Re: [Linaro-mm-sig] [PATCH v3 2/2] drm: protect drm_master pointers in drm_lease.c

On Sun, Jun 20, 2021 at 07:03:27PM +0800, Desmond Cheong Zhi Xi wrote:

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 as follows:

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@ffwll.ch Signed-off-by: Desmond Cheong Zhi Xi desmondcheongzx@gmail.com

I think this approach looks much clearer than the previous. I've found a few smaller things below still. I think at least ...

Cheers, Daniel

---

drivers/gpu/drm/drm_auth.c | 22 ++++++++++++++ drivers/gpu/drm/drm_lease.c | 57 ++++++++++++++++++++++++++----------- include/drm/drm_auth.h | 1 + include/drm/drm_file.h | 15 ++++++++-- 4 files changed, 75 insertions(+), 20 deletions(-)

diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c index 86d4b72e95cb..0c64a77c67a6 100644 --- a/drivers/gpu/drm/drm_auth.c +++ b/drivers/gpu/drm/drm_auth.c @@ -384,6 +384,28 @@ struct drm_master *drm_master_get(struct drm_master *master) } EXPORT_SYMBOL(drm_master_get); +/**

• • drm_file_get_master - reference @file_priv->master
• • @file_priv: DRM file private
• • Increments the reference count of @file_priv->master and returns

Does this format correctly? I'd go with "&drm_file.master of @file_priv".

• • @file_priv->master.
• • 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;
• mutex_lock(&file_priv->master->dev->master_mutex);
• master = drm_master_get(file_priv->master);
• mutex_unlock(&file_priv->master->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 da4f085fc09e..65eab82f8acc 100644 --- a/drivers/gpu/drm/drm_lease.c +++ b/drivers/gpu/drm/drm_lease.c @@ -107,10 +107,17 @@ static bool _drm_has_leased(struct drm_master *master, int id) */ bool _drm_lease_held(struct drm_file *file_priv, int id) {

• bool ret;
• struct drm_master *master;
• if (!file_priv || !file_priv->master)

So here we still have a ->master access outside of the locked code section. I think the best fix for that would be to move the NULL check into drm_file_get_master (where we grab the lock already anyway), and update the kerneldoc to state that it might return NULL.

Same with all the checks for ->master below.

return true;

• return _drm_lease_held_master(file_priv->master, id);

• master = drm_file_get_master(file_priv);
• ret = _drm_lease_held_master(master, id);
• drm_master_put(&master);
• return ret;

} /** @@ -132,10 +139,11 @@ bool drm_lease_held(struct drm_file *file_priv, int id) if (!file_priv || !file_priv->master || !file_priv->master->lessor) return true;

master->lessor dereferenced outside the lock or without holding a reference.

• master = file_priv->master;

• master = drm_file_get_master(file_priv); 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;

} @@ -158,7 +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;

Same here.

• master = file_priv->master;

• master = drm_file_get_master(file_priv); dev = master->dev;

count_in = count_out = 0; @@ -177,6 +185,7 @@ 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;

} @@ -490,7 +499,7 @@ 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 +511,6 @@ 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 +522,22 @@ 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) {

Here we check after grabbing the reference, so looks correct.

• 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 +548,15 @@ 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 +592,7 @@ 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 +602,8 @@ 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 +626,7 @@ 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 +637,7 @@ 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 +661,7 @@ 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 +681,7 @@ 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 +695,7 @@ 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 +723,7 @@ 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 +742,7 @@ 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 +752,7 @@ 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 +773,7 @@ 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