linux-kernel - Re: [PATCH] drm/gma500/cdv: Fix a wild pointer dereference in cdv_intel_dp_get

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Message-ID: <YaZP2HzTQw1QJxOK@intel.com>
Date:   Tue, 30 Nov 2021 18:22:48 +0200
From:   Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:     Zhou Qingyang <zhou1615@....edu>
Cc:     David Airlie <airlied@...ux.ie>, kjlu@....edu,
        linux-kernel@...r.kernel.org, Zhao Yakui <yakui.zhao@...el.com>,
        dri-devel@...ts.freedesktop.org, Dave Airlie <airlied@...hat.com>,
        Alan Cox <alan@...ux.intel.com>
Subject: Re: [PATCH] drm/gma500/cdv: Fix a wild pointer dereference in
 cdv_intel_dp_get_modes()

On Tue, Nov 30, 2021 at 09:23:28PM +0800, Zhou Qingyang wrote:
> In cdv_intel_dp_get_modes(), the third return value of
> drm_mode_duplicate() is assigned to mode and used in
> drm_mode_probed_add(). drm_mode_probed_add() passes mode->head to
> list_add_tail(). list_add_tail() will further call __list_add() and
> there is a dereference of mode->head in __list_add(), which could lead
> to a wild pointer dereference on failure of drm_mode_duplicate().
> 
> Fix this bug by adding a check of 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_GMA500=m show no new warnings,
> and our static analyzer no longer warns about this code.
> 
> Fixes: d112a8163f83 ("gma500/cdv: Add eDP support")
> Signed-off-by: Zhou Qingyang <zhou1615@....edu>
> ---
>  drivers/gpu/drm/gma500/cdv_intel_dp.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/gpu/drm/gma500/cdv_intel_dp.c b/drivers/gpu/drm/gma500/cdv_intel_dp.c
> index ba6ad1466374..b389008965a9 100644
> --- a/drivers/gpu/drm/gma500/cdv_intel_dp.c
> +++ b/drivers/gpu/drm/gma500/cdv_intel_dp.c
> @@ -1773,6 +1773,9 @@ static int cdv_intel_dp_get_modes(struct drm_connector *connector)
>  		if (intel_dp->panel_fixed_mode != NULL) {
>  			struct drm_display_mode *mode;
>  			mode = drm_mode_duplicate(dev, intel_dp->panel_fixed_mode);
> +			if (!mode)
> +				return -ENOMEM;

.get_modes() isn't supposed to return negative values.

> +
>  			drm_mode_probed_add(connector, mode);
>  			return 1;
>  		}
> -- 
> 2.25.1

-- 
Ville Syrjälä
Intel