[PATCH] net: mctp: hold a reference to the route device in mctp_route_lookup()

mctp_route_lookup() uses rt->dev without holding a reference on it.
mctp_route_lookup_single() returns the route under RCU only, so the
route's device can be torn down concurrently: mctp_dev_put() drops the
last reference and synchronously kfree()s mdev->addrs.  mctp_dev_saddr()
then reads rt->dev->addrs[0], giving a use-after-free reachable by an
unprivileged local AF_MCTP user on the receive/forwarding path (no
CAP_NET_RAW required):

  BUG: KASAN: slab-use-after-free in mctp_route_lookup
  Read of size 1 at addr ... by task mctp_uaf/...
   mctp_route_lookup
   mctp_pkttype_receive
  Freed by task ...:
   kfree
   mctp_dev_put
   mctp_dev_notify

In the same window mctp_dst_from_route() -> mctp_dev_hold() also
increments a refcount that has already reached zero
("refcount_t: addition on 0 ... mctp_dev_hold").

This reintroduces the use-after-free class of CVE-2023-3439: the source
address lookup was moved ahead of the point where the destination takes
its device reference.

Take a reference with refcount_inc_not_zero() before touching rt->dev,
skip a device that is already dead, and drop the reference once the
destination has taken its own.

Fixes: 22cb45afd221 ("net: mctp: perform source address lookups when we populate our dst")
Cc: [email protected]
Signed-off-by: Aldo Ariel Panzardo <[email protected]>
---
A reproducer is available on request.

 net/mctp/route.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/net/mctp/route.c b/net/mctp/route.c
index 1f3dccbb7a..b19c63a569 100644
--- a/net/mctp/route.c
+++ b/net/mctp/route.c
@@ -998,14 +998,29 @@ int mctp_route_lookup(struct net *net, unsigned int dnet,
 			mtu = mtu ?: rt->mtu;
 
 		if (rt->dst_type == MCTP_ROUTE_DIRECT) {
-			mctp_eid_t saddr = mctp_dev_saddr(rt->dev);
+			mctp_eid_t saddr;
+
+			/* rt->dev may be going away concurrently: its last
+			 * reference is dropped in mctp_dev_put(), which frees
+			 * mdev->addrs that mctp_dev_saddr() reads, and
+			 * mctp_dst_from_route() takes a reference on it.  Pin
+			 * it before use, and skip a device that is already
+			 * dead rather than resurrecting it.
+			 */
+			if (!refcount_inc_not_zero(&rt->dev->refs))
+				break;
+
+			saddr = mctp_dev_saddr(rt->dev);
 
 			/* cannot do gateway-ed routes without a src  */
-			if (saddr == MCTP_ADDR_NULL && depth != 0)
+			if (saddr == MCTP_ADDR_NULL && depth != 0) {
+				mctp_dev_put(rt->dev);
 				break;
+			}
 
 			if (dst)
 				mctp_dst_from_route(dst, daddr, saddr, mtu, rt);
+			mctp_dev_put(rt->dev);
 			rc = 0;
 			break;
 
-- 
2.43.0