[PATCH AUTOSEL 5.10 020/176] drm: protect drm_master pointers in drm_lease.c

Sasha Levin
2021-09-09T12:34:47Z

From: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>

[ Upstream commit 56f0729a510f92151682ff6c89f69724d5595d6e ]

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
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/gpu/drm/drm_auth.c | 25 ++++++++++++
drivers/gpu/drm/drm_lease.c | 81 ++++++++++++++++++++++++++++---------
include/drm/drm_auth.h | 1 +
include/drm/drm_file.h | 6 +++
4 files changed, 93 insertions(+), 20 deletions(-)

diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
index 0024ad93d24b..c7adbeaf10b1 100644
--- a/drivers/gpu/drm/drm_auth.c
+++ b/drivers/gpu/drm/drm_auth.c
@@ -376,6 +376,31 @@ 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 da4f085fc09e..aef22634005e 100644
--- a/drivers/gpu/drm/drm_lease.c
+++ b/drivers/gpu/drm/drm_lease.c
@@ -107,10 +107,19 @@ 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;
}

/**
@@ -129,13 +138,22 @@ 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;
}

@@ -155,10 +173,16 @@ 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;
@@ -177,6 +201,9 @@ 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;
}

@@ -490,7 +517,7 @@ 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 +529,6 @@ 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 +540,22 @@ 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);

@@ -535,14 +566,15 @@ 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");
@@ -578,6 +610,7 @@ 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;

@@ -587,6 +620,8 @@ 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;
}
@@ -609,7 +644,7 @@ 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 +655,7 @@ 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);
@@ -643,6 +679,7 @@ 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;
}
@@ -662,7 +699,7 @@ 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 +713,7 @@ 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);
@@ -703,6 +741,7 @@ 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;
}
@@ -721,7 +760,7 @@ 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;

@@ -731,6 +770,7 @@ 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);
@@ -751,6 +791,7 @@ 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 ca659ece3ee8..42d04607d091 100644
--- 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>`.
*/
--
2.30.2