crypto: ecdh - avoid buffer overflow in ecdh_set_secret()

Pavel reports that commit 17858b14 ("crypto: ecdh - avoid unaligned
accesses in ecdh_set_secret()") fixes one problem but introduces another:
the unconditional memcpy() introduced by that commit may overflow the
target buffer if the source data is invalid, which could be the result of
intentional tampering.

So check params.key_size explicitly against the size of the target buffer
before validating the key further.

Fixes: 17858b14 ("crypto: ecdh - avoid unaligned accesses in ecdh_set_secret()")
Reported-by: Pavel Machek <pavel@denx.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/ecdh.c

@@ -39,7 +39,8 @@ static int ecdh_set_secret(struct crypto_kpp *tfm, const void *buf,
 	struct ecdh params;
 	unsigned int ndigits;
 
-	if (crypto_ecdh_decode_key(buf, len, &params) < 0)
+	if (crypto_ecdh_decode_key(buf, len, &params) < 0 ||
+	    params.key_size > sizeof(ctx->private_key))
 		return -EINVAL;
 
 	ndigits = ecdh_supported_curve(params.curve_id);