On Tue, Mar 28, 2023 at 02:32:22AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > Sent: Friday, March 24, 2023 11:03 PM
> >
> > On Fri, Mar 24, 2023 at 01:37:51AM +0000, Tian, Kevin wrote:
> >
> > > If vfio races attach/detach then lots of things are messed.
> > >
> > > e.g. iommufd_device_detach() directly calls list_del(&idev->group_item)
> > > w/o checking whether the device has been attached.
> >
> > Yeah, you obviously can't race attach/detach or detach/replace
> >
> > > And with that race UAF could occur if we narrow down the lock scope
> > > to iommufd_hw_pagetable_attach():
> > >
> > > cpu0 cpu1
> > > vfio_iommufd_attach()
> > > iommufd_device_attach()
> > > iommufd_device_auto_get_domain()
> > > mutex_lock(&ioas->mutex);
> > > iommufd_hw_pagetable_alloc()
> > > hwpt = iommufd_object_alloc() //hwpt.users=1
> > > hwpt->domain = iommu_domain_alloc(idev->dev->bus);
> > > iommufd_hw_pagetable_attach() //hwpt.users=2
> > > vfio_iommufd_detach()
> > > iommufd_device_detach()
> > > mutex_lock(&idev->igroup->lock);
> > > hwpt = iommufd_hw_pagetable_detach()
> > > mutex_unlock(&idev->igroup->lock);
> > > iommufd_hw_pagetable_put(hwpt)
> > > iommufd_object_destroy_user(hwpt)
> > //hwpt.users=0
> > > iommufd_hw_pagetable_destroy(hwpt)
> > > iommu_domain_free(hwpt->domain);
> > > iopt_table_add_domain(&hwpt->ioas->iopt, hwpt->domain); //UAF
> >
> > You didn't balance the refcounts properly, the cpu1 put will get to
> > hwpt.users=1
> >
>
> iommufd_object_destroy_user() decrements the count twice if the value
> is two:
>
> refcount_dec(&obj->users);
> if (!refcount_dec_if_one(&obj->users)) {
Ohh, it isn't allowed to call iommufd_object_destroy_user() until
finalize has passed..
Jason
