From 408da1df18116c971c3392e21e50586688cd3fbf Mon Sep 17 00:00:00 2001 From: Aldo Ariel Panzardo Date: Wed, 12 Aug 2026 23:21:02 -0300 Subject: [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: stable@vger.kernel.org Signed-off-by: Aldo Ariel Panzardo Link: https://patch.msgid.link/20260813022102.2792032-1-qwe.aldo@gmail.com Signed-off-by: Jakub Kicinski --- 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 1f3dccbb7aed..b19c63a5691a 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;