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From: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
To: maarten.lankhorst@linux.intel.com, mripard@kernel.org,
	tzimmermann@suse.de, airlied@linux.ie, 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 4/9] drm: fix potential null ptr dereferences in drm_{auth,ioctl}
Date: Wed, 18 Aug 2021 23:37:08 +0800	[thread overview]
Message-ID: <53a63ac8-f2de-91f7-4e0f-20b0f3f61d52@gmail.com> (raw)
In-Reply-To: <YRzcuiQrLFsWowas@phenom.ffwll.local>

On 18/8/21 6:11 pm, Daniel Vetter wrote:
> On Wed, Aug 18, 2021 at 03:38:19PM +0800, Desmond Cheong Zhi Xi wrote:
>> There are three areas where we dereference struct drm_master without
>> checking if the pointer is non-NULL.
>>
>> 1. drm_getmagic is called from the ioctl_handler. Since
>> DRM_IOCTL_GET_MAGIC has no ioctl flags, drm_getmagic is run without
>> any check that drm_file.master has been set.
>>
>> 2. Similarly, drm_getunique is called from the ioctl_handler, but
>> DRM_IOCTL_GET_UNIQUE has no ioctl flags. So there is no guarantee that
>> drm_file.master has been set.
> 
> I think the above two are impossible, due to the refcounting rules for
> struct file.
> 

Right, will drop those two parts from the patch.

>> 3. drm_master_release can also be called without having a
>> drm_file.master set. Here is one error path:
>>    drm_open():
>>      drm_open_helper():
>>        drm_master_open():
>>          drm_new_set_master(); <--- returns -ENOMEM,
>>                                     drm_file.master not set
>>        drm_file_free():
>>          drm_master_release(); <--- NULL ptr dereference
>>                                     (file_priv->master->magic_map)
>>
>> Fix these by checking if the master pointers are NULL before use.
>>
>> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
>> ---
>>   drivers/gpu/drm/drm_auth.c  | 16 ++++++++++++++--
>>   drivers/gpu/drm/drm_ioctl.c |  5 +++++
>>   2 files changed, 19 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
>> index f9267b21556e..b7230604496b 100644
>> --- a/drivers/gpu/drm/drm_auth.c
>> +++ b/drivers/gpu/drm/drm_auth.c
>> @@ -95,11 +95,18 @@ EXPORT_SYMBOL(drm_is_current_master);
>>   int drm_getmagic(struct drm_device *dev, void *data, struct drm_file *file_priv)
>>   {
>>   	struct drm_auth *auth = data;
>> +	struct drm_master *master;
>>   	int ret = 0;
>>   
>>   	mutex_lock(&dev->master_mutex);
>> +	master = file_priv->master;
>> +	if (!master) {
>> +		mutex_unlock(&dev->master_mutex);
>> +		return -EINVAL;
>> +	}
>> +
>>   	if (!file_priv->magic) {
>> -		ret = idr_alloc(&file_priv->master->magic_map, file_priv,
>> +		ret = idr_alloc(&master->magic_map, file_priv,
>>   				1, 0, GFP_KERNEL);
>>   		if (ret >= 0)
>>   			file_priv->magic = ret;
>> @@ -355,8 +362,12 @@ void drm_master_release(struct drm_file *file_priv)
>>   
>>   	mutex_lock(&dev->master_mutex);
>>   	master = file_priv->master;
>> +
>> +	if (!master)
>> +		goto unlock;
> 
> This is a bit convoluted, since we're in the single-threaded release path
> we don't need any locking for file_priv related things. Therefore we can
> pull the master check out and just directly return.
> 
> But since it's a bit surprising maybe a comment that this can happen when
> drm_master_open in drm_open_helper fails?
> 

Sounds good. This can actually also happen in the failure path of 
mock_drm_getfile if anon_inode_getfile fails. I'll leave a short note 
about both of them.

> Another option, and maybe cleaner, would be to move the drm_master_release
> from drm_file_free into drm_close_helper. That would be fully symmetrical
> and should also fix the bug here?
> -Daniel
> 
Hmmm maybe the first option to move the check out of the lock might be 
better. If I'm not wrong, we would otherwise also need to move 
drm_master_release into drm_client_close.

> 
>> +
>>   	if (file_priv->magic)
>> -		idr_remove(&file_priv->master->magic_map, file_priv->magic);
>> +		idr_remove(&master->magic_map, file_priv->magic);
>>   
>>   	if (!drm_is_current_master_locked(file_priv))
>>   		goto out;
>> @@ -379,6 +390,7 @@ void drm_master_release(struct drm_file *file_priv)
>>   		drm_master_put(&file_priv->master);
>>   		spin_unlock(&dev->master_lookup_lock);
>>   	}
>> +unlock:
>>   	mutex_unlock(&dev->master_mutex);
>>   }
>>   
>> diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
>> index 26f3a9ede8fe..4d029d3061d9 100644
>> --- a/drivers/gpu/drm/drm_ioctl.c
>> +++ b/drivers/gpu/drm/drm_ioctl.c
>> @@ -121,6 +121,11 @@ int drm_getunique(struct drm_device *dev, void *data,
>>   
>>   	mutex_lock(&dev->master_mutex);
>>   	master = file_priv->master;
>> +	if (!master) {
>> +		mutex_unlock(&dev->master_mutex);
>> +		return -EINVAL;
>> +	}
>> +
>>   	if (u->unique_len >= master->unique_len) {
>>   		if (copy_to_user(u->unique, master->unique, master->unique_len)) {
>>   			mutex_unlock(&dev->master_mutex);
>> -- 
>> 2.25.1
>>
> 


  reply	other threads:[~2021-08-18 15:37 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
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 [this message]
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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