[PATCH net v3] rtnetlink: let rtnl_bridge_setlink checks IFLA_BRIDGE_MODE length

[Lin Ma <linma@zju.edu.cn>]

There are totally 9 ndo_bridge_setlink handlers in the current kernel,
which are 1) bnxt_bridge_setlink, 2) be_ndo_bridge_setlink 3)
i40e_ndo_bridge_setlink 4) ice_bridge_setlink 5)
ixgbe_ndo_bridge_setlink 6) mlx5e_bridge_setlink 7)
nfp_net_bridge_setlink 8) qeth_l2_bridge_setlink 9) br_setlink.

By investigating the code, we find that 1-7 parse and use nlattr
IFLA_BRIDGE_MODE but 3 and 4 forget to do the nla_len check. This can
lead to an out-of-attribute read and allow a malformed nlattr (e.g.,
length 0) to be viewed as a 2 byte integer.

To avoid such issues, also for other ndo_bridge_setlink handlers in the
future. This patch adds the nla_len check in rtnl_bridge_setlink and
does an early error return if length mismatches. To make it works, the
break is removed from the parsing for IFLA_BRIDGE_FLAGS to make sure
this nla_for_each_nested iterates every attribute.

Fixes: b1edc14a3fbf ("ice: Implement ice_bridge_getlink and ice_bridge_setlink")
Fixes: 51616018dd1b ("i40e: Add support for getlink, setlink ndo ops")
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Lin Ma <linma@zju.edu.cn>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
---
V1 -> V2: removes the break in parsing for IFLA_BRIDGE_FLAGS suggested
          by Hangbin Liu <liuhangbin@gmail.com>
V2 -> V3: add net subject prefix and Acked-by tag
 net/core/rtnetlink.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index 3ad4e030846d..aef25aa5cf1d 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -5140,13 +5140,17 @@ static int rtnl_bridge_setlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 	br_spec = nlmsg_find_attr(nlh, sizeof(struct ifinfomsg), IFLA_AF_SPEC);
 	if (br_spec) {
 		nla_for_each_nested(attr, br_spec, rem) {
-			if (nla_type(attr) == IFLA_BRIDGE_FLAGS) {
+			if (nla_type(attr) == IFLA_BRIDGE_FLAGS && !have_flags) {
 				if (nla_len(attr) < sizeof(flags))
 					return -EINVAL;
 
 				have_flags = true;
 				flags = nla_get_u16(attr);
-				break;
+			}
+
+			if (nla_type(attr) == IFLA_BRIDGE_MODE) {
+				if (nla_len(attr) < sizeof(u16))
+					return -EINVAL;
 			}
 		}
 	}
-- 
2.17.1