[PATCH] bridge: mrp: Fix out-of-bounds read in br_mrp_parse

From: Horatiu Vultur
Date: Mon May 25 2020 - 03:56:25 EST

The issue was reported by syzbot. When the function br_mrp_parse was
called with a valid net_bridge_port, the net_bridge was an invalid
pointer. Therefore the check br->stp_enabled could pass/fail
depending where it was pointing in memory.
The fix consists of setting the net_bridge pointer if the port is a
valid pointer.

Reported-by: syzbot+9c6f0f1f8e32223df9a4@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 6536993371fa ("bridge: mrp: Integrate MRP into the bridge")
Signed-off-by: Horatiu Vultur <horatiu.vultur@xxxxxxxxxxxxx>
net/bridge/br_mrp_netlink.c | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/net/bridge/br_mrp_netlink.c b/net/bridge/br_mrp_netlink.c
index 397e7f710772a..4a08a99519b04 100644
--- a/net/bridge/br_mrp_netlink.c
+++ b/net/bridge/br_mrp_netlink.c
@@ -27,6 +27,12 @@ int br_mrp_parse(struct net_bridge *br, struct net_bridge_port *p,
struct nlattr *tb[IFLA_BRIDGE_MRP_MAX + 1];
int err;

+ /* When this function is called for a port then the br pointer is
+ * invalid, therefor set the br to point correctly
+ */
+ if (p)
+ br = p->br;
if (br->stp_enabled != BR_NO_STP) {
NL_SET_ERR_MSG_MOD(extack, "MRP can't be enabled if STP is already enabled");
return -EINVAL;