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

On Tue, Jun 15, 2021 at 10:36:45AM +0800, Desmond Cheong Zhi Xi wrote:

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);

So maybe we have a bug somewhere, and the kerneldoc isn't 100% clear, but I thought file_priv->master is invariant over the lifetime of file_priv. So we don't need a lock to check anything here.

It's the drm_device->master derefence that gets us into trouble. Well also file_priv->is_owner is protected by dev->master_mutex.

So I think with your previous patch all the access here in drm_lease.c is ok and already protected? Or am I missing something?

Thanks, Daniel

• 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