SMB3: Work around mount failure when using SMB3 dialect to Macs

commit 7db0a6ef upstream.

Macs send the maximum buffer size in response on ioctl to validate
negotiate security information, which causes us to fail the mount
as the response buffer is larger than the expected response.

Changed ioctl response processing to allow for padding of validate
negotiate ioctl response and limit the maximum response size to
maximum buffer size.
Signed-off-by: Steve French <steve.french@primarydata.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

fs/cifs/smb2pdu.c

@@ -524,8 +524,12 @@ int smb3_validate_negotiate(const unsigned int xid, struct cifs_tcon *tcon)
 	}
 
 	if (rsplen != sizeof(struct validate_negotiate_info_rsp)) {
-		cifs_dbg(VFS, "invalid size of protocol negotiate response\n");
-		return -EIO;
+		cifs_dbg(VFS, "invalid protocol negotiate response size: %d\n",
+			 rsplen);
+
+		/* relax check since Mac returns max bufsize allowed on ioctl */
+		if (rsplen > CIFSMaxBufSize)
+			return -EIO;
 	}
 
 	/* check validate negotiate info response matches what we got earlier */
@@ -1293,8 +1297,12 @@ SMB2_ioctl(const unsigned int xid, struct cifs_tcon *tcon, u64 persistent_fid,
 	 * than one credit. Windows typically sets this smaller, but for some
 	 * ioctls it may be useful to allow server to send more. No point
 	 * limiting what the server can send as long as fits in one credit
+	 * Unfortunately - we can not handle more than CIFS_MAX_MSG_SIZE
+	 * (by default, note that it can be overridden to make max larger)
+	 * in responses (except for read responses which can be bigger.
+	 * We may want to bump this limit up
 	 */
-	req->MaxOutputResponse = cpu_to_le32(0xFF00); /* < 64K uses 1 credit */
+	req->MaxOutputResponse = cpu_to_le32(CIFSMaxBufSize);
 
 	if (is_fsctl)
 		req->Flags = cpu_to_le32(SMB2_0_IOCTL_IS_FSCTL);