datapath: Avoid OOB read when parsing flow nlattrs

Upstream commit:
    commit 04a4af334b971814eedf4e4a413343ad3287d9a9
    Author: Ross Lagerwall <[email protected]>
    Date:   Mon Jan 14 09:16:56 2019 +0000

    openvswitch: Avoid OOB read when parsing flow nlattrs

    For nested and variable attributes, the expected length of an attribute
    is not known and marked by a negative number.  This results in an OOB
    read when the expected length is later used to check if the attribute is
    all zeros. Fix this by using the actual length of the attribute rather
    than the expected length.

    Signed-off-by: Ross Lagerwall <[email protected]>
    Acked-by: Pravin B Shelar <[email protected]>
    Signed-off-by: David S. Miller <[email protected]>

Cc: Ross Lagerwall <[email protected]>
Signed-off-by: Greg Rose <[email protected]>
Signed-off-by: Ben Pfaff <[email protected]>

datapath/flow_netlink.c

@@ -502,7 +502,7 @@ static int __parse_flow_nlattrs(const struct nlattr *attr,
 			return -EINVAL;
 		}
 
-		if (!nz || !is_all_zero(nla_data(nla), expected_len)) {
+		if (!nz || !is_all_zero(nla_data(nla), nla_len(nla))) {
 			attrs |= 1ULL << type;
 			a[type] = nla;
 		}