linux-kernel - Re: [PATCH] drm: bridge: cdns-mhdp8546: Fix a NULL pointer dereference in cdns_mhdp_atomic

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Date:   Tue, 4 Jan 2022 15:17:32 +0100
From:   Robert Foss <robert.foss@...aro.org>
To:     Zhou Qingyang <zhou1615@....edu>
Cc:     kjlu@....edu, Andrzej Hajda <a.hajda@...sung.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Dave Airlie <airlied@...hat.com>,
        Lyude Paul <lyude@...hat.com>,
        Thierry Reding <treding@...dia.com>,
        Parshuram Thombare <pthombar@...ence.com>,
        Julia Lawall <Julia.Lawall@...ia.fr>,
        Jiapeng Chong <jiapeng.chong@...ux.alibaba.com>,
        Yu Kuai <yukuai3@...wei.com>,
        Tomi Valkeinen <tomi.valkeinen@...com>,
        Quentin Schulz <quentin.schulz@...e-electrons.com>,
        Yuti Amonkar <yamonkar@...ence.com>,
        Swapnil Jakhade <sjakhade@...ence.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm: bridge: cdns-mhdp8546: Fix a NULL pointer
 dereference in cdns_mhdp_atomic_enable()

Hey Zhou,

Thanks for submitting this patch.

On Tue, 30 Nov 2021 at 14:11, Zhou Qingyang <zhou1615@....edu> wrote:
>
> In cdns_mhdp_atomic_enable(), the return value of drm_mode_duplicate()
> is assigned to mhdp_state->current_mode and used in drm_mode_set_name().
> There is a dereference of it in drm_mode_set_name(), which could lead
> to a NULL pointer dereference on failure of drm_mode_duplicate().
>
> Fix this bug by adding a check of mhdp_state->current_mode.
>
> This bug was found by a static analyzer. The analysis employs
> differential checking to identify inconsistent security operations
> (e.g., checks or kfrees) between two code paths and confirms that the
> inconsistent operations are not recovered in the current function or
> the callers, so they constitute bugs.
>
> Note that, as a bug found by static analysis, it can be a false
> positive or hard to trigger. Multiple researchers have cross-reviewed
> the bug.
>
> Builds with CONFIG_DRM_CDNS_MHDP8546=m show no new warnings,
> and our static analyzer no longer warns about this code.
>
> Fixes: fb43aa0acdfd ("drm: bridge: Add support for Cadence MHDP8546 DPI/DP bridge")
> Signed-off-by: Zhou Qingyang <zhou1615@....edu>
> ---
>  drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c b/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
> index 5530fbf64f1e..347fbecf76a4 100644
> --- a/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
> +++ b/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
> @@ -2040,6 +2040,11 @@ static void cdns_mhdp_atomic_enable(struct drm_bridge *bridge,
>         mhdp_state = to_cdns_mhdp_bridge_state(new_state);
>
>         mhdp_state->current_mode = drm_mode_duplicate(bridge->dev, mode);
> +       if (!mhdp_state->current_mode) {
> +               ret = -ENOMEM;
> +               goto out;
> +       }
> +

This appears to be a problem that is handled in other drivers, but the
solution here does strike me as good.

The out-label will schedule modeset_retry_work to be executed if
ret==-ENOMEM. If drm_mode_duplicate() fails, we've had a memory
allocation issue, and failing is probably the correct solution here.
However cdns_mhdp_atomic_enable() does allow for signalling failures.

>         drm_mode_set_name(mhdp_state->current_mode);
>
>         dev_dbg(mhdp->dev, "%s: Enabling mode %s\n", __func__, mode->name);
> --
> 2.25.1
>