From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sean Christopherson Subject: Re: [PATCH v17 18/23] platform/x86: Intel SGX driver Date: Mon, 17 Dec 2018 10:36:13 -0800 Message-ID: <20181217183613.GD12491@linux.intel.com> References: <20181116010412.23967-1-jarkko.sakkinen@linux.intel.com> <20181116010412.23967-19-jarkko.sakkinen@linux.intel.com> <7d5cde02-4649-546b-0f03-2d6414bb80b5@intel.com> <20181217180102.GA12560@linux.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20181217180102.GA12560@linux.intel.com> Sender: linux-kernel-owner@vger.kernel.org To: Jarkko Sakkinen Cc: Dave Hansen , x86@kernel.org, platform-driver-x86@vger.kernel.org, linux-sgx@vger.kernel.org, nhorman@redhat.com, npmccallum@redhat.com, serge.ayoun@intel.com, shay.katz-zamir@intel.com, haitao.huang@linux.intel.com, andriy.shevchenko@linux.intel.com, tglx@linutronix.de, kai.svahn@intel.com, mark.shanahan@intel.com, luto@amacapital.net, Suresh Siddha , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , Darren Hart , Andy Shevchenko , "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" List-Id: platform-driver-x86.vger.kernel.org On Mon, Dec 17, 2018 at 08:01:02PM +0200, Jarkko Sakkinen wrote: > On Mon, Dec 17, 2018 at 09:45:40AM -0800, Dave Hansen wrote: > > > +struct sgx_encl *sgx_encl_alloc(struct sgx_secs *secs) > > > +{ > > ... > > > + kref_init(&encl->refcount); > > > + INIT_LIST_HEAD(&encl->add_page_reqs); > > > + INIT_RADIX_TREE(&encl->page_tree, GFP_KERNEL); > > > + mutex_init(&encl->lock); > > > + INIT_WORK(&encl->add_page_work, sgx_add_page_worker); > > > + > > > + encl->mm = current->mm; <---------------------------------> + encl->base = secs->base; > > > + encl->size = secs->size; > > > + encl->ssaframesize = secs->ssa_frame_size; > > > + encl->backing = backing; > > > + > > > + return encl; > > > +} > > > > How is this OK without taking a reference on the mm? It's subtle and the ordering is all kinds of weird, but technically we are taking a reference on mm when the mmu_notifier is registered in sgx_encl_create(). sgx_encl_alloc() and sgx_encl_create() are always called in tandem and with mm->mm_users > 0, so we'll never use encl->mm without holding a reference to mm. We need to comment the weirdness or maybe register the notifier before > > I have a feeling a bunch of your bugs with the mmu notifiers and so > > forth are because the refcounting is wrong here. Eh, not really. Maybe the mmu_notifier is more subtle, e.g. calling do_unmap() after mmput() would be quite obvious, but there's no fundamental bug, we just haven't needed to touch VMAs during release prior to moving away from shmem. > > Sean's SGX_ENCL_MM_RELEASED would, I think be unnecessary if you just > > take a refcount here and release it when the enclave is destroyed. > > Right, atomic_inc(encl->mm->count) here and once when releasing. > > The we would not even need the whole mmu notifier in the first place. I'm pretty sure doing mmget() would result in circular dependencies and a zombie enclave. In the do_exit() case where a task is abruptly killed: - __mmput() is never called because the enclave holds a ref - sgx_encl_release() is never be called because its VMAs hold refs - sgx_vma_close() is never called because __mmput()->exit_mmap() is blocked and the process itself is dead, i.e. won't unmap anything.