linux-kernel - nla_put_string() vs NLA

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Date:   Tue, 20 Feb 2018 22:00:26 -0800
From:   Kees Cook <keescook@...gle.com>
To:     Thomas Graf <tgraf@...g.ch>
Cc:     Johannes Berg <johannes@...solutions.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Daniel Micay <danielmicay@...il.com>
Subject: nla_put_string() vs NLA_STRING

Hi,

It seems that in at least one case[1], nla_put_string() is being used
on an NLA_STRING, which lacks a NULL terminator, which leads to
silliness when nla_put_string() uses strlen() to figure out the size:

/**
 * nla_put_string - Add a string netlink attribute to a socket buffer
 * @skb: socket buffer to add attribute to
 * @attrtype: attribute type
 * @str: NUL terminated string
*/
static inline int nla_put_string(struct sk_buff *skb, int attrtype,
const char *str)
{
    return nla_put(skb, attrtype, strlen(str) + 1, str);
}

This is a problem at least here:

struct regulatory_request {
...
char alpha2[2];
...

static const struct nla_policy nl80211_policy[NUM_NL80211_ATTR] = {
...
[NL80211_ATTR_REG_ALPHA2] = { .type = NLA_STRING, .len = 2 },
...

AIUI, working with NLA_STRING needs nla_strlcpy() to "extract" them,
and that takes the nla_policy size normally to bounds-check the copy.

So, this specific problem needs fixing (in at least two places calling
nla_put_string(msg, NL80211_ATTR_REG_ALPHA2, ...)). While I suspect
it's only ever written an extra byte from the following variable in
the structure which is an enum nl80211_dfs_regions, I worry there
might be a lot more of these (though I'd hope unterminated strings are
uncommon for internal representation). And more generally, it seems
like only the NLA _input_ functions actually check nla_policy details.
It seems that the output functions should do the same too, yes?

-Kees

[1] https://github.com/copperhead/linux-hardened/issues/72

-- 
Kees Cook
Pixel Security