[PATCH v2 2/2] drm: Protect drm_master pointers in drm_lease.c

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

This patch ensures that the device's master mutex is acquired before
accessing pointers to struct drm_master that are subsequently
dereferenced. Without the mutex, the struct drm_master may be freed
concurrently by another process calling drm_setmaster_ioctl(). This
could then lead to use-after-free errors.

Reported-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
---
 drivers/gpu/drm/drm_lease.c | 58 +++++++++++++++++++++++++++----------
 1 file changed, 43 insertions(+), 15 deletions(-)

diff --git a/drivers/gpu/drm/drm_lease.c b/drivers/gpu/drm/drm_lease.c
index da4f085fc09e..3e6f689236e5 100644
--- a/drivers/gpu/drm/drm_lease.c
+++ b/drivers/gpu/drm/drm_lease.c
@@ -107,10 +107,16 @@ static bool _drm_has_leased(struct drm_master *master, int id)
  */
 bool _drm_lease_held(struct drm_file *file_priv, int id)
 {
+	bool ret;
+
 	if (!file_priv || !file_priv->master)
 		return true;
 
-	return _drm_lease_held_master(file_priv->master, id);
+	mutex_lock(&file_priv->master->dev->master_mutex);
+	ret = _drm_lease_held_master(file_priv->master, id);
+	mutex_unlock(&file_priv->master->dev->master_mutex);
+
+	return ret;
 }
 
 /**
@@ -132,10 +138,12 @@ bool drm_lease_held(struct drm_file *file_priv, int id)
 	if (!file_priv || !file_priv->master || !file_priv->master->lessor)
 		return true;
 
+	mutex_lock(&file_priv->master->dev->master_mutex);
 	master = file_priv->master;
 	mutex_lock(&master->dev->mode_config.idr_mutex);
 	ret = _drm_lease_held_master(master, id);
 	mutex_unlock(&master->dev->mode_config.idr_mutex);
+	mutex_unlock(&file_priv->master->dev->master_mutex);
 	return ret;
 }
 
@@ -158,6 +166,7 @@ uint32_t drm_lease_filter_crtcs(struct drm_file *file_priv, uint32_t crtcs_in)
 	if (!file_priv || !file_priv->master || !file_priv->master->lessor)
 		return crtcs_in;
 
+	mutex_lock(&file_priv->master->dev->master_mutex);
 	master = file_priv->master;
 	dev = master->dev;
 
@@ -177,6 +186,7 @@ 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);
+	mutex_unlock(&file_priv->master->dev->master_mutex);
 	return crtcs_out;
 }
 
@@ -490,7 +500,7 @@ 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;
@@ -502,12 +512,6 @@ 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");
@@ -519,12 +523,23 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
 		return -EINVAL;
 	}
 
+	mutex_lock(&dev->master_mutex);
+	lessor = lessor_priv->master;
+	/* Do not allow sub-leases */
+	if (lessor->lessor) {
+		DRM_DEBUG_LEASE("recursive leasing not allowed\n");
+		ret = -EINVAL;
+		goto unlock;
+	}
+
 	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 unlock;
+	}
 
 	idr_init(&leases);
 
@@ -535,14 +550,15 @@ 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 unlock;
 	}
 
 	/* 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 unlock;
 	}
 
 	DRM_DEBUG_LEASE("Creating lease\n");
@@ -578,6 +594,7 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
 	/* Hook up the fd */
 	fd_install(fd, lessee_file);
 
+	mutex_unlock(&dev->master_mutex);
 	DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl succeeded\n");
 	return 0;
 
@@ -587,6 +604,8 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
 out_leases:
 	put_unused_fd(fd);
 
+unlock:
+	mutex_unlock(&dev->master_mutex);
 	DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl failed: %d\n", ret);
 	return ret;
 }
@@ -609,7 +628,7 @@ 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;
 
@@ -620,6 +639,8 @@ int drm_mode_list_lessees_ioctl(struct drm_device *dev,
 	if (!drm_core_check_feature(dev, DRIVER_MODESET))
 		return -EOPNOTSUPP;
 
+	mutex_lock(&dev->master_mutex);
+	lessor = lessor_priv->master;
 	DRM_DEBUG_LEASE("List lessees for %d\n", lessor->lessee_id);
 
 	mutex_lock(&dev->mode_config.idr_mutex);
@@ -643,6 +664,7 @@ int drm_mode_list_lessees_ioctl(struct drm_device *dev,
 		arg->count_lessees = count;
 
 	mutex_unlock(&dev->mode_config.idr_mutex);
+	mutex_unlock(&dev->master_mutex);
 
 	return ret;
 }
@@ -662,7 +684,7 @@ 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;
@@ -676,6 +698,8 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev,
 	if (!drm_core_check_feature(dev, DRIVER_MODESET))
 		return -EOPNOTSUPP;
 
+	mutex_lock(&dev->master_mutex);
+	lessee = lessee_priv->master;
 	DRM_DEBUG_LEASE("get lease for %d\n", lessee->lessee_id);
 
 	mutex_lock(&dev->mode_config.idr_mutex);
@@ -703,6 +727,7 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev,
 		arg->count_objects = count;
 
 	mutex_unlock(&dev->mode_config.idr_mutex);
+	mutex_unlock(&dev->master_mutex);
 
 	return ret;
 }
@@ -721,7 +746,7 @@ 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;
 
@@ -731,8 +756,10 @@ int drm_mode_revoke_lease_ioctl(struct drm_device *dev,
 	if (!drm_core_check_feature(dev, DRIVER_MODESET))
 		return -EOPNOTSUPP;
 
+	mutex_lock(&dev->master_mutex);
 	mutex_lock(&dev->mode_config.idr_mutex);
 
+	lessor = lessor_priv->master;
 	lessee = _drm_find_lessee(lessor, arg->lessee_id);
 
 	/* No such lessee */
@@ -751,6 +778,7 @@ int drm_mode_revoke_lease_ioctl(struct drm_device *dev,
 
 fail:
 	mutex_unlock(&dev->mode_config.idr_mutex);
+	mutex_unlock(&dev->master_mutex);
 
 	return ret;
 }
-- 
2.25.1