linux-kernel - Re: [PATCH] comedi: drivers: ni_routes: Use strcmp() instead of memcmp()

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Date:   Wed, 16 Feb 2022 15:03:45 +0000
From:   Ian Abbott <abbotti@....co.uk>
To:     Kees Cook <keescook@...omium.org>
Cc:     H Hartley Sweeten <hsweeten@...ionengravers.com>,
        "Spencer E . Olson" <olsonse@...ch.edu>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Lee Jones <lee.jones@...aro.org>,
        kernel test robot <oliver.sang@...el.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] comedi: drivers: ni_routes: Use strcmp() instead of
 memcmp()

On 15/02/2022 17:10, Kees Cook wrote:
> The family and device comparisons were using memcmp(), but this could
> lead to Out-of-bounds reads when the length was larger than the
> buffers being compared. Since these appear to always be NUL-terminated
> strings, just use strcmp() instead.
> 
> This was found with Clang under LTO:
> 
> [ 92.405851][    T1] kernel BUG at lib/string_helpers.c:980!
> ...
> [ 92.409141][ T1] RIP: 0010:fortify_panic (fbdev.c:?)
> ...
> [ 92.410056][ T1] ni_assign_device_routes (fbdev.c:?)
> [ 92.410056][ T1] ? unittest_enter (fbdev.c:?)
> [ 92.410056][ T1] ni_routes_unittest (ni_routes_test.c:?)
> [ 92.410056][ T1] ? unittest_enter (fbdev.c:?)
> [ 92.410056][ T1] __initstub__kmod_ni_routes_test__505_604_ni_routes_unittest6 (fbdev.c:?)
> [ 92.410056][ T1] do_one_initcall (fbdev.c:?)
> 
> Cc: Ian Abbott <abbotti@....co.uk>
> Cc: H Hartley Sweeten <hsweeten@...ionengravers.com>
> Cc: Spencer E. Olson <olsonse@...ch.edu>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Masahiro Yamada <masahiroy@...nel.org>
> Cc: Lee Jones <lee.jones@...aro.org>
> Reported-by: kernel test robot <oliver.sang@...el.com>
> Link: https://lore.kernel.org/lkml/20220210072821.GD4074@xsang-OptiPlex-9020
> Fixes: 4bb90c87abbe ("staging: comedi: add interface to ni routing table information")
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>   drivers/comedi/drivers/ni_routes.c | 6 ++----
>   1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/comedi/drivers/ni_routes.c b/drivers/comedi/drivers/ni_routes.c
> index f24eeb464eba..295a3a9ee0c9 100644
> --- a/drivers/comedi/drivers/ni_routes.c
> +++ b/drivers/comedi/drivers/ni_routes.c
> @@ -56,8 +56,7 @@ static const u8 *ni_find_route_values(const char *device_family)
>   	int i;
>   
>   	for (i = 0; ni_all_route_values[i]; ++i) {
> -		if (memcmp(ni_all_route_values[i]->family, device_family,
> -			   strnlen(device_family, 30)) == 0) {
> +		if (!strcmp(ni_all_route_values[i]->family, device_family)) {
>   			rv = &ni_all_route_values[i]->register_values[0][0];
>   			break;
>   		}
> @@ -75,8 +74,7 @@ ni_find_valid_routes(const char *board_name)
>   	int i;
>   
>   	for (i = 0; ni_device_routes_list[i]; ++i) {
> -		if (memcmp(ni_device_routes_list[i]->device, board_name,
> -			   strnlen(board_name, 30)) == 0) {
> +		if (!strcmp(ni_device_routes_list[i]->device, board_name)) {
>   			dr = ni_device_routes_list[i];
>   			break;
>   		}

Looks good, thanks!  I'm not sure why the tests used memcmp() like that. 
Indeed, all the strings are statically allocated and null-terminated.

Reviewed-by: Ian Abbott <abbotti@....co.uk>

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