Patch "drm/dp/mst: Validate port in drm_dp_payload_send_msg()" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

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

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
drm-dp-mst-validate-port-in-drm_dp_payload_send_msg.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "[email protected]" <[email protected]>
Date: Mon, 4 Apr 2016 19:58:47 -0400
Subject: drm/dp/mst: Validate port in drm_dp_payload_send_msg()
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From: [email protected] <[email protected]>

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.

---
 drivers/gpu/drm/drm_dp_mst_topology.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

@@ -1665,13 +1665,19 @@ static int drm_dp_payload_send_msg(struc
 	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(struc
 	kfree(txmsg);
 fail_put:
 	drm_dp_put_mst_branch_device(mstb);
+	drm_dp_put_port(port);
 	return ret;
 }
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/drm-dp-mst-restore-primary-hub-guid-on-resume.patch queue-4.4/drm-dp-mst-validate-port-in-drm_dp_payload_send_msg.patch queue-4.4/drm-i915-fix-race-condition-in-intel_dp_destroy_mst_connector.patch queue-4.4/drm-dp-mst-get-validated-port-ref-in-drm_dp_update_payload_part1.patch