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From:   Kees Cook <keescook@chromium.org>
Date:   Tue, 8 May 2018 16:14:25 -0700
Message-ID: <CAGXu5jLL7NeX+zUNqcYW_LDF87CPg-5Z4R5+FZ5uPDT-sTisng@mail.gmail.com>
Subject: Re: [PATCH v3 1/3] big key: get rid of stack array allocation
To:     Tycho Andersen <tycho@tycho.ws>
Cc:     David Howells <dhowells@redhat.com>, keyrings@vger.kernel.org,
        linux-security-module <linux-security-module@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        James Morris <jmorris@namei.org>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        "Jason A . Donenfeld" <Jason@zx2c4.com>,
        Eric Biggers <ebiggers3@gmail.com>
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On Tue, Apr 24, 2018 at 1:26 PM, Tycho Andersen <tycho@tycho.ws> wrote:
> We're interested in getting rid of all of the stack allocated arrays in the
> kernel [1]. This patch simply hardcodes the iv length to match that of the
> hardcoded cipher.
>
> [1]: https://lkml.org/lkml/2018/3/7/621
>
> v2: hardcode the length of the nonce to be the GCM AES IV length, and do a
>     sanity check in init(), Eric Biggers
> v3: * remember to free big_key_aead when sanity check fails
>     * define a constant for big key IV size so it can be changed along side
>       the algorithm in the code
>
> Signed-off-by: Tycho Andersen <tycho@tycho.ws>
> CC: David Howells <dhowells@redhat.com>
> CC: James Morris <jmorris@namei.org>
> CC: "Serge E. Hallyn" <serge@hallyn.com>
> CC: Jason A. Donenfeld <Jason@zx2c4.com>
> CC: Eric Biggers <ebiggers3@gmail.com>

Please consider this and patches 2 and 3:

Reviewed-by: Kees Cook <keescook@chromium.org>

James, are these something you can take into your tree?

Thanks!

-Kees

> ---
>  security/keys/big_key.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/security/keys/big_key.c b/security/keys/big_key.c
> index 933623784ccd..2806e70d7f8f 100644
> --- a/security/keys/big_key.c
> +++ b/security/keys/big_key.c
> @@ -22,6 +22,7 @@
>  #include <keys/user-type.h>
>  #include <keys/big_key-type.h>
>  #include <crypto/aead.h>
> +#include <crypto/gcm.h>
>
>  struct big_key_buf {
>         unsigned int            nr_pages;
> @@ -85,6 +86,7 @@ struct key_type key_type_big_key = {
>   * Crypto names for big_key data authenticated encryption
>   */
>  static const char big_key_alg_name[] = "gcm(aes)";
> +#define BIG_KEY_IV_SIZE                GCM_AES_IV_SIZE
>
>  /*
>   * Crypto algorithms for big_key data authenticated encryption
> @@ -109,7 +111,7 @@ static int big_key_crypt(enum big_key_op op, struct big_key_buf *buf, size_t dat
>          * an .update function, so there's no chance we'll wind up reusing the
>          * key to encrypt updated data. Simply put: one key, one encryption.
>          */
> -       u8 zero_nonce[crypto_aead_ivsize(big_key_aead)];
> +       u8 zero_nonce[BIG_KEY_IV_SIZE];
>
>         aead_req = aead_request_alloc(big_key_aead, GFP_KERNEL);
>         if (!aead_req)
> @@ -425,6 +427,13 @@ static int __init big_key_init(void)
>                 pr_err("Can't alloc crypto: %d\n", ret);
>                 return ret;
>         }
> +
> +       if (unlikely(crypto_aead_ivsize(big_key_aead) != BIG_KEY_IV_SIZE)) {
> +               WARN(1, "big key algorithm changed?");
> +               ret = -EINVAL;
> +               goto free_aead;
> +       }
> +
>         ret = crypto_aead_setauthsize(big_key_aead, ENC_AUTHTAG_SIZE);
>         if (ret < 0) {
>                 pr_err("Can't set crypto auth tag len: %d\n", ret);
> --
> 2.17.0
>


-- 
Kees Cook
Pixel Security