On Fri, Sep 22, 2023 at 8:34 AM Lukasz Majczak <lma@xxxxxxxxxxxx> wrote: > > As drm_dp_get_mst_branch_device_by_guid() is called from > drm_dp_get_mst_branch_device_by_guid(), mstb parameter has to be checked, > otherwise NULL dereference may occur in the call to > the memcpy() and cause following: > > [12579.365869] BUG: kernel NULL pointer dereference, address: 0000000000000049 > [12579.365878] #PF: supervisor read access in kernel mode > [12579.365880] #PF: error_code(0x0000) - not-present page > [12579.365882] PGD 0 P4D 0 > [12579.365887] Oops: 0000 [#1] PREEMPT SMP NOPTI > ... > [12579.365895] Workqueue: events_long drm_dp_mst_up_req_work > [12579.365899] RIP: 0010:memcmp+0xb/0x29 > [12579.365921] Call Trace: > [12579.365927] get_mst_branch_device_by_guid_helper+0x22/0x64 > [12579.365930] drm_dp_mst_up_req_work+0x137/0x416 > [12579.365933] process_one_work+0x1d0/0x419 > [12579.365935] worker_thread+0x11a/0x289 > [12579.365938] kthread+0x13e/0x14f > [12579.365941] ? process_one_work+0x419/0x419 > [12579.365943] ? kthread_blkcg+0x31/0x31 > [12579.365946] ret_from_fork+0x1f/0x30 > > As get_mst_branch_device_by_guid_helper() is recursive, moving condition > to the first line allow to remove a similar one for step over of NULL elements > inside a loop. > > Fixes: 5e93b8208d3c ("drm/dp/mst: move GUID storage from mgr, port to only mst branch") > Cc: <stable@xxxxxxxxxxxxxxx> # 4.14+ > Signed-off-by: Lukasz Majczak <lma@xxxxxxxxxxxx> > --- > > v2->v3: > * Fixed patch description. > > drivers/gpu/drm/display/drm_dp_mst_topology.c | 6 +++--- > 1 file changed, 3 insertions(+), 3 deletions(-) > > diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c b/drivers/gpu/drm/display/drm_dp_mst_topology.c > index ed96cfcfa304..8c929ef72c72 100644 > --- a/drivers/gpu/drm/display/drm_dp_mst_topology.c > +++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c > @@ -2574,14 +2574,14 @@ static struct drm_dp_mst_branch *get_mst_branch_device_by_guid_helper( > struct drm_dp_mst_branch *found_mstb; > struct drm_dp_mst_port *port; > > + if (!mstb) > + return NULL; > + > if (memcmp(mstb->guid, guid, 16) == 0) > return mstb; > > > list_for_each_entry(port, &mstb->ports, next) { > - if (!port->mstb) > - continue; > - > found_mstb = get_mst_branch_device_by_guid_helper(port->mstb, guid); > > if (found_mstb) > -- > 2.42.0.515.g380fc7ccd1-goog > Reviewed-by: Radoslaw Biernacki <rad@xxxxxxxxxxxx>