linux-kernel - Re: [PATCH 1/2] drm: Add a locked version of drm_is_current

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Date:   Mon, 14 Jun 2021 20:09:22 +0100
From:   Emil Velikov <emil.l.velikov@...il.com>
To:     Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>
Cc:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Dave Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>,
        ML dri-devel <dri-devel@...ts.freedesktop.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        skhan@...uxfoundation.org,
        linux-kernel-mentees@...ts.linuxfoundation.org
Subject: Re: [PATCH 1/2] drm: Add a locked version of drm_is_current_master

On Sat, 12 Jun 2021 at 13:55, Desmond Cheong Zhi Xi
<desmondcheongzx@...il.com> wrote:
>
> While checking the master status of the DRM file in
> drm_is_current_master(), the device's master mutex should be
> held. Without the mutex, the pointer fpriv->master may be freed
> concurrently by another process calling drm_setmaster_ioctl(). This
> could lead to use-after-free errors when the pointer is subsequently
> dereferenced in drm_lease_owner().
>
> The callers of drm_is_current_master() from drm_auth.c hold the
> device's master mutex, but external callers do not. Hence, we implement
> drm_is_current_master_locked() to be used within drm_auth.c, and
> modify drm_is_current_master() to grab the device's master mutex
> before checking the master status.
>
> Reported-by: Daniel Vetter <daniel.vetter@...ll.ch>
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>

Reviewed-by: Emil Velikov <emil.l.velikov@...il.com>

-Emil