drm: protect drm_master pointers in drm_lease.c

drm_file->master pointers should be protected by
drm_device.master_mutex or drm_file.master_lookup_lock when being
dereferenced.

However, in drm_lease.c, there are multiple instances where
drm_file->master is accessed and dereferenced while neither lock is
held. This makes drm_lease.c vulnerable to use-after-free bugs.

We address this issue in 2 ways:

1. Add a new drm_file_get_master() function that calls drm_master_get
on drm_file->master while holding on to
drm_file.master_lookup_lock. Since drm_master_get increments the
reference count of master, this prevents master from being freed until
we unreference it with drm_master_put.

2. 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>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210712043508.11584-6-desmondcheongzx@gmail.com

diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
index 30a2399..f00354b 100644 (file)
--- a/drivers/gpu/drm/drm_auth.c
+++ b/drivers/gpu/drm/drm_auth.c
@@ -389,6 +389,31 @@ 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;
+
+       spin_lock(&file_priv->master_lookup_lock);
+       if (!file_priv->master)
+               goto unlock;
+       master = drm_master_get(file_priv->master);
+
+unlock:
+       spin_unlock(&file_priv->master_lookup_lock);
+       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 00fb433..92eac73 100644 (file)
--- 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)
+               return true;
+       ret = _drm_lease_held_master(master, id);
+       drm_master_put(&master);
+
+       return ret;
 }
 
 /**
@@ -128,13 +137,22 @@ 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)
+               return true;
+       if (!master->lessor) {
+               ret = true;
+               goto out;
+       }
        mutex_lock(&master->dev->mode_config.idr_mutex);
        ret = _drm_lease_held_master(master, id);
        mutex_unlock(&master->dev->mode_config.idr_mutex);
+
+out:
+       drm_master_put(&master);
        return ret;
 }
 
@@ -154,10 +172,16 @@ 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)
+               return crtcs_in;
+       if (!master->lessor) {
+               crtcs_out = crtcs_in;
+               goto out;
+       }
        dev = master->dev;
 
        count_in = count_out = 0;
@@ -176,6 +200,9 @@ 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);
+
+out:
+       drm_master_put(&master);
        return crtcs_out;
 }
 
@@ -489,7 +516,7 @@ 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 +528,6 @@ 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 +539,22 @@ 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 +565,15 @@ 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 +609,7 @@ 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 +619,8 @@ out_lessee:
 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 +643,7 @@ 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 +654,7 @@ 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 +678,7 @@ 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 +698,7 @@ 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 +712,7 @@ 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 +740,7 @@ 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 +759,7 @@ 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 +769,7 @@ 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 +790,7 @@ 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 6bf8b2b..f99d341 100644 (file)
--- 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 9b82988..726cfe0 100644 (file)
--- a/include/drm/drm_file.h
+++ b/include/drm/drm_file.h
@@ -233,6 +233,12 @@ struct drm_file {
         * this only matches &drm_device.master if the master is the currently
         * active one.
         *
+        * When dereferencing this pointer, either hold struct
+        * &drm_device.master_mutex for the duration of the pointer's use, or
+        * 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>`.
         */