linux-hardening - Re: [RFC][PATCH v2 2/3] Modify return value of nla

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Date:   Mon, 19 Oct 2020 09:43:55 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     laniel_francis@...vacyrequired.com
Cc:     linux-hardening@...r.kernel.org, davem@...emloft.net
Subject: Re: [RFC][PATCH v2 2/3] Modify return value of nla_strlcpy to match
 that of strscpy.

On Mon, 19 Oct 2020 17:23:30 +0200 laniel_francis@...vacyrequired.com
wrote:
> diff --git a/include/net/pkt_cls.h b/include/net/pkt_cls.h
> index d4d461236351..85f4ac779399 100644
> --- a/include/net/pkt_cls.h
> +++ b/include/net/pkt_cls.h
> @@ -4,6 +4,7 @@
>  
>  #include <linux/pkt_cls.h>
>  #include <linux/workqueue.h>
> +#include <linux/errno.h>

Stray include.

>  #include <net/sch_generic.h>
>  #include <net/act_api.h>
>  #include <net/net_namespace.h>
> diff --git a/lib/nlattr.c b/lib/nlattr.c
> index 07156e581997..d692716bda78 100644
> --- a/lib/nlattr.c
> +++ b/lib/nlattr.c
> @@ -713,30 +713,39 @@ EXPORT_SYMBOL(nla_find);
>   * @dst: where to copy the string to
>   * @nla: attribute to copy the string from
>   * @dstsize: size of destination buffer
> + * @returns: -E2BIG if @dstsize is 0 or source buffer length greater than

I don't think this is correct format for kdoc.

> + * @dstsize, otherwise it returns the number of copied characters (not
> + * including the trailing %NUL).
>   *
>   * Copies at most dstsize - 1 bytes into the destination buffer.
> - * The result is always a valid NUL-terminated string. Unlike
> - * strlcpy the destination buffer is always padded out.
> - *
> - * Returns the length of the source buffer.
> + * Unlike strlcpy the destination buffer is always padded out.
>   */
> -size_t nla_strlcpy(char *dst, const struct nlattr *nla, size_t dstsize)
> +ssize_t nla_strlcpy(char *dst, const struct nlattr *nla, size_t dstsize)
>  {
> +	size_t len;
> +	ssize_t ret;
>  	size_t srclen = nla_len(nla);
>  	char *src = nla_data(nla);

Sort local variables long to short.

>  
> +	if (dstsize == 0 || WARN_ON_ONCE(dstsize > INT_MAX))

You can make it > U16_MAX, attr len is 16 bit.

> +		return -E2BIG;
> +
>  	if (srclen > 0 && src[srclen - 1] == '\0')
>  		srclen--;
>  
> -	if (dstsize > 0) {
> -		size_t len = (srclen >= dstsize) ? dstsize - 1 : srclen;
> -
> -		memcpy(dst, src, len);
> -		/* Zero pad end of dst. */
> -		memset(dst + len, 0, dstsize - len);
> +	if (srclen >= dstsize) {
> +		len = dstsize - 1;
> +		ret = -E2BIG;
> +	} else {
> +		len = srclen;
> +		ret = len;
>  	}
>  
> -	return srclen;
> +	memcpy(dst, src, len);
> +	/* Zero pad end of dst. */
> +	memset(dst + len, 0, dstsize - len);
> +
> +	return ret;
>  }
>  EXPORT_SYMBOL(nla_strlcpy);
>  

> diff --git a/net/sched/cls_api.c b/net/sched/cls_api.c
> index 41a55c6cbeb8..f0bf64393cbf 100644
> --- a/net/sched/cls_api.c
> +++ b/net/sched/cls_api.c
> @@ -223,7 +223,7 @@ static inline u32 tcf_auto_prio(struct tcf_proto *tp)
>  static bool tcf_proto_check_kind(struct nlattr *kind, char *name)
>  {
>  	if (kind)
> -		return nla_strlcpy(name, kind, IFNAMSIZ) >= IFNAMSIZ;
> +		return nla_strlcpy(name, kind, IFNAMSIZ) > 0;

Bug.

>  	memset(name, 0, IFNAMSIZ);
>  	return false;
>  }