[PATCH v4 2/2] drm: protect drm_master pointers in drm_lease.c
From: Desmond Cheong Zhi Xi
Date: Wed Jun 23 2021 - 07:40:31 EST
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>
---
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
+ * 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