crypto: rsa-pkcs1pad - Handle leading zero for decryption

As the software RSA implementation now produces fixed-length
output, we need to eliminate leading zeros in the calling code
instead.

This patch does just that for pkcs1pad decryption while signature
verification was fixed in an earlier patch.

Fixes: 9b45b7bb ("crypto: rsa - Generate fixed-length output")
Reported-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/rsa-pkcs1pad.c

@@ -298,41 +298,48 @@ static int pkcs1pad_decrypt_complete(struct akcipher_request *req, int err)
 	struct crypto_akcipher *tfm = crypto_akcipher_reqtfm(req);
 	struct pkcs1pad_ctx *ctx = akcipher_tfm_ctx(tfm);
 	struct pkcs1pad_request *req_ctx = akcipher_request_ctx(req);
+	unsigned int dst_len;
 	unsigned int pos;
-
-	if (err == -EOVERFLOW)
-		/* Decrypted value had no leading 0 byte */
-		err = -EINVAL;
+	u8 *out_buf;
 
 	if (err)
 		goto done;
 
-	if (req_ctx->child_req.dst_len != ctx->key_size - 1) {
-		err = -EINVAL;
+	err = -EINVAL;
+	dst_len = req_ctx->child_req.dst_len;
+	if (dst_len < ctx->key_size - 1)
 		goto done;
+
+	out_buf = req_ctx->out_buf;
+	if (dst_len == ctx->key_size) {
+		if (out_buf[0] != 0x00)
+			/* Decrypted value had no leading 0 byte */
+			goto done;
+
+		dst_len--;
+		out_buf++;
 	}
 
-	if (req_ctx->out_buf[0] != 0x02) {
-		err = -EINVAL;
+	if (out_buf[0] != 0x02)
 		goto done;
-	}
-	for (pos = 1; pos < req_ctx->child_req.dst_len; pos++)
-		if (req_ctx->out_buf[pos] == 0x00)
+
+	for (pos = 1; pos < dst_len; pos++)
+		if (out_buf[pos] == 0x00)
 			break;
-	if (pos < 9 || pos == req_ctx->child_req.dst_len) {
-		err = -EINVAL;
+	if (pos < 9 || pos == dst_len)
 		goto done;
-	}
 	pos++;
 
-	if (req->dst_len < req_ctx->child_req.dst_len - pos)
+	err = 0;
+
+	if (req->dst_len < dst_len - pos)
 		err = -EOVERFLOW;
-	req->dst_len = req_ctx->child_req.dst_len - pos;
+	req->dst_len = dst_len - pos;
 
 	if (!err)
 		sg_copy_from_buffer(req->dst,
 				sg_nents_for_len(req->dst, req->dst_len),
-				req_ctx->out_buf + pos, req->dst_len);
+				out_buf + pos, req->dst_len);
 
 done:
 	kzfree(req_ctx->out_buf);