crypto: shash - Add init_tfm/exit_tfm and verify descsize

The shash interface supports a dynamic descsize field because of
the presence of fallbacks (it's just padlock-sha actually, perhaps
we can remove it one day).  As it is the API does not verify the
setting of descsize at all.  It is up to the individual algorithms
to ensure that descsize does not exceed the specified maximum value
of HASH_MAX_DESCSIZE (going above would cause stack corruption).

In order to allow the API to impose this limit directly, this patch
adds init_tfm/exit_tfm hooks to the shash_alg structure.  We can
then verify the descsize setting in the API directly.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/shash.c

@@ -385,15 +385,41 @@ int crypto_init_shash_ops_async(struct crypto_tfm *tfm)
 	return 0;
 }
 
+static void crypto_shash_exit_tfm(struct crypto_tfm *tfm)
+{
+	struct crypto_shash *hash = __crypto_shash_cast(tfm);
+	struct shash_alg *alg = crypto_shash_alg(hash);
+
+	alg->exit_tfm(hash);
+}
+
 static int crypto_shash_init_tfm(struct crypto_tfm *tfm)
 {
 	struct crypto_shash *hash = __crypto_shash_cast(tfm);
 	struct shash_alg *alg = crypto_shash_alg(hash);
+	int err;
 
 	hash->descsize = alg->descsize;
 
 	shash_set_needkey(hash, alg);
 
+	if (alg->exit_tfm)
+		tfm->exit = crypto_shash_exit_tfm;
+
+	if (!alg->init_tfm)
+		return 0;
+
+	err = alg->init_tfm(hash);
+	if (err)
+		return err;
+
+	/* ->init_tfm() may have increased the descsize. */
+	if (WARN_ON_ONCE(hash->descsize > HASH_MAX_DESCSIZE)) {
+		if (alg->exit_tfm)
+			alg->exit_tfm(hash);
+		return -EINVAL;
+	}
+
 	return 0;
 }
 

include/crypto/hash.h

@@ -169,6 +169,17 @@ struct shash_desc {
  * @export: see struct ahash_alg
  * @import: see struct ahash_alg
  * @setkey: see struct ahash_alg
+ * @init_tfm: Initialize the cryptographic transformation object.
+ *	      This function is called only once at the instantiation
+ *	      time, right after the transformation context was
+ *	      allocated. In case the cryptographic hardware has
+ *	      some special requirements which need to be handled
+ *	      by software, this function shall check for the precise
+ *	      requirement of the transformation and put any software
+ *	      fallbacks in place.
+ * @exit_tfm: Deinitialize the cryptographic transformation object.
+ *	      This is a counterpart to @init_tfm, used to remove
+ *	      various changes set in @init_tfm.
  * @digestsize: see struct ahash_alg
  * @statesize: see struct ahash_alg
  * @descsize: Size of the operational state for the message digest. This state
@@ -189,6 +200,8 @@ struct shash_alg {
 	int (*import)(struct shash_desc *desc, const void *in);
 	int (*setkey)(struct crypto_shash *tfm, const u8 *key,
 		      unsigned int keylen);
+	int (*init_tfm)(struct crypto_shash *tfm);
+	void (*exit_tfm)(struct crypto_shash *tfm);
 
 	unsigned int descsize;