbafa70bde2 ("string: Introduce strtomem() and strtomem_pad()")
> Cc: Andy Shevchenko <andy@kernel.org>
> Cc: linux-hardening@vger.kernel.org
> Signed-off-by: Kees Cook <keescook@chromium.org>

Thanks for addressing this, looks good

Tested-by: Arnd Bergmann <arnd@arndb.de>