linux-kernel - Re: [PATCH] drm/dp_mst: Fix NULL deref in get_mst_branch_device_by_guid

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Date:   Wed, 16 Aug 2023 11:07:56 +0200
From:   Lukasz Majczak <lma@...ihalf.com>
To:     David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>
Cc:     Guenter Roeck <linux@...ck-us.net>, upstream@...ihalf.com,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH] drm/dp_mst: Fix NULL deref in get_mst_branch_device_by_guid_helper()

czw., 3 sie 2023 o 11:23 Lukasz Majczak <lma@...ihalf.com> napisał(a):
>
> Check mgr->mst_primary, before passing it to
> the get_mst_branch_device_by_guid_helper(), otherwise NULL dereference
> may occur in the call to memcpy() and cause:
>
> [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
>
> Similar check is done in e.g: drm_dp_mst_topology_get_mstb_validated().
>
> Fixes: 5e93b8208d3c ("drm/dp/mst: move GUID storage from mgr, port to only mst branch")
> Cc: <stable@...r.kernel.org> # 4.14+
> Signed-off-by: Lukasz Majczak <lma@...ihalf.com>
> ---
>  drivers/gpu/drm/display/drm_dp_mst_topology.c | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 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..703cd97b1d11 100644
> --- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> @@ -2595,19 +2595,19 @@ static struct drm_dp_mst_branch *
>  drm_dp_get_mst_branch_device_by_guid(struct drm_dp_mst_topology_mgr *mgr,
>                                      const uint8_t *guid)
>  {
> -       struct drm_dp_mst_branch *mstb;
> +       struct drm_dp_mst_branch *mstb = NULL;
>         int ret;
>
>         /* find the port by iterating down */
>         mutex_lock(&mgr->lock);
> -
> -       mstb = get_mst_branch_device_by_guid_helper(mgr->mst_primary, guid);
> -       if (mstb) {
> -               ret = drm_dp_mst_topology_try_get_mstb(mstb);
> -               if (!ret)
> -                       mstb = NULL;
> +       if (mgr->mst_primary) {
> +               mstb = get_mst_branch_device_by_guid_helper(mgr->mst_primary, guid);
> +               if (mstb) {
> +                       ret = drm_dp_mst_topology_try_get_mstb(mstb);
> +                       if (!ret)
> +                               mstb = NULL;
> +               }
>         }
> -
>         mutex_unlock(&mgr->lock);
>         return mstb;
>  }
> --
> 2.41.0.640.ga95def55d0-goog
>
Hi,

Is there anything more I should do regarding these changes?

Best regards,
Lukasz