[PATCH 16/18] crypto: rsa-psspad: Implement signature verify callback

[Varad Gautam]

The RSA output must be processed as per the EMSA-PSS-VERIFY operation
from RFC8017, which forms the core of the PSS signature verification.

Implement the verification callback, which operates on the RSA output
buffer.

Reference: https://tools.ietf.org/html/rfc8017#section-9.1.2
Signed-off-by: Varad Gautam <varad.gautam@xxxxxxxx>
---
 crypto/rsa-psspad.c | 107 +++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 106 insertions(+), 1 deletion(-)

diff --git a/crypto/rsa-psspad.c b/crypto/rsa-psspad.c
index 1ca17b8b93f2..2ce750c5a23f 100644
--- a/crypto/rsa-psspad.c
+++ b/crypto/rsa-psspad.c
@@ -8,6 +8,7 @@
 
 #include <crypto/hash.h>
 #include <crypto/internal/akcipher.h>
+#include <crypto/internal/rsa.h>
 #include <crypto/internal/rsa-common.h>
 #include <crypto/public_key.h>
 
@@ -106,7 +107,111 @@ static int pkcs1_mgf1(u8 *seed, unsigned int seed_len,
 
 static int psspad_verify_complete(struct akcipher_request *req, int err)
 {
-	return -EOPNOTSUPP;
+	struct crypto_akcipher *tfm = crypto_akcipher_reqtfm(req);
+	struct rsapad_tfm_ctx *ctx = akcipher_tfm_ctx(tfm);
+	struct rsapad_akciper_req_ctx *req_ctx = akcipher_request_ctx(req);
+	struct akcipher_instance *inst = akcipher_alg_instance(tfm);
+	struct rsapad_inst_ctx *ictx = akcipher_instance_ctx(inst);
+	const struct rsa_asn1_template *digest_info = ictx->digest_info;
+	struct crypto_shash *hash_tfm = NULL;
+	struct shash_desc *desc = NULL;
+	struct rsa_mpi_key *pkey = akcipher_tfm_ctx(ctx->child);
+
+	u8 *em, *h, *salt, *maskeddb;
+	unsigned int em_len, em_bits, h_len, s_len, maskeddb_len;
+	u8 *m_hash, *db_mask, *db, *h_;
+	static u8 zeroes[8] = { 0 };
+	unsigned int pos;
+
+	if (err)
+		goto out;
+
+	err = -EINVAL;
+	if (!digest_info)
+		goto out;
+
+	em = req_ctx->out_buf;
+	em_len = ctx->key_size;
+	em_bits = mpi_get_nbits(pkey->n) - 1;
+	if ((em_bits & 0x7) == 0) {
+		em_len--;
+		em++;
+	}
+
+	h_len = req->dst_len;
+	s_len = ictx->salt_len;
+
+	if (em_len < h_len + s_len + 2)
+		goto out;
+
+	if (em[em_len - 1] != 0xbc)
+		goto out;
+
+	maskeddb = em;
+	maskeddb_len = em_len - h_len - 1;
+	h = em + maskeddb_len;
+
+	if (em[0] & ~((u8) 0xff >> (8 * em_len - em_bits)))
+		goto out;
+
+	db_mask = kzalloc(maskeddb_len, GFP_KERNEL);
+	if (!db_mask) {
+		err = -ENOMEM;
+		goto out;
+	}
+
+	err = psspad_setup_shash(&hash_tfm, &desc, digest_info->name);
+	if (err < 0)
+		goto out_db_mask;
+
+	err = pkcs1_mgf1(h, h_len, desc, db_mask, maskeddb_len);
+	if (err < 0)
+		goto out_shash;
+
+	for (pos = 0; pos < maskeddb_len; pos++)
+		maskeddb[pos] ^= db_mask[pos];
+	db = maskeddb;
+
+	db[0] &= ((u8) 0xff >> (8 * em_len - em_bits));
+
+	err = -EINVAL;
+	for (pos = 0; pos < em_len - h_len - s_len - 2; pos++) {
+		if (db[pos] != 0)
+			goto out_shash;
+	}
+	if (db[pos] != 0x01)
+		goto out_shash;
+
+	salt = db + (maskeddb_len - s_len);
+
+	m_hash = req_ctx->out_buf + ctx->key_size;
+	sg_pcopy_to_buffer(req->src,
+			   sg_nents_for_len(req->src, req->src_len + req->dst_len),
+			   m_hash,
+			   req->dst_len, ctx->key_size);
+
+	err = crypto_shash_init(desc);
+	if (!err)
+		err = crypto_shash_update(desc, zeroes, 8);
+	if (!err)
+		err = crypto_shash_update(desc, m_hash, h_len);
+	if (!err)
+		err = crypto_shash_finup(desc, salt, s_len, m_hash);
+	if (err < 0)
+		goto out_shash;
+
+	h_ = m_hash;
+
+	if (memcmp(h_, h, h_len) != 0)
+		err = -EKEYREJECTED;
+
+out_shash:
+	psspad_free_shash(hash_tfm, desc);
+out_db_mask:
+	kfree(db_mask);
+out:
+	kfree_sensitive(req_ctx->out_buf);
+	return err;
 }
 
 static void psspad_verify_complete_cb(struct crypto_async_request *child_async_req,
-- 
2.30.2