Re: [PATCH v1 2/3] Modify return value of nla_strlcpy to match that of strscpy.

[Kees Cook <keescook@chromium.org>]

On Fri, Oct 16, 2020 at 02:52:15PM +0200, laniel_francis@privacyrequired.com wrote:
> From: Francis Laniel <laniel_francis@privacyrequired.com>
> 
> This patch solves part 2 of issue: https://github.com/KSPP/linux/issues/110

As with the others, this needs a full commit log (imagine someone
doesn't have access to read that issue, etc).

> 
> Signed-off-by: Francis Laniel <laniel_francis@privacyrequired.com>
> ---
>  include/net/netlink.h |  2 +-
>  include/net/pkt_cls.h |  3 ++-
>  lib/nlattr.c          | 31 ++++++++++++++++++++-----------
>  net/sched/act_api.c   |  2 +-
>  net/sched/sch_api.c   |  2 +-
>  5 files changed, 25 insertions(+), 15 deletions(-)
> 
> diff --git a/include/net/netlink.h b/include/net/netlink.h
> index 7356f41d23ba..446ca182e13d 100644
> --- a/include/net/netlink.h
> +++ b/include/net/netlink.h
> @@ -506,7 +506,7 @@ int __nla_parse(struct nlattr **tb, int maxtype, const struct nlattr *head,
>  		struct netlink_ext_ack *extack);
>  int nla_policy_len(const struct nla_policy *, int);
>  struct nlattr *nla_find(const struct nlattr *head, int len, int attrtype);
> -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);
>  char *nla_strdup(const struct nlattr *nla, gfp_t flags);
>  int nla_memcpy(void *dest, const struct nlattr *src, int count);
>  int nla_memcmp(const struct nlattr *nla, const void *data, size_t size);
> diff --git a/include/net/pkt_cls.h b/include/net/pkt_cls.h
> index d4d461236351..f42db07c399b 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>
>  #include <net/sch_generic.h>
>  #include <net/act_api.h>
>  #include <net/net_namespace.h>
> @@ -512,7 +513,7 @@ tcf_change_indev(struct net *net, struct nlattr *indev_tlv,
>  	char indev[IFNAMSIZ];
>  	struct net_device *dev;
>  
> -	if (nla_strlcpy(indev, indev_tlv, IFNAMSIZ) >= IFNAMSIZ) {
> +	if (nla_strlcpy(indev, indev_tlv, IFNAMSIZ) == -E2BIG) {

I would make these tests all be "< 0" rather than specifically -E2BIG.

>  		NL_SET_ERR_MSG_ATTR(extack, indev_tlv,
>  				    "Interface name too long");
>  		return -EINVAL;
> diff --git a/lib/nlattr.c b/lib/nlattr.c
> index ab96a5f4b9b8..83dd233bbe3e 100644
> --- a/lib/nlattr.c
> +++ b/lib/nlattr.c
> @@ -713,29 +713,38 @@ 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
> + * @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.
> + * The result is always a valid NUL-terminated string.
>   */
> -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);
>  
> +	if (dstsize == 0 || WARN_ON_ONCE(dstsize > INT_MAX))
> +		return -E2BIG;

Cool, yeah, good to include.

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

Otherwise, I think this looks good. (Though same note about
zero-padding.)

>  
> diff --git a/net/sched/act_api.c b/net/sched/act_api.c
> index f66417d5d2c3..ed411d6c29fc 100644
> --- a/net/sched/act_api.c
> +++ b/net/sched/act_api.c
> @@ -935,7 +935,7 @@ struct tc_action *tcf_action_init_1(struct net *net, struct tcf_proto *tp,
>  			NL_SET_ERR_MSG(extack, "TC action kind must be specified");
>  			goto err_out;
>  		}
> -		if (nla_strlcpy(act_name, kind, IFNAMSIZ) >= IFNAMSIZ) {
> +		if (nla_strlcpy(act_name, kind, IFNAMSIZ) == -E2BIG) {
>  			NL_SET_ERR_MSG(extack, "TC action name too long");
>  			goto err_out;
>  		}
> diff --git a/net/sched/sch_api.c b/net/sched/sch_api.c
> index 2a76a2f5ed88..3e89c666d1e8 100644
> --- a/net/sched/sch_api.c
> +++ b/net/sched/sch_api.c
> @@ -1170,7 +1170,7 @@ static struct Qdisc *qdisc_create(struct net_device *dev,
>  #ifdef CONFIG_MODULES
>  	if (ops == NULL && kind != NULL) {
>  		char name[IFNAMSIZ];
> -		if (nla_strlcpy(name, kind, IFNAMSIZ) < IFNAMSIZ) {
> +		if (nla_strlcpy(name, kind, IFNAMSIZ) != -E2BIG) {
>  			/* We dropped the RTNL semaphore in order to
>  			 * perform the module load.  So, even if we
>  			 * succeeded in loading the module we have to
> -- 
> 2.20.1
> 

-- 
Kees Cook