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

From: Daniel Vetter
Date: Wed Jun 23 2021 - 03:41:39 EST

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In reply to: Desmond Cheong Zhi Xi: "Re: [PATCH v3 2/2] drm: protect drm_master pointers in drm_lease.c"
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On Wed, Jun 23, 2021 at 5:49 AM Desmond Cheong Zhi Xi
<desmondcheongzx@xxxxxxxxx> wrote:
>
> On 21/6/21 10:47 pm, Daniel Vetter wrote:
> > On Sun, Jun 20, 2021 at 07:03:27PM +0800, Desmond Cheong Zhi Xi wrote:
> >> 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".
> >
>
> Got it. "file_priv->master" was bolded, but no link to drm_file.master
> was generated. I'll update this.
>
> >> + * @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.
> >
>
> Moving the check into drm_file_get_master sounds good. Grabbing the lock
> before performing the NULL check poses a little chicken-and-egg problem
> though.
>
> It's true that without the lock, even if file_priv->master passes the
> NULL check, it could be freed in the time between the check and grabbing
> the lock.
>
> However, based on the original code, it seems there's the possibility
> that file_priv->master might be NULL. In this case, grabbing the lock
> results in a null ptr dereference because we get the mutex via
> &file_priv->master->dev->master_mutex.
>
> By this reasoning, I think the safer method is still to perform the NULL
> check before grabbing the lock.

file_priv->dev->master_mutex should also work and avoid the trouble.

Please also cc intel-gfx list, there's a CI system there to test your
patches. Since patch 1 of this series had pretty bad deadlock that I
didn't see would be good to make sure we get more test coverage on
these.

Thanks, Daniel

>
> >> 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
> >>
> >
>
> Thanks for the feedback, Daniel. I'll send out an updated patch to
> address these issues.
>
> Best wishes,
> Desmond

--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

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