4.4-stable review patch. If anyone has any objections, please let me know.
------------------
From: Stanislav Lisovskiy <stanislav.lisovskiy@xxxxxxxxx>
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@xxxxxxxxxx>
Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
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@xxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
drivers/gpu/drm/drm_dp_mst_topology.c | 3 +++
1 file changed, 3 insertions(+)
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -1225,6 +1225,9 @@ static struct drm_dp_mst_branch *drm_dp_
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;
