[PATCH] netlink: don't compare the nul-termination in nla_strcmp

[PATCH] netlink: don't compare the nul-termination in nla_strcmp

[Pablo Neira Ayuso]

nla_strcmp compares the string length plus one, so it's implicitly
including the nul-termination in the comparison.

 int nla_strcmp(const struct nlattr *nla, const char *str)
 {
        int len = strlen(str) + 1;
        ...
                d = memcmp(nla_data(nla), str, len);

However, if NLA_STRING is used, userspace can send us a string without
the nul-termination. This is a problem since the string
comparison will not match as the last byte may be not the
nul-termination.

Fix this by skipping the comparison of the nul-termination if the
attribute data is nul-terminated. Suggested by Thomas Graf.

Cc: Florian Westphal <fw@strlen.de>
Cc: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
---
 lib/nlattr.c |   10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/lib/nlattr.c b/lib/nlattr.c
index 18eca78..53beba5 100644
--- a/lib/nlattr.c
+++ b/lib/nlattr.c
@@ -303,9 +303,15 @@ int nla_memcmp(const struct nlattr *nla, const void *data,
  */
 int nla_strcmp(const struct nlattr *nla, const char *str)
 {
-	int len = strlen(str) + 1;
-	int d = nla_len(nla) - len;
+	int len = strlen(str);
+	char *buf = nla_data(nla);
+	int attrlen = nla_len(nla);
+	int d;
 
+	if (buf[attrlen - 1] == '\0')
+		attrlen--;
+
+	d = attrlen - len;
 	if (d == 0)
 		d = memcmp(nla_data(nla), str, len);
 
-- 
1.7.10.4

Re: [PATCH] netlink: don't compare the nul-termination in nla_strcmp

[Thomas Graf]

On 03/31/14 at 05:34pm, Pablo Neira Ayuso wrote:
> nla_strcmp compares the string length plus one, so it's implicitly
> including the nul-termination in the comparison.
> 
>  int nla_strcmp(const struct nlattr *nla, const char *str)
>  {
>         int len = strlen(str) + 1;
>         ...
>                 d = memcmp(nla_data(nla), str, len);
> 
> However, if NLA_STRING is used, userspace can send us a string without
> the nul-termination. This is a problem since the string
> comparison will not match as the last byte may be not the
> nul-termination.
> 
> Fix this by skipping the comparison of the nul-termination if the
> attribute data is nul-terminated. Suggested by Thomas Graf.
> 
> Cc: Florian Westphal <fw@strlen.de>
> Cc: Thomas Graf <tgraf@suug.ch>
> Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
> ---
>  lib/nlattr.c |   10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/nlattr.c b/lib/nlattr.c
> index 18eca78..53beba5 100644
> --- a/lib/nlattr.c
> +++ b/lib/nlattr.c
> @@ -303,9 +303,15 @@ int nla_memcmp(const struct nlattr *nla, const void *data,
>   */
>  int nla_strcmp(const struct nlattr *nla, const char *str)
>  {
> -	int len = strlen(str) + 1;
> -	int d = nla_len(nla) - len;
> +	int len = strlen(str);
> +	char *buf = nla_data(nla);
> +	int attrlen = nla_len(nla);
> +	int d;
>  
> +	if (buf[attrlen - 1] == '\0')
> +		attrlen--;

Perhaps check for attrlen > 0? It should never happen if the
attribute has been validated against NLA_STRING or
NLA_NUL_STRING but not all attributes get validated.