crypto: chacha20poly1305 - validate the digest size

If the rfc7539 template was instantiated with a hash algorithm with
digest size larger than 16 bytes (POLY1305_DIGEST_SIZE), then the digest
overran the 'tag' buffer in 'struct chachapoly_req_ctx', corrupting the
subsequent memory, including 'cryptlen'.  This caused a crash during
crypto_skcipher_decrypt().

Fix it by, when instantiating the template, requiring that the
underlying hash algorithm has the digest size expected for Poly1305.

Reproducer:

    #include <linux/if_alg.h>
    #include <sys/socket.h>
    #include <unistd.h>

    int main()
    {
            int algfd, reqfd;
            struct sockaddr_alg addr = {
                    .salg_type = "aead",
                    .salg_name = "rfc7539(chacha20,sha256)",
            };
            unsigned char buf[32] = { 0 };

            algfd = socket(AF_ALG, SOCK_SEQPACKET, 0);
            bind(algfd, (void *)&addr, sizeof(addr));
            setsockopt(algfd, SOL_ALG, ALG_SET_KEY, buf, sizeof(buf));
            reqfd = accept(algfd, 0, 0);
            write(reqfd, buf, 16);
            read(reqfd, buf, 16);
    }
Reported-by: syzbot <syzkaller@googlegroups.com>
Fixes: 71ebc4d1 ("crypto: chacha20poly1305 - Add a ChaCha20-Poly1305 AEAD construction, RFC7539")
Cc: <stable@vger.kernel.org> # v4.2+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/chacha20poly1305.c

@@ -610,6 +610,11 @@ static int chachapoly_create(struct crypto_template *tmpl, struct rtattr **tb,
 						    algt->mask));
 	if (IS_ERR(poly))
 		return PTR_ERR(poly);
+	poly_hash = __crypto_hash_alg_common(poly);
+
+	err = -EINVAL;
+	if (poly_hash->digestsize != POLY1305_DIGEST_SIZE)
+		goto out_put_poly;
 
 	err = -ENOMEM;
 	inst = kzalloc(sizeof(*inst) + sizeof(*ctx), GFP_KERNEL);
@@ -618,7 +623,6 @@ static int chachapoly_create(struct crypto_template *tmpl, struct rtattr **tb,
 
 	ctx = aead_instance_ctx(inst);
 	ctx->saltlen = CHACHAPOLY_IV_SIZE - ivsize;
-	poly_hash = __crypto_hash_alg_common(poly);
 	err = crypto_init_ahash_spawn(&ctx->poly, poly_hash,
 				      aead_crypto_instance(inst));
 	if (err)