drm/dp_mst: Fix NULL deref in drm_dp_get_one_sb_msg()

While we don't need this function to store an mstb anywhere for UP
requests since we process them asynchronously, we do need to make sure
that we don't try to write to **mstb for UP requests otherwise we'll
cause a NULL pointer deref:

    RIP: 0010:drm_dp_get_one_sb_msg+0x4b/0x460 [drm_kms_helper]
    Call Trace:
     ? vprintk_emit+0x16a/0x230
     ? drm_dp_mst_hpd_irq+0x133/0x1010 [drm_kms_helper]
     drm_dp_mst_hpd_irq+0x133/0x1010 [drm_kms_helper]
     ? __drm_dbg+0x87/0x90 [drm]
     ? intel_dp_hpd_pulse+0x24b/0x400 [i915]
     intel_dp_hpd_pulse+0x24b/0x400 [i915]
     i915_digport_work_func+0xd6/0x160 [i915]
     process_one_work+0x1a9/0x370
     worker_thread+0x4d/0x3a0
     kthread+0xf9/0x130
     ? process_one_work+0x370/0x370
     ? kthread_park+0x90/0x90
     ret_from_fork+0x35/0x40

So, fix this.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Fixes: fbc821c4a506 ("drm/mst: Support simultaneous down replies")
Cc: Wayne Lin <Wayne.Lin@amd.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Wayne Lin <waynelin@amd.com>
Cc: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200406193352.1245985-1-lyude@redhat.com
Reviewed-by: Sean Paul <sean@poorly.run>

diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index 3331de086580b952a24bcc5df9592cef2009ef5a..6994076cbdc7e2ff766e3a9488004cc5e5bf20e8 100644 (file)
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -3702,7 +3702,8 @@ static bool drm_dp_get_one_sb_msg(struct drm_dp_mst_topology_mgr *mgr, bool up,
        int basereg = up ? DP_SIDEBAND_MSG_UP_REQ_BASE :
                           DP_SIDEBAND_MSG_DOWN_REP_BASE;
 
-       *mstb = NULL;
+       if (!up)
+               *mstb = NULL;
        *seqno = -1;
 
        len = min(mgr->max_dpcd_transaction_bytes, 16);