Subject: Re: [PATCH] ppp: mppe: Remove VLA usage

From: Eric Biggers <[email-masked]>
Date: Mon, 16 Jul 2018 10:36:32 -0700

On Sun, Jul 15, 2018 at 09:05:16PM -0700, Kees Cook wrote:
> In the quest to remove all stack VLA usage from the kernel[1], this
> removes the discouraged use of AHASH_REQUEST_ON_STACK (and associated
> VLA) by switching to shash directly and keeping the associated descriptor
> allocated with the regular state on the heap.
> 
> [1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=[email-masked]
> 
> Signed-off-by: Kees Cook <[email-masked]>
> ---
>  drivers/net/ppp/ppp_mppe.c | 57 +++++++++++++++++++++-----------------
>  1 file changed, 31 insertions(+), 26 deletions(-)
> 
> diff --git a/drivers/net/ppp/ppp_mppe.c b/drivers/net/ppp/ppp_mppe.c
> index 6c7fd98cb00a..5b4b81027a75 100644
> --- a/drivers/net/ppp/ppp_mppe.c
> +++ b/drivers/net/ppp/ppp_mppe.c
> @@ -96,7 +96,7 @@ static inline void sha_pad_init(struct sha_pad *shapad)
>   */
>  struct ppp_mppe_state {
>  	struct crypto_skcipher *arc4;
> -	struct crypto_ahash *sha1;
> +	struct shash_desc *sha1;
>  	unsigned char *sha1_digest;
>  	unsigned char master_key[MPPE_MAX_KEY_LEN];
>  	unsigned char session_key[MPPE_MAX_KEY_LEN];
> @@ -136,25 +136,16 @@ struct ppp_mppe_state {
>   */
>  static void get_new_key_from_sha(struct ppp_mppe_state * state)
>  {
> -	AHASH_REQUEST_ON_STACK(req, state->sha1);
> -	struct scatterlist sg[4];
> -	unsigned int nbytes;
> -
> -	sg_init_table(sg, 4);
> -
> -	nbytes = setup_sg(&sg[0], state->master_key, state->keylen);
> -	nbytes += setup_sg(&sg[1], sha_pad->sha_pad1,
> -			   sizeof(sha_pad->sha_pad1));
> -	nbytes += setup_sg(&sg[2], state->session_key, state->keylen);
> -	nbytes += setup_sg(&sg[3], sha_pad->sha_pad2,
> -			   sizeof(sha_pad->sha_pad2));
> -
> -	ahash_request_set_tfm(req, state->sha1);
> -	ahash_request_set_callback(req, 0, NULL, NULL);
> -	ahash_request_set_crypt(req, sg, state->sha1_digest, nbytes);
> -
> -	crypto_ahash_digest(req);
> -	ahash_request_zero(req);
> +	crypto_shash_init(state->sha1);
> +	crypto_shash_update(state->sha1, state->master_key,
> +			    state->keylen);
> +	crypto_shash_update(state->sha1, sha_pad->sha_pad1,
> +			    sizeof(sha_pad->sha_pad1));
> +	crypto_shash_update(state->sha1, state->session_key,
> +			    state->keylen);
> +	crypto_shash_update(state->sha1, sha_pad->sha_pad2,
> +			    sizeof(sha_pad->sha_pad2));
> +	crypto_shash_final(state->sha1, state->sha1_digest);
>  }
>  
>  /*
> @@ -200,6 +191,7 @@ static void mppe_rekey(struct ppp_mppe_state * state, int initial_key)
>  static void *mppe_alloc(unsigned char *options, int optlen)
>  {
>  	struct ppp_mppe_state *state;
> +	struct crypto_shash *shash;
>  	unsigned int digestsize;
>  
>  	if (optlen != CILEN_MPPE + sizeof(state->master_key) ||
> @@ -217,13 +209,21 @@ static void *mppe_alloc(unsigned char *options, int optlen)
>  		goto out_free;
>  	}
>  
> -	state->sha1 = crypto_alloc_ahash("sha1", 0, CRYPTO_ALG_ASYNC);
> -	if (IS_ERR(state->sha1)) {
> -		state->sha1 = NULL;
> +	shash = crypto_alloc_shash("sha1", 0, 0);
> +	if (IS_ERR(shash))
> +		goto out_free;
> +
> +	state->sha1 = kmalloc(sizeof(*state->sha1) +
> +				     crypto_shash_descsize(shash),
> +			      GFP_KERNEL);
> +	if (!state->sha1) {
> +		crypto_free_shash(shash);
>  		goto out_free;
>  	}
> +	state->sha1->tfm = shash;
> +	state->sha1->flags = 0;
>  
> -	digestsize = crypto_ahash_digestsize(state->sha1);
> +	digestsize = crypto_shash_digestsize(shash);
>  	if (digestsize < MPPE_MAX_KEY_LEN)
>  		goto out_free;
>  
> @@ -246,7 +246,11 @@ static void *mppe_alloc(unsigned char *options, int optlen)
>  
>  out_free:
>  	kfree(state->sha1_digest);
> -	crypto_free_ahash(state->sha1);
> +	if (state->sha1) {
> +		if (state->sha1->tfm)
> +			crypto_free_shash(state->sha1->tfm);

It's not necessary to check for NULL before calling crypto_free_shash().
Otherwise this looks good, though I dislike how the error codes aren't checked
in get_new_key_from_sha() (of course, they weren't before this patch either).

> +		kzfree(state->sha1);
> +	}
>  	crypto_free_skcipher(state->arc4);
>  	kfree(state);
>  out:
> @@ -261,7 +265,8 @@ static void mppe_free(void *arg)
>  	struct ppp_mppe_state *state = (struct ppp_mppe_state *) arg;
>  	if (state) {
>  		kfree(state->sha1_digest);
> -		crypto_free_ahash(state->sha1);
> +		crypto_free_shash(state->sha1->tfm);
> +		kzfree(state->sha1);
>  		crypto_free_skcipher(state->arc4);
>  		kfree(state);
>  	}
> -- 
> 2.17.1
> 
> 
> -- 
> Kees Cook
> Pixel Security