linux-kernel - Re: [PATCH v3 3/9] drm: check for null master in drm_is_current_master

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Date:   Wed, 18 Aug 2021 12:04:06 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>
Cc:     maarten.lankhorst@...ux.intel.com, mripard@...nel.org,
        tzimmermann@...e.de, airlied@...ux.ie, daniel@...ll.ch,
        sumit.semwal@...aro.org, christian.koenig@....com, axboe@...nel.dk,
        oleg@...hat.com, tglx@...utronix.de, dvyukov@...gle.com,
        walter-zh.wu@...iatek.com, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, intel-gfx@...ts.freedesktop.org,
        linux-media@...r.kernel.org, linaro-mm-sig@...ts.linaro.org,
        skhan@...uxfoundation.org, gregkh@...uxfoundation.org,
        linux-kernel-mentees@...ts.linuxfoundation.org
Subject: Re: [PATCH v3 3/9] drm: check for null master in
 drm_is_current_master_locked

On Wed, Aug 18, 2021 at 03:38:18PM +0800, Desmond Cheong Zhi Xi wrote:
> There is a window after calling drm_master_release, and before a file
> is freed, where drm_file can have is_master set to true, but both the
> drm_file and drm_device have no master.
> 
> This could result in wrongly approving permissions in
> drm_is_current_master_locked. Add a check that fpriv->master is
> non-NULl to guard against this scenario.
> 
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>

This should be impossible, drm_master_release is only called when the
struct file is released, which means all ioctls and anything else have
finished (they hold a temporary reference).

fpriv->master can change (if the drm_file becomes newly minted master and
wasnt one before through the setmaster ioctl), but it cannot become NULL
before it's completely gone from the system.
-Daniel


> ---
>  drivers/gpu/drm/drm_auth.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
> index 8c0e0dba1611..f9267b21556e 100644
> --- a/drivers/gpu/drm/drm_auth.c
> +++ b/drivers/gpu/drm/drm_auth.c
> @@ -66,7 +66,8 @@ static bool drm_is_current_master_locked(struct drm_file *fpriv)
>  	lockdep_assert_once(lockdep_is_held(&fpriv->minor->dev->master_lookup_lock) ||
>  			    lockdep_is_held(&fpriv->minor->dev->master_mutex));
>  
> -	return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
> +	return (fpriv->is_master && fpriv->master &&
> +		drm_lease_owner(fpriv->master) == fpriv->minor->dev->master);
>  }
>  
>  /**
> -- 
> 2.25.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch