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

[Desmond Cheong Zhi Xi]

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@xxxxxxxx>
> > Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
>
> 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;
 }