drm/dp/mst: Validate port in drm_dp_payload_send_msg()

commit deba0a2af9592b2022a0bce7b085a318b53ce1db upstream.

With the joys of things running concurrently, there's always a chance
that the port we get passed in drm_dp_payload_send_msg() isn't actually
valid anymore. Because of this, we need to make sure we validate the
reference to the port before we use it otherwise we risk running into
various race conditions. For instance, on the Dell MST monitor I have
here for testing, hotplugging it enough times causes us to kernel panic:

[drm:intel_mst_enable_dp] 1
[drm:drm_dp_update_payload_part2] payload 0 1
[drm:intel_get_hpd_pins] hotplug event received, stat 0x00200000, dig 0x10101011, pins 0x00000020
[drm:intel_hpd_irq_handler] digital hpd port B - short
[drm:intel_dp_hpd_pulse] got hpd irq on port B - short
[drm:intel_dp_check_mst_status] got esi 00 10 00
[drm:drm_dp_update_payload_part2] payload 1 1
general protection fault: 0000 [#1] SMP
…
Call Trace:
 [<ffffffffa012b632>] drm_dp_update_payload_part2+0xc2/0x130 [drm_kms_helper]
 [<ffffffffa032ef08>] intel_mst_enable_dp+0xf8/0x180 [i915]
 [<ffffffffa0310dbd>] haswell_crtc_enable+0x3ed/0x8c0 [i915]
 [<ffffffffa030c84d>] intel_atomic_commit+0x5ad/0x1590 [i915]
 [<ffffffffa01db877>] ? drm_atomic_set_crtc_for_connector+0x57/0xe0 [drm]
 [<ffffffffa01dc4e7>] drm_atomic_commit+0x37/0x60 [drm]
 [<ffffffffa0130a3a>] drm_atomic_helper_set_config+0x7a/0xb0 [drm_kms_helper]
 [<ffffffffa01cc482>] drm_mode_set_config_internal+0x62/0x100 [drm]
 [<ffffffffa01d02ad>] drm_mode_setcrtc+0x3cd/0x4e0 [drm]
 [<ffffffffa01c18e3>] drm_ioctl+0x143/0x510 [drm]
 [<ffffffffa01cfee0>] ? drm_mode_setplane+0x1b0/0x1b0 [drm]
 [<ffffffff810f79a7>] ? hrtimer_start_range_ns+0x1b7/0x3a0
 [<ffffffff81212962>] do_vfs_ioctl+0x92/0x570
 [<ffffffff81590852>] ? __sys_recvmsg+0x42/0x80
 [<ffffffff81212eb9>] SyS_ioctl+0x79/0x90
 [<ffffffff816b4e32>] entry_SYSCALL_64_fastpath+0x1a/0xa4
RIP  [<ffffffffa012b026>] drm_dp_payload_send_msg+0x146/0x1f0 [drm_kms_helper]

Which occurs because of the hotplug event shown in the log, which ends
up causing DRM's dp helpers to drop the port we're updating the payload
on and panic.

Signed-off-by: Lyude <cpaul@redhat.com>
Reviewed-by: David Airlie <airlied@linux.ie>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 8 insertions, 1 deletions

diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index 39d7e2e15c11..a4a3de372b69 100644
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -1665,13 +1665,19 @@ static int drm_dp_payload_send_msg(struct drm_dp_mst_topology_mgr *mgr,
 	struct drm_dp_mst_branch *mstb;
 	int len, ret, port_num;
 
+	port = drm_dp_get_validated_port_ref(mgr, port);
+	if (!port)
+		return -EINVAL;
+
 	port_num = port->port_num;
 	mstb = drm_dp_get_validated_mstb_ref(mgr, port->parent);
 	if (!mstb) {
 		mstb = drm_dp_get_last_connected_port_and_mstb(mgr, port->parent, &port_num);
 
-		if (!mstb)
+		if (!mstb) {
+			drm_dp_put_port(port);
 			return -EINVAL;
+		}
 	}
 
 	txmsg = kzalloc(sizeof(*txmsg), GFP_KERNEL);
@@ -1697,6 +1703,7 @@ static int drm_dp_payload_send_msg(struct drm_dp_mst_topology_mgr *mgr,
 	kfree(txmsg);
 fail_put:
 	drm_dp_put_mst_branch_device(mstb);
+	drm_dp_put_port(port);
 	return ret;
 }