Re: [PATCH v8 4/7] drm: avoid races with modesetting rights

[Desmond Cheong Zhi Xi]

On 26/8/21 8:59 pm, Daniel Vetter wrote:
> On Thu, Aug 26, 2021 at 10:01:19AM +0800, Desmond Cheong Zhi Xi wrote:
> > In drm_client_modeset.c and drm_fb_helper.c,
> > drm_master_internal_{acquire,release} are used to avoid races with DRM
> > userspace. These functions hold onto drm_device.master_rwsem while
> > committing, and bail if there's already a master.
> >
> > However, there are other places where modesetting rights can race. A
> > time-of-check-to-time-of-use error can occur if an ioctl that changes
> > the modeset has its rights revoked after it validates its permissions,
> > but before it completes.
> >
> > There are four places where modesetting permissions can change:
> >
> > - DROP_MASTER ioctl removes rights for a master and its leases
> >
> > - REVOKE_LEASE ioctl revokes rights for a specific lease
> >
> > - SET_MASTER ioctl sets the device master if the master role hasn't
> > been acquired yet
> >
> > - drm_open which can create a new master for a device if one does not
> > currently exist
> >
> > These races can be avoided using drm_device.master_rwsem: users that
> > perform modesetting should hold a read lock on the new
> > drm_device.master_rwsem, and users that change these permissions
> > should hold a write lock.
> >
> > To avoid deadlocks with master_rwsem, for ioctls that need to check
> > for modesetting permissions, but also need to hold a write lock on
> > master_rwsem to protect some other attribute (or recurses to some
> > function that holds a write lock, like drm_mode_create_lease_ioctl
> > which eventually calls drm_master_open), we remove the DRM_MASTER flag
> > and push the master_rwsem lock and permissions check into the ioctl.
> >
> > Reported-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> > Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
> > ---
> >   drivers/gpu/drm/drm_auth.c  |  4 ++++
> >   drivers/gpu/drm/drm_ioctl.c | 20 +++++++++++++++-----
> >   drivers/gpu/drm/drm_lease.c | 35 ++++++++++++++++++++++++-----------
> >   include/drm/drm_device.h    |  5 +++++
> >   4 files changed, 48 insertions(+), 16 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
> > index 73ade0513ccb..65065f7e1499 100644
> > --- a/drivers/gpu/drm/drm_auth.c
> > +++ b/drivers/gpu/drm/drm_auth.c
> > @@ -120,6 +120,10 @@ int drm_authmagic(struct drm_device *dev, void *data,
> >   	DRM_DEBUG("%u\n", auth->magic);
> >   
> >   	down_write(&dev->master_rwsem);
> > +	if (unlikely(!drm_is_current_master(file_priv))) {
> > +		up_write(&dev->master_rwsem);
> > +		return -EACCES;
> > +	}
> >   	file = idr_find(&file_priv->master->magic_map, auth->magic);
> >   	if (file) {
> >   		file->authenticated = 1;
> > diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
> > index 158629d88319..8bea39ffc5c0 100644
> > --- a/drivers/gpu/drm/drm_ioctl.c
> > +++ b/drivers/gpu/drm/drm_ioctl.c
> > @@ -386,6 +386,10 @@ static int drm_setversion(struct drm_device *dev, void *data, struct drm_file *f
> >   	int if_version, retcode = 0;
> >   
> >   	down_write(&dev->master_rwsem);
> > +	if (unlikely(!drm_is_current_master(file_priv))) {
> > +		retcode = -EACCES;
> > +		goto unlock;
> > +	}
> >   	if (sv->drm_di_major != -1) {
> >   		if (sv->drm_di_major != DRM_IF_MAJOR ||
> >   		    sv->drm_di_minor < 0 || sv->drm_di_minor > DRM_IF_MINOR) {
> > @@ -420,8 +424,9 @@ static int drm_setversion(struct drm_device *dev, void *data, struct drm_file *f
> >   	sv->drm_di_minor = DRM_IF_MINOR;
> >   	sv->drm_dd_major = dev->driver->major;
> >   	sv->drm_dd_minor = dev->driver->minor;
> > -	up_write(&dev->master_rwsem);
> >   
> > +unlock:
> > +	up_write(&dev->master_rwsem);
> >   	return retcode;
> >   }
> >   
> > @@ -574,12 +579,12 @@ static const struct drm_ioctl_desc drm_ioctls[] = {
> >   	DRM_IOCTL_DEF(DRM_IOCTL_GET_STATS, drm_getstats, 0),
> >   	DRM_IOCTL_DEF(DRM_IOCTL_GET_CAP, drm_getcap, DRM_RENDER_ALLOW),
> >   	DRM_IOCTL_DEF(DRM_IOCTL_SET_CLIENT_CAP, drm_setclientcap, 0),
> > -	DRM_IOCTL_DEF(DRM_IOCTL_SET_VERSION, drm_setversion, DRM_MASTER),
> > +	DRM_IOCTL_DEF(DRM_IOCTL_SET_VERSION, drm_setversion, 0),
>
> Random bikeshed, if you're bored: In newer code we've given ioctl
> callbacks an _ioctl suffix, so they'r easier to spot. Could do that in a
> follow-up if you want.

Makes sense, this was a small pain point when auditing the ioctl 
functions. I'll do this on a rainier day.

> >   
> >   	DRM_IOCTL_DEF(DRM_IOCTL_SET_UNIQUE, drm_invalid_op, DRM_AUTH|DRM_MASTER|DRM_ROOT_ONLY),
> >   	DRM_IOCTL_DEF(DRM_IOCTL_BLOCK, drm_noop, DRM_AUTH|DRM_MASTER|DRM_ROOT_ONLY),
> >   	DRM_IOCTL_DEF(DRM_IOCTL_UNBLOCK, drm_noop, DRM_AUTH|DRM_MASTER|DRM_ROOT_ONLY),
> > -	DRM_IOCTL_DEF(DRM_IOCTL_AUTH_MAGIC, drm_authmagic, DRM_MASTER),
> > +	DRM_IOCTL_DEF(DRM_IOCTL_AUTH_MAGIC, drm_authmagic, 0),
> >   
> >   	DRM_LEGACY_IOCTL_DEF(DRM_IOCTL_ADD_MAP, drm_legacy_addmap_ioctl, DRM_AUTH|DRM_MASTER|DRM_ROOT_ONLY),
> >   	DRM_LEGACY_IOCTL_DEF(DRM_IOCTL_RM_MAP, drm_legacy_rmmap_ioctl, DRM_AUTH),
> > @@ -706,10 +711,10 @@ static const struct drm_ioctl_desc drm_ioctls[] = {
> >   		      DRM_RENDER_ALLOW),
> >   	DRM_IOCTL_DEF(DRM_IOCTL_CRTC_GET_SEQUENCE, drm_crtc_get_sequence_ioctl, 0),
> >   	DRM_IOCTL_DEF(DRM_IOCTL_CRTC_QUEUE_SEQUENCE, drm_crtc_queue_sequence_ioctl, 0),
> > -	DRM_IOCTL_DEF(DRM_IOCTL_MODE_CREATE_LEASE, drm_mode_create_lease_ioctl, DRM_MASTER),
> > +	DRM_IOCTL_DEF(DRM_IOCTL_MODE_CREATE_LEASE, drm_mode_create_lease_ioctl, 0),
> >   	DRM_IOCTL_DEF(DRM_IOCTL_MODE_LIST_LESSEES, drm_mode_list_lessees_ioctl, DRM_MASTER),
> >   	DRM_IOCTL_DEF(DRM_IOCTL_MODE_GET_LEASE, drm_mode_get_lease_ioctl, DRM_MASTER),
> > -	DRM_IOCTL_DEF(DRM_IOCTL_MODE_REVOKE_LEASE, drm_mode_revoke_lease_ioctl, DRM_MASTER),
> > +	DRM_IOCTL_DEF(DRM_IOCTL_MODE_REVOKE_LEASE, drm_mode_revoke_lease_ioctl, 0),
> >   };
> >   
> >   #define DRM_CORE_IOCTL_COUNT	ARRAY_SIZE( drm_ioctls )
> > @@ -776,6 +781,9 @@ long drm_ioctl_kernel(struct file *file, drm_ioctl_t *func, void *kdata,
> >   	if (unlikely(drm_core_check_feature(dev, DRIVER_LEGACY)) && !(flags & DRM_UNLOCKED))
> >   		mutex_lock(&drm_global_mutex);
> >   
> > +	if (unlikely(flags & DRM_MASTER))
> > +		down_read(&dev->master_rwsem);
> > +
> >   	retcode = drm_ioctl_permit(flags, file_priv);
> >   	if (unlikely(retcode))
> >   		goto out;
> > @@ -783,6 +791,8 @@ long drm_ioctl_kernel(struct file *file, drm_ioctl_t *func, void *kdata,
> >   	retcode = func(dev, kdata, file_priv);
> >   
> >   out:
> > +	if (unlikely(flags & DRM_MASTER))
> > +		up_read(&dev->master_rwsem);
> >   	if (unlikely(drm_core_check_feature(dev, DRIVER_LEGACY)) && !(flags & DRM_UNLOCKED))
> >   		mutex_unlock(&drm_global_mutex);
> >   	return retcode;
> > diff --git a/drivers/gpu/drm/drm_lease.c b/drivers/gpu/drm/drm_lease.c
> > index dee4f24a1808..bed6f7636cbe 100644
> > --- a/drivers/gpu/drm/drm_lease.c
> > +++ b/drivers/gpu/drm/drm_lease.c
> > @@ -500,6 +500,18 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
> >   		return -EINVAL;
> >   	}
> >   
> > +	/* Clone the lessor file to create a new file for us */
> > +	DRM_DEBUG_LEASE("Allocating lease file\n");
> > +	lessee_file = file_clone_open(lessor_file);
> > +	if (IS_ERR(lessee_file))
> > +		return PTR_ERR(lessee_file);
> > +
> > +	down_read(&dev->master_rwsem);
> > +	if (unlikely(!drm_is_current_master(lessor_priv))) {
> > +		ret = -EACCES;
> > +		goto out_file;
> > +	}
> > +
> >   	lessor = drm_file_get_master(lessor_priv);
> >   	/* Do not allow sub-leases */
> >   	if (lessor->lessor) {
> > @@ -547,14 +559,6 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
> >   		goto out_leases;
> >   	}
> >   
> > -	/* Clone the lessor file to create a new file for us */
> > -	DRM_DEBUG_LEASE("Allocating lease file\n");
> > -	lessee_file = file_clone_open(lessor_file);
> > -	if (IS_ERR(lessee_file)) {
> > -		ret = PTR_ERR(lessee_file);
> > -		goto out_lessee;
> > -	}
> > -
> >   	lessee_priv = lessee_file->private_data;
> >   	/* Change the file to a master one */
> >   	drm_master_put(&lessee_priv->master);
> > @@ -571,17 +575,19 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
> >   	fd_install(fd, lessee_file);
> >   
> >   	drm_master_put(&lessor);
>
> Hm if we're unlucky this might be the last reference (against nasty
> userspace only), and that could then perhaps collide with us still holding
> the lock. It looks like we're fine, the only thing that drm_master_put
> might be locking is
>
> 	mutex_lock(&dev->mode_config.idr_mutex);
>
> through drm_lease_destroy(). We don't care about legacy drivers and hence
> the dev->struct_mutex (plus that lock is a complete mess anyway).
>
> Maybe another patch to add a might_lock to drm_master_put, just to be
> safe?

Sounds good.

I agree that we should be safe for now, since we're taking efforts to 
make master_rwsem an outer lock, especially after we take your later 
suggestion to remove the lock from drm_file_get_master and 
drm_is_current_master.

> > +	up_read(&dev->master_rwsem);
> >   	DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl succeeded\n");
> >   	return 0;
> >   
> > -out_lessee:
> > -	drm_master_put(&lessee);
> > -
> >   out_leases:
> >   	put_unused_fd(fd);
> >   
> >   out_lessor:
> >   	drm_master_put(&lessor);
> > +
> > +out_file:
> > +	up_read(&dev->master_rwsem);
> > +	fput(lessee_file);
> >   	DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl failed: %d\n", ret);
> >   	return ret;
> >   }
> > @@ -705,6 +711,11 @@ int drm_mode_revoke_lease_ioctl(struct drm_device *dev,
> >   	if (!drm_core_check_feature(dev, DRIVER_MODESET))
> >   		return -EOPNOTSUPP;
> >   
> > +	down_write(&dev->master_rwsem);
> > +	if (unlikely(!drm_is_current_master(lessor_priv))) {
> > +		ret = -EACCES;
> > +		goto unlock;
> > +	}
> >   	lessor = drm_file_get_master(lessor_priv);
> >   	mutex_lock(&dev->mode_config.idr_mutex);
> >   
> > @@ -728,5 +739,7 @@ int drm_mode_revoke_lease_ioctl(struct drm_device *dev,
> >   	mutex_unlock(&dev->mode_config.idr_mutex);
> >   	drm_master_put(&lessor);
> >   
> > +unlock:
> > +	up_write(&dev->master_rwsem);
> >   	return ret;
> >   }
> > diff --git a/include/drm/drm_device.h b/include/drm/drm_device.h
> > index 142fb2f6e74d..7d32bb69e6db 100644
> > --- a/include/drm/drm_device.h
> > +++ b/include/drm/drm_device.h
> > @@ -151,6 +151,11 @@ struct drm_device {
> >   	 * Lock for &drm_device.master, &drm_file.was_master,
> >   	 * &drm_file.is_master, &drm_file.master, &drm_master.unique,
> >   	 * &drm_master.unique_len, and &drm_master.magic_map.
> > +	 *
> > +	 * Additionally, synchronizes modesetting rights between multiple users.
>
> Current modern drives only use this for modesetting rights, but this is
> about anything which is exclusively owned by the drm_master. So maybe
> reword to "synchronizes access rights to exclusive resources like
> modesetting access".

Right, will do.

> With that clarified:
>
> Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
>
> The other suggestions are for follow-up patches, if you feel like.
> -Daniel
>
> > +	 * Users that can change the modeset or display state must hold a read
> > +	 * lock on @master_rwsem, and users that change modesetting rights
> > +	 * should hold a write lock.
> >   	 */
> >   	struct rw_semaphore master_rwsem;
> >   
> > --
> > 2.25.1