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Subject[PATCH net-next] ppp: mppe: Remove VLA usage
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=qPXydAacU1RqZWA@mail.gmail.com

Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
---
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

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