Subject: [PATCH net-next] ppp: mppe: Remove VLA usage

From: Kees Cook <[email-masked]>
Date: Fri, 3 Aug 2018 09:37:45 -0700

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]>
Acked-by: Arnd Bergmann <[email-masked]>
---
Paul suggested this go via netdev directly:
https://lkml.kernel.org/r/20180803031315.GA30807@fergus
---
 drivers/net/ppp/ppp_mppe.c | 56 ++++++++++++++++++++------------------
 1 file changed, 30 insertions(+), 26 deletions(-)

diff --git a/drivers/net/ppp/ppp_mppe.c b/drivers/net/ppp/ppp_mppe.c
index 6c7fd98cb00a..a205750b431b 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 +246,10 @@ static void *mppe_alloc(unsigned char *options, int optlen)
 
 out_free:
 	kfree(state->sha1_digest);
-	crypto_free_ahash(state->sha1);
+	if (state->sha1) {
+		crypto_free_shash(state->sha1->tfm);
+		kzfree(state->sha1);
+	}
 	crypto_free_skcipher(state->arc4);
 	kfree(state);
 out:
@@ -261,7 +264,8 @@ 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