Re: [Intel-gfx] [PATCH v5 3/3] drm: protect drm_master pointers in drm_lease.c

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

On 30/6/21 12:07 am, Daniel Vetter wrote:
> On Tue, Jun 29, 2021 at 11:37:06AM +0800, Desmond Cheong Zhi Xi wrote:
>> Currently, direct copies of drm_file->master pointers should be
>> protected by drm_device.master_mutex when being dereferenced. This is
>> because drm_file->master is not invariant for the lifetime of
>> drm_file. If drm_file is not the creator of master, then
>> drm_file->is_master is false, and a call to drm_setmaster_ioctl will
>> invoke drm_new_set_master, which then allocates a new master for
>> drm_file and puts the old master.
>>
>> Thus, without holding drm_device.master_mutex, the old value of
>> drm_file->master could be freed while it is being used by another
>> concurrent process.
>>
>> In drm_lease.c, there are multiple instances where drm_file->master is
>> accessed and dereferenced while drm_device.master_mutex is not
>> held. This makes drm_lease.c vulnerable to use-after-free bugs.
>>
>> We address this issue in 3 ways:
>>
>> 1. Clarify in the kerneldoc that drm_file->master is protected by
>> drm_device.master_mutex.
>>
>> 2. Add a new drm_file_get_master() function that calls drm_master_get
>> on drm_file->master while holding on to drm_device.master_mutex. Since
>> drm_master_get increments the reference count of master, this
>> prevents master from being freed until we unreference it with
>> drm_master_put.
>>
>> 3. In each case where drm_file->master is directly accessed and
>> eventually dereferenced in drm_lease.c, we wrap the access in a call
>> to the new drm_file_get_master function, then unreference the master
>> pointer once we are done using it.
>>
>> Reported-by: Daniel Vetter <daniel.vetter@ffwll.ch>
>> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
> 
> Series looks very nice, let's see what intel-gfx-ci says. You should get a
> mail, but results are also here:
> 
> https://patchwork.freedesktop.org/series/91969/#rev2
> 
> One tiny comment below.
> 
>> ---
>>   drivers/gpu/drm/drm_auth.c  | 25 ++++++++++++
>>   drivers/gpu/drm/drm_lease.c | 77 +++++++++++++++++++++++++++----------
>>   include/drm/drm_auth.h      |  1 +
>>   include/drm/drm_file.h      | 15 ++++++--
>>   4 files changed, 95 insertions(+), 23 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
>> index ab1863c5a5a0..c36a0b72be26 100644
>> --- a/drivers/gpu/drm/drm_auth.c
>> +++ b/drivers/gpu/drm/drm_auth.c
>> @@ -384,6 +384,31 @@ struct drm_master *drm_master_get(struct drm_master *master)
>>   }
>>   EXPORT_SYMBOL(drm_master_get);
>>   
>> +/**
>> + * drm_file_get_master - reference &drm_file.master of @file_priv
>> + * @file_priv: DRM file private
>> + *
>> + * Increments the reference count of @file_priv's &drm_file.master and returns
>> + * the &drm_file.master. If @file_priv has no &drm_file.master, returns NULL.
>> + *
>> + * Master pointers returned from this function should be unreferenced using
>> + * drm_master_put().
>> + */
>> +struct drm_master *drm_file_get_master(struct drm_file *file_priv)
>> +{
>> +	struct drm_master *master = NULL;
>> +
>> +	mutex_lock(&file_priv->minor->dev->master_mutex);
>> +	if (!file_priv->master)
>> +		goto unlock;
>> +	master = drm_master_get(file_priv->master);
>> +
>> +unlock:
>> +	mutex_unlock(&file_priv->minor->dev->master_mutex);
>> +	return master;
>> +}
>> +EXPORT_SYMBOL(drm_file_get_master);
>> +
>>   static void drm_master_destroy(struct kref *kref)
>>   {
>>   	struct drm_master *master = container_of(kref, struct drm_master, refcount);
>> diff --git a/drivers/gpu/drm/drm_lease.c b/drivers/gpu/drm/drm_lease.c
>> index 00fb433bcef1..cdcc87fa9685 100644
>> --- a/drivers/gpu/drm/drm_lease.c
>> +++ b/drivers/gpu/drm/drm_lease.c
>> @@ -106,10 +106,19 @@ static bool _drm_has_leased(struct drm_master *master, int id)
>>    */
>>   bool _drm_lease_held(struct drm_file *file_priv, int id)
>>   {
>> -	if (!file_priv || !file_priv->master)
>> +	bool ret;
>> +	struct drm_master *master;
>> +
>> +	if (!file_priv)
>>   		return true;
>>   
>> -	return _drm_lease_held_master(file_priv->master, id);
>> +	master = drm_file_get_master(file_priv);
>> +	if (master == NULL)
>> +		return true;
>> +	ret = _drm_lease_held_master(master, id);
>> +	drm_master_put(&master);
>> +
>> +	return ret;
>>   }
>>   
>>   /**
>> @@ -128,13 +137,20 @@ bool drm_lease_held(struct drm_file *file_priv, int id)
>>   	struct drm_master *master;
>>   	bool ret;
>>   
>> -	if (!file_priv || !file_priv->master || !file_priv->master->lessor)
>> +	if (!file_priv)
>>   		return true;
>>   
>> -	master = file_priv->master;
>> +	master = drm_file_get_master(file_priv);
>> +	if (master == NULL)
>> +		return true;
>> +	if (!master->lessor) {
>> +		drm_master_put(&master);
>> +		return true;
>> +	}
>>   	mutex_lock(&master->dev->mode_config.idr_mutex);
>>   	ret = _drm_lease_held_master(master, id);
>>   	mutex_unlock(&master->dev->mode_config.idr_mutex);
>> +	drm_master_put(&master);
>>   	return ret;
>>   }
>>   
>> @@ -154,10 +170,16 @@ uint32_t drm_lease_filter_crtcs(struct drm_file *file_priv, uint32_t crtcs_in)
>>   	int count_in, count_out;
>>   	uint32_t crtcs_out = 0;
>>   
>> -	if (!file_priv || !file_priv->master || !file_priv->master->lessor)
>> +	if (!file_priv)
>>   		return crtcs_in;
>>   
>> -	master = file_priv->master;
>> +	master = drm_file_get_master(file_priv);
>> +	if (master == NULL)
>> +		return crtcs_in;
>> +	if (!master->lessor) {
>> +		drm_master_put(&master);
>> +		return crtcs_in;
>> +	}
>>   	dev = master->dev;
>>   
>>   	count_in = count_out = 0;
>> @@ -176,6 +198,7 @@ uint32_t drm_lease_filter_crtcs(struct drm_file *file_priv, uint32_t crtcs_in)
>>   		count_in++;
>>   	}
>>   	mutex_unlock(&master->dev->mode_config.idr_mutex);
>> +	drm_master_put(&master);
>>   	return crtcs_out;
>>   }
>>   
>> @@ -489,7 +512,7 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
>>   	size_t object_count;
>>   	int ret = 0;
>>   	struct idr leases;
>> -	struct drm_master *lessor = lessor_priv->master;
>> +	struct drm_master *lessor;
>>   	struct drm_master *lessee = NULL;
>>   	struct file *lessee_file = NULL;
>>   	struct file *lessor_file = lessor_priv->filp;
>> @@ -501,12 +524,6 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
>>   	if (!drm_core_check_feature(dev, DRIVER_MODESET))
>>   		return -EOPNOTSUPP;
>>   
>> -	/* Do not allow sub-leases */
>> -	if (lessor->lessor) {
>> -		DRM_DEBUG_LEASE("recursive leasing not allowed\n");
>> -		return -EINVAL;
>> -	}
>> -
>>   	/* need some objects */
>>   	if (cl->object_count == 0) {
>>   		DRM_DEBUG_LEASE("no objects in lease\n");
>> @@ -518,12 +535,22 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
>>   		return -EINVAL;
>>   	}
>>   
>> +	lessor = drm_file_get_master(lessor_priv);
>> +	/* Do not allow sub-leases */
>> +	if (lessor->lessor) {
>> +		DRM_DEBUG_LEASE("recursive leasing not allowed\n");
>> +		ret = -EINVAL;
>> +		goto out_lessor;
>> +	}
>> +
>>   	object_count = cl->object_count;
>>   
>>   	object_ids = memdup_user(u64_to_user_ptr(cl->object_ids),
>>   			array_size(object_count, sizeof(__u32)));
>> -	if (IS_ERR(object_ids))
>> -		return PTR_ERR(object_ids);
>> +	if (IS_ERR(object_ids)) {
>> +		ret = PTR_ERR(object_ids);
>> +		goto out_lessor;
>> +	}
>>   
>>   	idr_init(&leases);
>>   
>> @@ -534,14 +561,15 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
>>   	if (ret) {
>>   		DRM_DEBUG_LEASE("lease object lookup failed: %i\n", ret);
>>   		idr_destroy(&leases);
>> -		return ret;
>> +		goto out_lessor;
>>   	}
>>   
>>   	/* Allocate a file descriptor for the lease */
>>   	fd = get_unused_fd_flags(cl->flags & (O_CLOEXEC | O_NONBLOCK));
>>   	if (fd < 0) {
>>   		idr_destroy(&leases);
>> -		return fd;
>> +		ret = fd;
>> +		goto out_lessor;
>>   	}
>>   
>>   	DRM_DEBUG_LEASE("Creating lease\n");
>> @@ -577,6 +605,7 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
>>   	/* Hook up the fd */
>>   	fd_install(fd, lessee_file);
>>   
>> +	drm_master_put(&lessor);
>>   	DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl succeeded\n");
>>   	return 0;
>>   
>> @@ -586,6 +615,8 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
>>   out_leases:
>>   	put_unused_fd(fd);
>>   
>> +out_lessor:
>> +	drm_master_put(&lessor);
>>   	DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl failed: %d\n", ret);
>>   	return ret;
>>   }
>> @@ -608,7 +639,7 @@ int drm_mode_list_lessees_ioctl(struct drm_device *dev,
>>   	struct drm_mode_list_lessees *arg = data;
>>   	__u32 __user *lessee_ids = (__u32 __user *) (uintptr_t) (arg->lessees_ptr);
>>   	__u32 count_lessees = arg->count_lessees;
>> -	struct drm_master *lessor = lessor_priv->master, *lessee;
>> +	struct drm_master *lessor, *lessee;
>>   	int count;
>>   	int ret = 0;
>>   
>> @@ -619,6 +650,7 @@ int drm_mode_list_lessees_ioctl(struct drm_device *dev,
>>   	if (!drm_core_check_feature(dev, DRIVER_MODESET))
>>   		return -EOPNOTSUPP;
>>   
>> +	lessor = drm_file_get_master(lessor_priv);
>>   	DRM_DEBUG_LEASE("List lessees for %d\n", lessor->lessee_id);
>>   
>>   	mutex_lock(&dev->mode_config.idr_mutex);
>> @@ -642,6 +674,7 @@ int drm_mode_list_lessees_ioctl(struct drm_device *dev,
>>   		arg->count_lessees = count;
>>   
>>   	mutex_unlock(&dev->mode_config.idr_mutex);
>> +	drm_master_put(&lessor);
>>   
>>   	return ret;
>>   }
>> @@ -661,7 +694,7 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev,
>>   	struct drm_mode_get_lease *arg = data;
>>   	__u32 __user *object_ids = (__u32 __user *) (uintptr_t) (arg->objects_ptr);
>>   	__u32 count_objects = arg->count_objects;
>> -	struct drm_master *lessee = lessee_priv->master;
>> +	struct drm_master *lessee;
>>   	struct idr *object_idr;
>>   	int count;
>>   	void *entry;
>> @@ -675,6 +708,7 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev,
>>   	if (!drm_core_check_feature(dev, DRIVER_MODESET))
>>   		return -EOPNOTSUPP;
>>   
>> +	lessee = drm_file_get_master(lessee_priv);
>>   	DRM_DEBUG_LEASE("get lease for %d\n", lessee->lessee_id);
>>   
>>   	mutex_lock(&dev->mode_config.idr_mutex);
>> @@ -702,6 +736,7 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev,
>>   		arg->count_objects = count;
>>   
>>   	mutex_unlock(&dev->mode_config.idr_mutex);
>> +	drm_master_put(&lessee);
>>   
>>   	return ret;
>>   }
>> @@ -720,7 +755,7 @@ int drm_mode_revoke_lease_ioctl(struct drm_device *dev,
>>   				void *data, struct drm_file *lessor_priv)
>>   {
>>   	struct drm_mode_revoke_lease *arg = data;
>> -	struct drm_master *lessor = lessor_priv->master;
>> +	struct drm_master *lessor;
>>   	struct drm_master *lessee;
>>   	int ret = 0;
>>   
>> @@ -730,6 +765,7 @@ int drm_mode_revoke_lease_ioctl(struct drm_device *dev,
>>   	if (!drm_core_check_feature(dev, DRIVER_MODESET))
>>   		return -EOPNOTSUPP;
>>   
>> +	lessor = drm_file_get_master(lessor_priv);
>>   	mutex_lock(&dev->mode_config.idr_mutex);
>>   
>>   	lessee = _drm_find_lessee(lessor, arg->lessee_id);
>> @@ -750,6 +786,7 @@ int drm_mode_revoke_lease_ioctl(struct drm_device *dev,
>>   
>>   fail:
>>   	mutex_unlock(&dev->mode_config.idr_mutex);
>> +	drm_master_put(&lessor);
>>   
>>   	return ret;
>>   }
>> diff --git a/include/drm/drm_auth.h b/include/drm/drm_auth.h
>> index 6bf8b2b78991..f99d3417f304 100644
>> --- a/include/drm/drm_auth.h
>> +++ b/include/drm/drm_auth.h
>> @@ -107,6 +107,7 @@ struct drm_master {
>>   };
>>   
>>   struct drm_master *drm_master_get(struct drm_master *master);
>> +struct drm_master *drm_file_get_master(struct drm_file *file_priv);
>>   void drm_master_put(struct drm_master **master);
>>   bool drm_is_current_master(struct drm_file *fpriv);
>>   
>> diff --git a/include/drm/drm_file.h b/include/drm/drm_file.h
>> index b81b3bfb08c8..e9931fca4ab7 100644
>> --- a/include/drm/drm_file.h
>> +++ b/include/drm/drm_file.h
>> @@ -226,9 +226,18 @@ struct drm_file {
>>   	/**
>>   	 * @master:
>>   	 *
>> -	 * Master this node is currently associated with. Only relevant if
>> -	 * drm_is_primary_client() returns true. Note that this only
>> -	 * matches &drm_device.master if the master is the currently active one.
>> +	 * Master this node is currently associated with. Protected by struct
>> +	 * &drm_device.master_mutex.
>> +	 *
>> +	 * Only relevant if drm_is_primary_client() returns true. Note that
>> +	 * this only matches &drm_device.master if the master is the currently
>> +	 * active one.
>> +	 *
>> +	 * When obtaining a copy of this pointer, it is recommended to either
> 
> I found this a bit confusing, since I generally don't think of
> dereferencing the pointer as "taking a copy". That's more for the entire
> datastructure when you have a memcpy() call, or kmemdup() or something
> like that. Also "it is recommended" is a bit weak if you get a
> use-after-free if you dont :-)
> 
> So instead "When dererencing this pointer either hold ... or use
> drm_file_get_master() ...."
> 
> Cheers, Daniel
> 
>> +	 * hold struct &drm_device.master_mutex for the duration of the
>> +	 * pointer's use, or to use drm_file_get_master() if struct
>> +	 * &drm_device.master_mutex is not currently held and there is no other
>> +	 * need to hold it. This prevents @master from being freed during use.
>>   	 *
>>   	 * See also @authentication and @is_master and the :ref:`section on
>>   	 * primary nodes and authentication <drm_primary_node>`.
>> -- 
>> 2.25.1
>>
> 

Hi Daniel,

Thanks for the suggestion, I'll clarify the kerneldoc accordingly.

Regarding the results from intel-gfx-ci, it seems that the patch is 
inverting the lock hierarchy for
&dev->master_mutex --> &dev->mode_config.idr_mutex

Currently the dmesg warnings share a common call trace:
drm_file_get_master+0x1b/0x70
_drm_lease_held+0x21/0x70
__drm_mode_object_find+0xd1/0xe0

Looking at the functions that lock &dev->mode_config.idr_mutex, this 
should be the only instance of this lock order inversion.

I'm thinking the call to _drm_lease_held can be moved outside of the 
&dev->mode_config.idr_mutex lock in __drm_mode_object. Any thoughts?

diff --git a/drivers/gpu/drm/drm_mode_object.c 
b/drivers/gpu/drm/drm_mode_object.c
index b26588b52795..63d35f1f98dd 100644
--- a/drivers/gpu/drm/drm_mode_object.c
+++ b/drivers/gpu/drm/drm_mode_object.c
@@ -146,16 +146,18 @@ struct drm_mode_object 
*__drm_mode_object_find(struct drm_device *dev,
         if (obj && obj->id != id)
                 obj = NULL;

-       if (obj && drm_mode_object_lease_required(obj->type) &&
-           !_drm_lease_held(file_priv, obj->id))
-               obj = NULL;
-
         if (obj && obj->free_cb) {
                 if (!kref_get_unless_zero(&obj->refcount))
                         obj = NULL;
         }
         mutex_unlock(&dev->mode_config.idr_mutex);

+       if (obj && drm_mode_object_lease_required(obj->type) &&
+               !_drm_lease_held(file_priv, obj->id)) {
+               drm_mode_object_put(obj);
+               obj = NULL;
+       }
+
         return obj;
  }

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