drm/dp_mst: Check if primary mstb is null

commit 23d8003907d094f77cf959228e2248d6db819fa7 upstream.

Unfortunately drm_dp_get_mst_branch_device which is called from both
drm_dp_mst_handle_down_rep and drm_dp_mst_handle_up_rep seem to rely
on that mgr->mst_primary is not NULL, which seem to be wrong as it can be
cleared with simultaneous mode set, if probing fails or in other case.
mgr->lock mutex doesn't protect against that as it might just get
assigned to NULL right before, not simultaneously.

There are currently bugs 107738, 108616 bugs which crash in
drm_dp_get_mst_branch_device, caused by this issue.

v2: Refactored the code, as it was nicely noticed.
    Fixed Bugzilla bug numbers(second was 108616, but not 108816)
    and added links.

[changed title and added stable cc]
Signed-off-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Cc: stable@vger.kernel.org
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108616
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107738
Link: https://patchwork.freedesktop.org/patch/msgid/20181109090012.24438-1-stanislav.lisovskiy@intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index c022ab6e84bdd714901a50bf539ddee849fabeff..2a4cf6837324696202ced1f89e56822b9c941da0 100644 (file)
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -1232,6 +1232,9 @@ static struct drm_dp_mst_branch *drm_dp_get_mst_branch_device(struct drm_dp_mst_
        mutex_lock(&mgr->lock);
        mstb = mgr->mst_primary;
 
+       if (!mstb)
+               goto out;
+
        for (i = 0; i < lct - 1; i++) {
                int shift = (i % 2) ? 0 : 4;
                int port_num = (rad[i / 2] >> shift) & 0xf;