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From: Daniel Vetter <daniel@ffwll.ch>
To: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Cc: maarten.lankhorst@linux.intel.com, mripard@kernel.org,
	tzimmermann@suse.de, airlied@linux.ie, daniel@ffwll.ch,
	sumit.semwal@linaro.org, christian.koenig@amd.com,
	axboe@kernel.dk, oleg@redhat.com, tglx@linutronix.de,
	dvyukov@google.com, walter-zh.wu@mediatek.com,
	dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
	intel-gfx@lists.freedesktop.org, linux-media@vger.kernel.org,
	linaro-mm-sig@lists.linaro.org, skhan@linuxfoundation.org,
	gregkh@linuxfoundation.org,
	linux-kernel-mentees@lists.linuxfoundation.org
Subject: Re: [PATCH v3 3/9] drm: check for null master in drm_is_current_master_locked
Date: Wed, 18 Aug 2021 12:04:06 +0200	[thread overview]
Message-ID: <YRzbFnkricE65Hn5@phenom.ffwll.local> (raw)
In-Reply-To: <20210818073824.1560124-4-desmondcheongzx@gmail.com>

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@gmail.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

  reply	other threads:[~2021-08-18 10:04 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-18  7:38 [PATCH v3 0/9] drm, kernel: update locking for DRM Desmond Cheong Zhi Xi
2021-08-18  7:38 ` [PATCH v3 1/9] drm: move master_lookup_lock into drm_device Desmond Cheong Zhi Xi
2021-08-18  7:38 ` [PATCH v3 2/9] drm: hold master_lookup_lock when releasing a drm_file's master Desmond Cheong Zhi Xi
2021-08-18 10:05   ` Daniel Vetter
2021-08-18 14:50     ` Desmond Cheong Zhi Xi
2021-08-18  7:38 ` [PATCH v3 3/9] drm: check for null master in drm_is_current_master_locked Desmond Cheong Zhi Xi
2021-08-18 10:04   ` Daniel Vetter [this message]
2021-08-18  7:38 ` [PATCH v3 4/9] drm: fix potential null ptr dereferences in drm_{auth,ioctl} Desmond Cheong Zhi Xi
2021-08-18 10:11   ` Daniel Vetter
2021-08-18 15:37     ` Desmond Cheong Zhi Xi
2021-08-18 16:33       ` [Intel-gfx] [PATCH v3 4/9] drm: fix potential null ptr dereferences in drm_{auth, ioctl} Daniel Vetter
2021-08-19  5:31         ` Desmond Cheong Zhi Xi
2021-08-18  7:38 ` [PATCH v3 5/9] drm: protect magic_map,unique{_len} with master_lookup_lock Desmond Cheong Zhi Xi
2021-08-18 10:43   ` Daniel Vetter
2021-08-18  7:38 ` [PATCH v3 6/9] drm: convert drm_device.master_mutex into a rwsem Desmond Cheong Zhi Xi
2021-08-18  7:38 ` [PATCH v3 7/9] drm: update global mutex lock in the ioctl handler Desmond Cheong Zhi Xi
2021-08-18 11:02   ` Daniel Vetter
2021-08-19 10:52     ` Desmond Cheong Zhi Xi
2021-08-19 11:21       ` [Intel-gfx] " Daniel Vetter
2021-08-18  7:38 ` [PATCH v3 8/9] kernel: export task_work_add Desmond Cheong Zhi Xi
2021-08-18 11:06   ` Daniel Vetter
2021-08-19  9:26   ` Christoph Hellwig
2021-08-19  9:40     ` Desmond Cheong Zhi Xi
2021-08-18  7:38 ` [PATCH v3 9/9] drm: avoid races with modesetting rights Desmond Cheong Zhi Xi

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