[RFC,0/2] mac80211: use crypto shash for AES cmac

Commit Message

Ard Biesheuvel Feb. 4, 2017, 2:24 p.m. UTC

On 4 February 2017 at 11:35, Malinen, Jouni <[e-mail]> wrote:
> On Fri, Feb 03, 2017 at 09:55:36PM +0000, Ard Biesheuvel wrote:
>> OK, that looks like something I could figure out how to use. But are
>> you saying the CMAC code is never called in practice?
>
> It will get called if there is a frame that were to need to use BIP.
> There are some APs that do send broadcast addressed Deauthentication and
> Disassociation frames when terminating the network and those would get
> here. In theory, someone could come up with a new Action frame use case
> as well with a group addressed Robust Action frame, but no such thing is
> defined today. Anyway, this will be needed for certification purposes
> and to block DoS with broadcast addressed Deauthentication and
> Disassociation frames even if there is not really a common use case
> using BIP frames today.
>
>> I did spot something peculiar when looking at the code: if I am
>> reading the following sequence correctly (from
>> fils_encrypt_assoc_req())
>
>> addr[0] = mgmt->sa;
> ...
>> addr[4] = capab;
>> len[4] = encr - capab;
>>
>> crypt_len = skb->data + skb->len - encr;
>> skb_put(skb, AES_BLOCK_SIZE);
>> return aes_siv_encrypt(assoc_data->fils_kek, assoc_data->fils_kek_len,
>>       encr, crypt_len, 1, addr, len, encr);
>>
>> the addr[]/len[] arrays are populated with 5 (addr, len) pairs, but
>> only one is actually passed into aes_siv_encrypt()? This is actually
>> the main reason I stopped looking into whether I could convert it to
>> CMAC, because I couldn't figure it out.
>
> Argh.. Thanks for finding this!
>
> That is indeed supposed to have num_elems == 5. This "works" since the
> other end of the exchange is actually in user space (hostapd) and a
> similar error is there..
>

Interesting.

> This comes from the development history of FILS support.. The draft
> standard changed this AES-SIV AAD design from P802.11ai/D7.0 to D8.0
> from a single AAD vector with concatenated fields to separate vectors.
> Clearly I added the vectors here in addr[] and len[] arrays, but forgot
> to update the num_elems parameter in this direction (STA->AP); the other
> direction for Association Response frame is actually using correct
> number of AAD vectors.
>
> I'll fix this in hostapd and mac80211.
>

There is another issue I spotted: the skcipher you allocate may be of
the async variant, which may return from skcipher_encrypt() with
-EINPROGRESS as soon as it has queued the request. Since you don't
deal with that result, you should allocate a sync cipher explicitly:


> I did not actually remember that the AP side was still in hostapd for
> FILS AEAD in case of mac80211, but with that in mind, the answer to your
> earlier question about testing these becomes much simpler.. All you need
> to do is this with hostap.git hwsim tests:
>
> hostap/tests/hwsim/vm$ ./vm-run.sh ap_cipher_bip fils_sk_erp
> Starting test run in a virtual machine
> ./run-all.sh: passing the following args to run-tests.py: ap_cipher_bip fils_sk_erp
> START ap_cipher_bip 1/2
> PASS ap_cipher_bip 0.703048 2017-02-04 11:21:35.341174
> START fils_sk_erp 2/2
> PASS fils_sk_erp 0.667927 2017-02-04 11:21:36.029205
> passed all 2 test case(s)
> ALL-PASSED
>
>
> ap_cipher_bip uses wlantest to verify the BIP (AES-128-CMAC) protection
> in the frames and fils_sk_erp goes through FILS AEAD exchange with one
> side of the operation in mac80211 and the other one in hostapd. This
> should be able to discover if something is broken in the AES related
> changes in crypto.
>
> And this particular example run here was with your two patches applied,
> to the proposed net/mac80211/aes_cmac.c change seems to work fine.

Thanks for the instructions and thanks for testing. If I manage to run
this locally, I will follow up with an alternative patch #1 that
switches FILS to use cmac(aes) as well (which looks reasonably
feasible now that I understand the code)

Regards,
Ard.

Comments

Jouni Malinen Feb. 4, 2017, 2:39 p.m. UTC | #1

On Sat, Feb 04, 2017 at 02:24:36PM +0000, Ard Biesheuvel wrote:
> There is another issue I spotted: the skcipher you allocate may be of
> the async variant, which may return from skcipher_encrypt() with
> -EINPROGRESS as soon as it has queued the request. Since you don't
> deal with that result, you should allocate a sync cipher explicitly:

> diff --git a/net/mac80211/fils_aead.c b/net/mac80211/fils_aead.c
> -       tfm2 = crypto_alloc_skcipher("ctr(aes)", 0, 0);
> +       tfm2 = crypto_alloc_skcipher("ctr(aes)", 0, CRYPTO_ALG_ASYNC);

> -       tfm2 = crypto_alloc_skcipher("ctr(aes)", 0, 0);
> +       tfm2 = crypto_alloc_skcipher("ctr(aes)", 0, CRYPTO_ALG_ASYNC);

Thanks! Can you send this as a full contribution or do you want me to
do that? I did run this through all the FILS test cases with
mac80211_hwsim.

> Thanks for the instructions and thanks for testing. If I manage to run
> this locally, I will follow up with an alternative patch #1 that
> switches FILS to use cmac(aes) as well (which looks reasonably
> feasible now that I understand the code)

If you have any issues in getting the hwsim test setup working, please
let me know. I'm trying to make it easy for anyone to run those tests in
hopes of improving quality of Linux WLAN contributions and what gets
applied into cfg80211 or mac80211 (or hostap.git for that matter). In
particular the latest step-by-step guide I added for the VM version (*)
was hoping to show how that can be done in 15 minutes from scratch..


(*) http://w1.fi/cgit/hostap/plain/tests/hwsim/vm/example-vm-setup.txt

Ard Biesheuvel Feb. 4, 2017, 2:44 p.m. UTC | #2

On 4 February 2017 at 14:39, Malinen, Jouni <[e-mail]> wrote:
> On Sat, Feb 04, 2017 at 02:24:36PM +0000, Ard Biesheuvel wrote:
>> There is another issue I spotted: the skcipher you allocate may be of
>> the async variant, which may return from skcipher_encrypt() with
>> -EINPROGRESS as soon as it has queued the request. Since you don't
>> deal with that result, you should allocate a sync cipher explicitly:
>
>> diff --git a/net/mac80211/fils_aead.c b/net/mac80211/fils_aead.c
>> -       tfm2 = crypto_alloc_skcipher("ctr(aes)", 0, 0);
>> +       tfm2 = crypto_alloc_skcipher("ctr(aes)", 0, CRYPTO_ALG_ASYNC);
>
>> -       tfm2 = crypto_alloc_skcipher("ctr(aes)", 0, 0);
>> +       tfm2 = crypto_alloc_skcipher("ctr(aes)", 0, CRYPTO_ALG_ASYNC);
>
> Thanks! Can you send this as a full contribution or do you want me to
> do that?

Please go ahead if you don't mind doing it

> I did run this through all the FILS test cases with
> mac80211_hwsim.
>

Well, even async ciphers can act synchronously: the SIMD based async
ciphers will only enqueue the request for deferred processing when
called in interrupt context (on most architectures) but if you happen
to run on a platform that has a h/w accelerator for ctr(aes), you are
quite likely to see failures here.

>> Thanks for the instructions and thanks for testing. If I manage to run
>> this locally, I will follow up with an alternative patch #1 tha here
>> switches FILS to use cmac(aes) as well (which looks reasonably
>> feasible now that I understand the code)
>
> If you have any issues in getting the hwsim test setup working, please
> let me know. I'm trying to make it easy for anyone to run those tests in
> hopes of improving quality of Linux WLAN contributions and what gets
> applied into cfg80211 or mac80211 (or hostap.git for that matter). In
> particular the latest step-by-step guide I added for the VM version (*)
> was hoping to show how that can be done in 15 minutes from scratch..
>
>
> (*) http://w1.fi/cgit/hostap/plain/tests/hwsim/vm/example-vm-setup.txt
>

I will take a look on Monday

Patch

diff --git a/net/mac80211/fils_aead.c b/net/mac80211/fils_aead.c
index ecfdd97758a3..a31be5262283 100644
--- a/net/mac80211/fils_aead.c
+++ b/net/mac80211/fils_aead.c
@@ -124,7 +124,7 @@  static int aes_siv_encrypt(const u8 *key, size_t key_len,

        /* CTR */

-       tfm2 = crypto_alloc_skcipher("ctr(aes)", 0, 0);
+       tfm2 = crypto_alloc_skcipher("ctr(aes)", 0, CRYPTO_ALG_ASYNC);
        if (IS_ERR(tfm2)) {
                kfree(tmp);
                return PTR_ERR(tfm2);
@@ -183,7 +183,7 @@  static int aes_siv_decrypt(const u8 *key, size_t key_len,

        /* CTR */

-       tfm2 = crypto_alloc_skcipher("ctr(aes)", 0, 0);
+       tfm2 = crypto_alloc_skcipher("ctr(aes)", 0, CRYPTO_ALG_ASYNC);
        if (IS_ERR(tfm2))
                return PTR_ERR(tfm2);
        /* K2 for CTR */