cifs: fix unicode string area word alignment in session setup

The handling of unicode string area alignment is wrong.
decode_unicode_ssetup improperly assumes that it will always be preceded
by a pad byte. This isn't the case if the string area is already
word-aligned.

This problem, combined with the bad buffer sizing for the serverDomain
string can cause memory corruption. The bad alignment can make it so
that the alignment of the characters is off. This can make them
translate to characters that are greater than 2 bytes each. If this
happens we can overflow the allocation.

Fix this by fixing the alignment in CIFS_SessSetup instead so we can
verify it against the head of the response. Also, clean up the
workaround for improperly terminated strings by checking for a
odd-length unicode buffers and then forcibly terminating them.

Finally, resize the buffer for serverDomain. Now that we've fixed
the alignment, it's probably fine, but a malicious server could
overflow it.

A better solution for handling these strings is still needed, but
this should be a suitable bandaid.

Signed-off-by: Jeff Layton <[e-mail]>
CC: Stable <[e-mail]>
Signed-off-by: Steve French <[e-mail]>

fs/cifs/sess.c

···
 	int words_left, len;
 	char *data = *pbcc_area;
 
-
-
 	cFYI(1, ("bleft %d", bleft));
 
-
-	/* SMB header is unaligned, so cifs servers word align start of
-	   Unicode strings */
-	data++;
-	bleft--; /* Windows servers do not always double null terminate
-		    their final Unicode string - in which case we
-		    now will not attempt to decode the byte of junk
-		    which follows it */
+	/*
+	 * Windows servers do not always double null terminate their final
+	 * Unicode string. Check to see if there are an uneven number of bytes
+	 * left. If so, then add an extra NULL pad byte to the end of the
+	 * response.
+	 *
+	 * See section 2.7.2 in "Implementing CIFS" for details
+	 */
+	if (bleft % 2) {
+		data[bleft] = 0;
+		++bleft;
+	}
 
 	words_left = bleft / 2;
 
 	/* save off server operating system */
 	len = UniStrnlen((wchar_t *) data, words_left);
 
-/* We look for obvious messed up bcc or strings in response so we do not go off
-   the end since (at least) WIN2K and Windows XP have a major bug in not null
-   terminating last Unicode string in response  */
 	if (len >= words_left)
 		return rc;
 
···
 		return rc;
 
 	kfree(ses->serverDomain);
-	ses->serverDomain = kzalloc(2 * (len + 1), GFP_KERNEL); /* BB FIXME wrong length */
-	if (ses->serverDomain != NULL) {
+	ses->serverDomain = kzalloc((4 * len) + 2, GFP_KERNEL);
+	if (ses->serverDomain != NULL)
 		cifs_strfromUCS_le(ses->serverDomain, (__le16 *)data, len,
 				   nls_cp);
-		ses->serverDomain[2*len] = 0;
-		ses->serverDomain[(2*len) + 1] = 0;
-	}
 	data += 2 * (len + 1);
 	words_left -= len + 1;
 
···
 	}
 
 	/* BB check if Unicode and decode strings */
-	if (smb_buf->Flags2 & SMBFLG2_UNICODE)
+	if (smb_buf->Flags2 & SMBFLG2_UNICODE) {
+		/* unicode string area must be word-aligned */
+		if (((unsigned long) bcc_ptr - (unsigned long) smb_buf) % 2) {
+			++bcc_ptr;
+			--bytes_remaining;
+		}
 		rc = decode_unicode_ssetup(&bcc_ptr, bytes_remaining,
-						   ses, nls_cp);
-	else
+					   ses, nls_cp);
+	} else {
 		rc = decode_ascii_ssetup(&bcc_ptr, bytes_remaining,
 					 ses, nls_cp);
+	}
 
 ssetup_exit:
 	if (spnego_key) {