Re: [Intel-gfx] [PATCH v3 4/9] drm: fix potential null ptr dereferences in drm_{auth, ioctl}

[Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>]

On 19/8/21 12:33 am, Daniel Vetter wrote:
> On Wed, Aug 18, 2021 at 5:37 PM Desmond Cheong Zhi Xi
> <desmondcheongzx@gmail.com> wrote:
>>
>> 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.
> 
> Do we have to?
> 
> If I haven't missed anything, the drm_client stuff only calls
> drm_file_alloc and doesn't set up a master. So this should work?
> -Daniel
> 

Ahhhh ok yes, I understand what's going on better now. I think you're 
right, moving drm_master_release into drm_close_helper should fix all 
the places it can go wrong.

>>
>>>
>>>> +
>>>>       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
>>>>
>>>
>>
> 
>