linux-kernel - Re: nla_put_string() vs NLA

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Date:   Thu, 22 Feb 2018 20:54:49 +0100
From:   Johannes Berg <johannes@...solutions.net>
To:     Kees Cook <keescook@...gle.com>, Thomas Graf <tgraf@...g.ch>
Cc:     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: Re: nla_put_string() vs NLA_STRING

On Tue, 2018-02-20 at 22:00 -0800, Kees Cook wrote:

> 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:

Fun! I'm not a big fan of the whole NLA_STRING thing with or without
NUL terminator anyway, it's a bit confusing at times :-)

> 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 },
> ...

Yeah, this is clearly stupid. We already fixed one of these, see commit
a5fe8e7695dc ("regulatory: add NUL to alpha2"). I'll fix up the second
one too.

> 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, 

Only one, since the other has alpha2[3] already :-)

And in that case, yes, on little endian and only if the dfs region is
non-zero, though the dfs region was added later so dunno what else
there was - but certainly this struct would have always contained some
enum value that had zero-bytes.

> I worry there
> might be a lot more of these (though I'd hope unterminated strings are
> uncommon for internal representation).

Generally they are, I'd argue.

> 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?

It doesn't really work that way - there's no real guarantee that the
policy is symmetric on input/output.

johannes