[PATCH] drm/mst: Fix MST sideband up-reply failure handling

[Lyude Paul]

Patch mostly looks good to me, one comment below

On Fri, 2019-05-24 at 00:24 +0300, Imre Deak wrote:
> Fix the breakage resulting in the stacktrace below, due to tx queue
> being full when trying to send an up-reply. txmsg->seqno is -1 in this
> case leading to a corruption of the mstb object by
> 
> 	txmsg->dst->tx_slots[txmsg->seqno] = NULL;
> 
> in process_single_up_tx_qlock().
> 
> [  +0,005162] [drm:process_single_tx_qlock [drm_kms_helper]]
> set_hdr_from_dst_qlock: failed to find slot
> [  +0,000015] [drm:drm_dp_send_up_ack_reply.constprop.19 [drm_kms_helper]]
> failed to send msg in q -11
> [  +0,000939] BUG: kernel NULL pointer dereference, address:
> 00000000000005a0
> [  +0,006982] #PF: supervisor write access in kernel mode
> [  +0,005223] #PF: error_code(0x0002) - not-present page
> [  +0,005135] PGD 0 P4D 0
> [  +0,002581] Oops: 0002 [#1] PREEMPT SMP NOPTI
> [  +0,004359] CPU: 1 PID: 1200 Comm: kworker/u16:3 Tainted:
> G     U            5.2.0-rc1+ #410
> [  +0,008433] Hardware name: Intel Corporation Ice Lake Client
> Platform/IceLake U DDR4 SODIMM PD RVP, BIOS ICLSFWR1.R00.3175.A00.1904261428
> 04/26/2019
> [  +0,013323] Workqueue: i915-dp i915_digport_work_func [i915]
> [  +0,005676] RIP: 0010:queue_work_on+0x19/0x70
> [  +0,004372] Code: ff ff ff 0f 1f 40 00 66 2e 0f 1f 84 00 00 00 00 00 41 56
> 49 89 f6 41 55 41 89 fd 41 54 55 53 48 89 d3 9c 5d fa e8 e7 81 0c 00 <f0> 48
> 0f ba 2b 00 73 31 45 31 e4 f7 c5 00 02 00 00 74 13 e8 cf 7f
> [  +0,018750] RSP: 0018:ffffc900007dfc50 EFLAGS: 00010006
> [  +0,005222] RAX: 0000000000000046 RBX: 00000000000005a0 RCX:
> 0000000000000001
> [  +0,007133] RDX: 000000000001b608 RSI: 0000000000000000 RDI:
> ffffffff82121972
> [  +0,007129] RBP: 0000000000000202 R08: 0000000000000000 R09:
> 0000000000000001
> [  +0,007129] R10: 0000000000000000 R11: 0000000000000000 R12:
> ffff88847bfa5096
> [  +0,007131] R13: 0000000000000010 R14: ffff88849c08f3f8 R15:
> 0000000000000000
> [  +0,007128] FS:  0000000000000000(0000) GS:ffff88849dc80000(0000)
> knlGS:0000000000000000
> [  +0,008083] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [  +0,005749] CR2: 00000000000005a0 CR3: 0000000005210006 CR4:
> 0000000000760ee0
> [  +0,007128] PKRU: 55555554
> [  +0,002722] Call Trace:
> [  +0,002458]  drm_dp_mst_handle_up_req+0x517/0x540 [drm_kms_helper]
> [  +0,006197]  ? drm_dp_mst_hpd_irq+0x5b/0x9c0 [drm_kms_helper]
> [  +0,005764]  drm_dp_mst_hpd_irq+0x5b/0x9c0 [drm_kms_helper]
> [  +0,005623]  ? intel_dp_hpd_pulse+0x205/0x370 [i915]
> [  +0,005018]  intel_dp_hpd_pulse+0x205/0x370 [i915]
> [  +0,004836]  i915_digport_work_func+0xbb/0x140 [i915]
> [  +0,005108]  process_one_work+0x245/0x610
> [  +0,004027]  worker_thread+0x37/0x380
> [  +0,003684]  ? process_one_work+0x610/0x610
> [  +0,004184]  kthread+0x119/0x130
> [  +0,003240]  ? kthread_park+0x80/0x80
> [  +0,003668]  ret_from_fork+0x24/0x50
> 
> Cc: Lyude Paul <lyude at redhat.com>
> Cc: Dave Airlie <airlied at redhat.com>
> Signed-off-by: Imre Deak <imre.deak at intel.com>
> ---
>  drivers/gpu/drm/drm_dp_mst_topology.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c
> b/drivers/gpu/drm/drm_dp_mst_topology.c
> index da1abca1b9e9..24c325f4a616 100644
> --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> @@ -1996,7 +1996,11 @@ static void process_single_up_tx_qlock(struct
> drm_dp_mst_topology_mgr *mgr,
>  	if (ret != 1)
>  		DRM_DEBUG_KMS("failed to send msg in q %d\n", ret);
>  
> -	txmsg->dst->tx_slots[txmsg->seqno] = NULL;
> +	if (txmsg->seqno != -1) {
> +		WARN_ON((unsigned)txmsg->seqno >
> +			ARRAY_SIZE(txmsg->dst->tx_slots));

Not 100% sure on this nitpick myself but, if we know that txmsg->seqno is
about to go out of bounds shouldn't we also try to take action to stop it?
like

if (!WARN_ON((unsigned)txmsg->seqno > ARRAY_SIZE(txmsg->dst->tx_slots)))
        txmsg->dst->tx_slots[txmsg->seqno] = NULL;



> +		txmsg->dst->tx_slots[txmsg->seqno] = NULL;
> +	}
>  }
>  
>  static void drm_dp_queue_down_tx(struct drm_dp_mst_topology_mgr *mgr,
-- 
Cheers,
	Lyude Paul