[PATCH linux-next] cifs: Cleanup and clarify CalcNTLMv2_response()

[Tim Gardner]

Use of a structure aids in the understanding of this
seemingly simple bit of code. The addition of a couple
of comments also helps.

Cc: Jeff Layton <jlayton@xxxxxxxxxx>
Cc: Steve French <sfrench@xxxxxxxxx>
Signed-off-by: Tim Gardner <timg@xxxxxxx>
---

I'd just like to be sure that the destructive copy is
really what was intended. I can't tell from reading the MSDN
section 3.3.2 NTLM v2 Authentication. It sort of makes
my head hurt.

rtg

 fs/cifs/cifsencrypt.c |   21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/fs/cifs/cifsencrypt.c b/fs/cifs/cifsencrypt.c
index fc6f4f3..fecbfd0 100644
--- a/fs/cifs/cifsencrypt.c
+++ b/fs/cifs/cifsencrypt.c
@@ -548,7 +548,9 @@ static int
 CalcNTLMv2_response(const struct cifs_ses *ses, char *ntlmv2_hash)
 {
 	int rc;
-	unsigned int offset = CIFS_SESS_KEY_SIZE + 8;
+	struct ntlmv2_resp *resp = (struct ntlmv2_resp *)
+		(ses->auth_key.response + CIFS_SESS_KEY_SIZE);
+	unsigned int hashed_len = ses->auth_key.len - (CIFS_SESS_KEY_SIZE + 8);
 
 	if (!ses->server->secmech.sdeschmacmd5) {
 		cifs_dbg(VFS, "%s: can't generate ntlmv2 hash\n", __func__);
@@ -569,21 +571,30 @@ CalcNTLMv2_response(const struct cifs_ses *ses, char *ntlmv2_hash)
 		return rc;
 	}
 
+	/*
+	 * The cryptkey is part of the buffer that feeds the MD5 hash, but
+	 * gets over written later by the digest. See crypto_shash_final()
+	 * below.
+	 */
 	if (ses->server->negflavor == CIFS_NEGFLAVOR_EXTENDED)
-		memcpy(ses->auth_key.response + offset,
+		memcpy(&resp->ntlmv2_hash[CIFS_SERVER_CHALLENGE_SIZE],
 			ses->ntlmssp->cryptkey, CIFS_SERVER_CHALLENGE_SIZE);
 	else
-		memcpy(ses->auth_key.response + offset,
+		memcpy(&resp->ntlmv2_hash[CIFS_SERVER_CHALLENGE_SIZE],
 			ses->server->cryptkey, CIFS_SERVER_CHALLENGE_SIZE);
 	rc = crypto_shash_update(&ses->server->secmech.sdeschmacmd5->shash,
-		ses->auth_key.response + offset, ses->auth_key.len - offset);
+		&resp->ntlmv2_hash[8], hashed_len);
 	if (rc) {
 		cifs_dbg(VFS, "%s: Could not update with response\n", __func__);
 		return rc;
 	}
 
+	/*
+	 * Yes - this is destructive. The 16 byte MD5 digest clobbers the
+	 * cryptkey that was just copied into &resp->ntlmv2_hash[8].
+	 */
 	rc = crypto_shash_final(&ses->server->secmech.sdeschmacmd5->shash,
-		ses->auth_key.response + CIFS_SESS_KEY_SIZE);
+		resp->ntlmv2_hash);
 	if (rc)
 		cifs_dbg(VFS, "%s: Could not generate md5 hash\n", __func__);
 
-- 
1.7.9.5

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