CIFS: Fix signing for SMB2/3

It seems Ronnie's preamble removal broke signing.

the signing functions are called when:

A) we send a request (to sign it)
B) when we recv a response (to check the signature).

On code path A, the smb2 header is in iov[1] but on code path B, the
smb2 header is in iov[0] (and there's only one vector).

So we have different iov indexes for the smb2 header but the signing
function always use index 1. Fix this by checking the nb of io vectors
in the signing function as a hint.

Signed-off-by: Aurelien Aptel <[e-mail]>
Reviewed-by: Ronnie Sahlberg <[e-mail]>
Signed-off-by: Steve French <[e-mail]>

fs/cifs/cifsencrypt.c

···
 #include <crypto/aead.h>
 
 int __cifs_calc_signature(struct smb_rqst *rqst,
+			int start,
 			struct TCP_Server_Info *server, char *signature,
 			struct shash_desc *shash)
 {
···
 	struct kvec *iov = rqst->rq_iov;
 	int n_vec = rqst->rq_nvec;
 
-	if (n_vec < 2 || iov[0].iov_len != 4)
-		return -EIO;
-
-	for (i = 1; i < n_vec; i++) {
+	for (i = start; i < n_vec; i++) {
 		if (iov[i].iov_len == 0)
 			continue;
 		if (iov[i].iov_base == NULL) {
···
 		return rc;
 	}
 
-	return __cifs_calc_signature(rqst, server, signature,
+	return __cifs_calc_signature(rqst, 1, server, signature,
 				     &server->secmech.sdescmd5->shash);
 }
 

fs/cifs/cifsproto.h

···
 			   struct cifs_sb_info *cifs_sb,
 			   const unsigned char *path, char *pbuf,
 			   unsigned int *pbytes_written);
-int __cifs_calc_signature(struct smb_rqst *rqst,
+int __cifs_calc_signature(struct smb_rqst *rqst, int start,
 			struct TCP_Server_Info *server, char *signature,
 			struct shash_desc *shash);
 enum securityEnum cifs_select_sectype(struct TCP_Server_Info *,

fs/cifs/smb2transport.c

···
 	unsigned char smb2_signature[SMB2_HMACSHA256_SIZE];
 	unsigned char *sigptr = smb2_signature;
 	struct kvec *iov = rqst->rq_iov;
-	struct smb2_sync_hdr *shdr = (struct smb2_sync_hdr *)iov[1].iov_base;
+	int iov_hdr_index = rqst->rq_nvec > 1 ? 1 : 0;
+	struct smb2_sync_hdr *shdr =
+		(struct smb2_sync_hdr *)iov[iov_hdr_index].iov_base;
 	struct cifs_ses *ses;
 
 	ses = smb2_find_smb_ses(server, shdr->SessionId);
···
 		return rc;
 	}
 
-	rc = __cifs_calc_signature(rqst, server, sigptr,
+	rc = __cifs_calc_signature(rqst, iov_hdr_index,  server, sigptr,
 		&server->secmech.sdeschmacsha256->shash);
 
 	if (!rc)
···
 	unsigned char smb3_signature[SMB2_CMACAES_SIZE];
 	unsigned char *sigptr = smb3_signature;
 	struct kvec *iov = rqst->rq_iov;
-	struct smb2_sync_hdr *shdr = (struct smb2_sync_hdr *)iov[1].iov_base;
+	int iov_hdr_index = rqst->rq_nvec > 1 ? 1 : 0;
+	struct smb2_sync_hdr *shdr =
+		(struct smb2_sync_hdr *)iov[iov_hdr_index].iov_base;
 	struct cifs_ses *ses;
 
 	ses = smb2_find_smb_ses(server, shdr->SessionId);
···
 		return rc;
 	}
 
-	rc = __cifs_calc_signature(rqst, server, sigptr,
+	rc = __cifs_calc_signature(rqst, iov_hdr_index, server, sigptr,
 				   &server->secmech.sdesccmacaes->shash);
 
 	if (!rc)