netdev - Re: [PATCH] nfp: replace deprecated strncpy with strscpy

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Date: Wed, 11 Oct 2023 16:50:24 -0700
From: Kees Cook <keescook@...omium.org>
To: Justin Stitt <justinstitt@...gle.com>
Cc: Louis Peens <louis.peens@...igine.com>,
	Jakub Kicinski <kuba@...nel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	oss-drivers@...igine.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] nfp: replace deprecated strncpy with strscpy

On Wed, Oct 11, 2023 at 09:48:39PM +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.
> 
> We expect res->name to be NUL-terminated based on its usage with format
> strings:
> |       dev_err(cpp->dev.parent, "Dangling area: %d:%d:%d:0x%0llx-0x%0llx%s%s\n",
> |               NFP_CPP_ID_TARGET_of(res->cpp_id),
> |               NFP_CPP_ID_ACTION_of(res->cpp_id),
> |               NFP_CPP_ID_TOKEN_of(res->cpp_id),
> |               res->start, res->end,
> |               res->name ? " " : "",
> |               res->name ? res->name : "");
> ... and with strcmp()
> |       if (!strcmp(res->name, NFP_RESOURCE_TBL_NAME)) {
> 
> Moreover, NUL-padding is not required as `res` is already
> zero-allocated:
> |       res = kzalloc(sizeof(*res), GFP_KERNEL);
> 
> 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.
> 
> Let's also opt to use the more idiomatic strscpy() usage of (dest, src,
> sizeof(dest)) rather than (dest, src, SOME_LEN).
> 
> Typically the pattern of 1) allocate memory for string, 2) copy string
> into freshly-allocated memory is a candidate for kmemdup_nul() but in
> this case we are allocating the entirety of the `res` struct and that
> should stay as is. As mentioned above, simple 1:1 replacement of strncpy
> -> 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 [2]
> Link: https://github.com/KSPP/linux/issues/90
> Cc: linux-hardening@...r.kernel.org
> Signed-off-by: Justin Stitt <justinstitt@...gle.com>
> ---
> Note: build-tested only.
> 
> Found with: $ rg "strncpy\("
> ---
>  drivers/net/ethernet/netronome/nfp/nfpcore/nfp_resource.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/netronome/nfp/nfpcore/nfp_resource.c b/drivers/net/ethernet/netronome/nfp/nfpcore/nfp_resource.c
> index ce7492a6a98f..279ea0b56955 100644
> --- a/drivers/net/ethernet/netronome/nfp/nfpcore/nfp_resource.c
> +++ b/drivers/net/ethernet/netronome/nfp/nfpcore/nfp_resource.c
> @@ -159,7 +159,7 @@ nfp_resource_acquire(struct nfp_cpp *cpp, const char *name)
>  	if (!res)
>  		return ERR_PTR(-ENOMEM);
>  
> -	strncpy(res->name, name, NFP_RESOURCE_ENTRY_NAME_SZ);
> +	strscpy(res->name, name, sizeof(res->name));

struct nfp_resource {
        char name[NFP_RESOURCE_ENTRY_NAME_SZ + 1];

Yup, this is doing the implicit string size reduction.

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

-- 
Kees Cook