Subject: [PATCH v7 03/14] PKCS#7: Introduce pkcs7_get_digest()

From: Thiago Jung Bauermann <[email-masked]>
Date: Tue, 22 May 2018 21:12:42 -0300

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 <[email-masked]>
Reviewed-by: Mimi Zohar <[email-masked]>
Cc: David Howells <[email-masked]>
Cc: Herbert Xu <[email-masked]>
Cc: "David S. Miller" <[email-masked]>
---
 crypto/asymmetric_keys/pkcs7_verify.c | 25 +++++++++++++++++++++++++
 include/crypto/pkcs7.h                |  3 +++
 2 files changed, 28 insertions(+)

diff --git a/crypto/asymmetric_keys/pkcs7_verify.c b/crypto/asymmetric_keys/pkcs7_verify.c
index 97c77f66b20d..e61f121bfc87 100644
--- a/crypto/asymmetric_keys/pkcs7_verify.c
+++ b/crypto/asymmetric_keys/pkcs7_verify.c
@@ -33,6 +33,10 @@ static int pkcs7_digest(struct pkcs7_message *pkcs7,
 
 	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;
 
@@ -122,6 +126,27 @@ static int pkcs7_digest(struct pkcs7_message *pkcs7,
 	return ret;
 }
 
+int pkcs7_get_digest(struct pkcs7_message *pkcs7, const u8 **buf, u8 *len)
+{
+	struct pkcs7_signed_info *sinfo = pkcs7->signed_infos;
+	int 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;
+
+	return 0;
+}
+
 /*
  * Find the key (X.509 certificate) to use to verify a PKCS#7 message.  PKCS#7
  * uses the issuer's name and the issuing certificate serial number for
diff --git a/include/crypto/pkcs7.h b/include/crypto/pkcs7.h
index 6f51d0cb6d12..cfaea9c37f4a 100644
--- a/include/crypto/pkcs7.h
+++ b/include/crypto/pkcs7.h
@@ -46,4 +46,7 @@ extern int pkcs7_verify(struct pkcs7_message *pkcs7,
 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,
+			    u8 *len);
+
 #endif /* _CRYPTO_PKCS7_H */