[PATCH] drm/dp_mst: Fix NULL deref in drm_dp_get_one_sb_msg()

[PATCH] drm/dp_mst: Fix NULL deref in drm_dp_get_one_sb_msg()

[Lyude Paul]

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>
---
 drivers/gpu/drm/drm_dp_mst_topology.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index 1ff49547b2e8..8751278b3941 100644
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -3703,7 +3703,8 @@ 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);
-- 
2.25.1

Re: [PATCH] drm/dp_mst: Fix NULL deref in drm_dp_get_one_sb_msg()

[Sean Paul]

On Mon, Apr 6, 2020 at 3:34 PM Lyude Paul <lyude@redhat.com> wrote:
>
> 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.

Ugggh, what a fail! I found this in Feb and posted the patch in
20200218171522.GF253734@art_vandelay. I had to migrate my workstation
due to WFH order and didn't apply the patch before pushing. Messy
messy messy.

Thanks for fixing!

Reviewed-by: Sean Paul <sean@poorly.run>

>
> 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>
> ---
>  drivers/gpu/drm/drm_dp_mst_topology.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
> index 1ff49547b2e8..8751278b3941 100644
> --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> @@ -3703,7 +3703,8 @@ 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);
> --
> 2.25.1
>