We can thank KASAN for finding this, otherwise I probably would have spent hours on it. This fixes a somewhat harder to trigger kernel panic, occuring while enabling MST where the port we were currently updating the payload on would have all of it's refs dropped before we finished what we were doing:

================================================================== BUG: KASAN: use-after-free in drm_dp_update_payload_part1+0xb3f/0xdb0 [drm_kms_helper] at addr ffff8800d29de018 Read of size 4 by task Xorg/973 ============================================================================= BUG kmalloc-2048 (Tainted: G B W ): kasan: bad access detected -----------------------------------------------------------------------------

INFO: Allocated in drm_dp_add_port+0x1aa/0x1ed0 [drm_kms_helper] age=16477 cpu=0 pid=2175 ___slab_alloc+0x472/0x490 __slab_alloc+0x20/0x40 kmem_cache_alloc_trace+0x151/0x190 drm_dp_add_port+0x1aa/0x1ed0 [drm_kms_helper] drm_dp_send_link_address+0x526/0x960 [drm_kms_helper] drm_dp_check_and_send_link_address+0x1ac/0x210 [drm_kms_helper] drm_dp_mst_link_probe_work+0x77/0xd0 [drm_kms_helper] process_one_work+0x562/0x1350 worker_thread+0xd9/0x1390 kthread+0x1c5/0x260 ret_from_fork+0x22/0x40 INFO: Freed in drm_dp_free_mst_port+0x50/0x60 [drm_kms_helper] age=7521 cpu=0 pid=2175 __slab_free+0x17f/0x2d0 kfree+0x169/0x180 drm_dp_free_mst_port+0x50/0x60 [drm_kms_helper] drm_dp_destroy_connector_work+0x2b8/0x490 [drm_kms_helper] process_one_work+0x562/0x1350 worker_thread+0xd9/0x1390 kthread+0x1c5/0x260 ret_from_fork+0x22/0x40

which on this T460s, would eventually lead to kernel panics in somewhat random places later in intel_mst_enable_dp() if we got lucky enough.

Signed-off-by: Lyude cpaul@redhat.com --- drivers/gpu/drm/drm_dp_mst_topology.c | 8 ++++++++ 1 file changed, 8 insertions(+)

diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c index 297eb8e..71ea052 100644 --- a/drivers/gpu/drm/drm_dp_mst_topology.c +++ b/drivers/gpu/drm/drm_dp_mst_topology.c @@ -1796,6 +1796,11 @@ int drm_dp_update_payload_part1(struct drm_dp_mst_topology_mgr *mgr) req_payload.start_slot = cur_slots; if (mgr->proposed_vcpis[i]) { port = container_of(mgr->proposed_vcpis[i], struct drm_dp_mst_port, vcpi); + port = drm_dp_get_validated_port_ref(mgr, port); + if (!port) { + mutex_unlock(&mgr->payload_lock); + return -EINVAL; + } req_payload.num_slots = mgr->proposed_vcpis[i]->num_slots; req_payload.vcpi = mgr->proposed_vcpis[i]->vcpi; } else { @@ -1823,6 +1828,9 @@ int drm_dp_update_payload_part1(struct drm_dp_mst_topology_mgr *mgr) mgr->payloads[i].payload_state = req_payload.payload_state; } cur_slots += req_payload.num_slots; + + if (port) + drm_dp_put_port(port); }

for (i = 0; i < mgr->max_payloads; i++) {