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

Daniel Vetter
2021-06-17T17:12:23Z

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
>

--
Daniel Vetter
Software Engineer, Intel Corporation