Re: [PATCH 7/7] mac80211: Switch to new AEAD interface

Johannes Berg
2015-06-01T14:05:23Z

On Mon, 2015-06-01 at 15:49 +0200, Stephan Mueller wrote:

> >The contents, now, that's a more interesting question. I believe it can
> >never be all zeroes, since association request frames are not
> >encrypted/protected and thus at least one byte in here must be non-zero.
> >The MAC addresses are also very likely non-zero, but technically
> >00:00:00:00:00:00 is a valid MAC address I believe.
>
> So, even when having a malicious AP, that value is never zero? The driver of
> the question is the following code in the patch set:
>
> + sg_set_buf(&sg[0], &aad[2], be16_to_cpup((__be16 *)aad));
>
> ...
>
> + aead_request_set_crypt(aead_req, sg, sg, data_len, b_0);
>
> ...
>
> crypto_aead_encrypt(aead_req);
>
>
> When I played around with the aead_request_set_crypt, I saw a crash in the
> scatterlist handling of the crypto API when the first SGL entry has a zero
> length.

Wait, I guess that's a *third* way for this to be "zero" a valid pointer
but zero length data?

Oh, no - you're referring to the CCM/GCM cases only, I guess, i.e. this
part:

- sg_init_one(&assoc, &aad[2], be16_to_cpup((__be16 *)aad));
+ sg_set_buf(&sg[0], &aad[2], be16_to_cpup((__be16 *)aad));

I was looking at GMAC and that has a constant for the length :-)

Ok - here the length is kinda passed a part of the AAD buffer, but this
is really just some arcane code that should be fixed to use a proper
struct. The value there, even though it is __be16 and looks like it came
from the data, is actually created locally, see ccmp_special_blocks()
and gcmp_special_blocks().

johannes