PKCS#7: Introduce pkcs7_get_digest()

IMA will need to access the digest of the PKCS7 message (as calculated by
the kernel) before the signature is verified, so introduce
pkcs7_get_digest() for that purpose.

Also, modify pkcs7_digest() to detect when the digest was already
calculated so that it doesn't have to do redundant work. Verifying that
sinfo->sig->digest isn't NULL is sufficient because both places which
allocate sinfo->sig (pkcs7_parse_message() and pkcs7_note_signed_info())
use kzalloc() so sig->digest is always initialized to zero.

Signed-off-by: Thiago Jung Bauermann <[e-mail]>
Reviewed-by: Mimi Zohar <[e-mail]>
Cc: David Howells <[e-mail]>
Cc: David Woodhouse <[e-mail]>
Cc: Herbert Xu <[e-mail]>
Cc: "David S. Miller" <[e-mail]>
Signed-off-by: Mimi Zohar <[e-mail]>

crypto/asymmetric_keys/pkcs7_verify.c

···
 #include <linux/err.h>
 #include <linux/asn1.h>
 #include <crypto/hash.h>
+#include <crypto/hash_info.h>
 #include <crypto/public_key.h>
 #include "pkcs7_parser.h"
 
···
 	int ret;
 
 	kenter(",%u,%s", sinfo->index, sinfo->sig->hash_algo);
+
+	/* The digest was calculated already. */
+	if (sig->digest)
+		return 0;
 
 	if (!sinfo->sig->hash_algo)
 		return -ENOPKG;
···
 	crypto_free_shash(tfm);
 	kleave(" = %d", ret);
 	return ret;
+}
+
+int pkcs7_get_digest(struct pkcs7_message *pkcs7, const u8 **buf, u32 *len,
+		     enum hash_algo *hash_algo)
+{
+	struct pkcs7_signed_info *sinfo = pkcs7->signed_infos;
+	int i, ret;
+
+	/*
+	 * This function doesn't support messages with more than one signature.
+	 */
+	if (sinfo == NULL || sinfo->next != NULL)
+		return -EBADMSG;
+
+	ret = pkcs7_digest(pkcs7, sinfo);
+	if (ret)
+		return ret;
+
+	*buf = sinfo->sig->digest;
+	*len = sinfo->sig->digest_size;
+
+	for (i = 0; i < HASH_ALGO__LAST; i++)
+		if (!strcmp(hash_algo_name[i], sinfo->sig->hash_algo)) {
+			*hash_algo = i;
+			break;
+		}
+
+	return 0;
 }
 
 /*

include/crypto/pkcs7.h

···
 #define _CRYPTO_PKCS7_H
 
 #include <linux/verification.h>
+#include <linux/hash_info.h>
 #include <crypto/public_key.h>
 
 struct key;
···
 
 extern int pkcs7_supply_detached_data(struct pkcs7_message *pkcs7,
 				      const void *data, size_t datalen);
+
+extern int pkcs7_get_digest(struct pkcs7_message *pkcs7, const u8 **buf,
+			    u32 *len, enum hash_algo *hash_algo);
 
 #endif /* _CRYPTO_PKCS7_H */