From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jason Gunthorpe Subject: Re: [PATCH v4 0/1] Use HMM for ODP v4 Date: Wed, 22 May 2019 19:39:06 -0300 Message-ID: <20190522223906.GA15389@ziepe.ca> References: <20190411181314.19465-1-jglisse@redhat.com> <20190506195657.GA30261@ziepe.ca> <20190521205321.GC3331@redhat.com> <20190522005225.GA30819@ziepe.ca> <20190522174852.GA23038@redhat.com> <20190522201247.GH6054@ziepe.ca> <20190522220419.GB20179@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20190522220419.GB20179@redhat.com> Sender: linux-kernel-owner@vger.kernel.org To: Jerome Glisse Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-rdma@vger.kernel.org, Leon Romanovsky , Doug Ledford , Artemy Kovalyov , Moni Shoua , Mike Marciniszyn , Kaike Wan , Dennis Dalessandro List-Id: linux-rdma@vger.kernel.org On Wed, May 22, 2019 at 06:04:20PM -0400, Jerome Glisse wrote: > On Wed, May 22, 2019 at 05:12:47PM -0300, Jason Gunthorpe wrote: > > On Wed, May 22, 2019 at 01:48:52PM -0400, Jerome Glisse wrote: > > > > > static void put_per_mm(struct ib_umem_odp *umem_odp) > > > { > > > struct ib_ucontext_per_mm *per_mm = umem_odp->per_mm; > > > @@ -325,9 +283,10 @@ static void put_per_mm(struct ib_umem_odp *umem_odp) > > > up_write(&per_mm->umem_rwsem); > > > > > > WARN_ON(!RB_EMPTY_ROOT(&per_mm->umem_tree.rb_root)); > > > - mmu_notifier_unregister_no_release(&per_mm->mn, per_mm->mm); > > > + hmm_mirror_unregister(&per_mm->mirror); > > > put_pid(per_mm->tgid); > > > - mmu_notifier_call_srcu(&per_mm->rcu, free_per_mm); > > > + > > > + kfree(per_mm); > > > > Notice that mmu_notifier only uses SRCU to fence in-progress ops > > callbacks, so I think hmm internally has the bug that this ODP > > approach prevents. > > > > hmm should follow the same pattern ODP has and 'kfree_srcu' the hmm > > struct, use container_of in the mmu_notifier callbacks, and use the > > otherwise vestigal kref_get_unless_zero() to bail: > > > > From 0cb536dc0150ba964a1d655151d7b7a84d0f915a Mon Sep 17 00:00:00 2001 > > From: Jason Gunthorpe > > Date: Wed, 22 May 2019 16:52:52 -0300 > > Subject: [PATCH] hmm: Fix use after free with struct hmm in the mmu notifiers > > > > mmu_notifier_unregister_no_release() is not a fence and the mmu_notifier > > system will continue to reference hmm->mn until the srcu grace period > > expires. > > > > CPU0 CPU1 > > __mmu_notifier_invalidate_range_start() > > srcu_read_lock > > hlist_for_each () > > // mn == hmm->mn > > hmm_mirror_unregister() > > hmm_put() > > hmm_free() > > mmu_notifier_unregister_no_release() > > hlist_del_init_rcu(hmm-mn->list) > > mn->ops->invalidate_range_start(mn, range); > > mm_get_hmm() > > mm->hmm = NULL; > > kfree(hmm) > > mutex_lock(&hmm->lock); > > > > Use SRCU to kfree the hmm memory so that the notifiers can rely on hmm > > existing. Get the now-safe hmm struct through container_of and directly > > check kref_get_unless_zero to lock it against free. > > It is already badly handled with BUG_ON() You can't crash the kernel because userspace forced a race, and no it isn't handled today because there is no RCU locking in mm_get_hmm nor is there a kfree_rcu for the struct hmm to make the kref_get_unless_zero work without use-after-free. > i just need to convert those to return and to use > mmu_notifier_call_srcu() to free hmm struct. Isn't that what this patch does? > The way race is avoided is because mm->hmm will either be NULL or > point to another hmm struct before an existing hmm is free. There is no locking on mm->hmm so it is useless to prevent races. > Also if range_start/range_end use kref_get_unless_zero() but right > now this is BUG_ON if it turn out to be NULL, it should just return > on NULL. Still needs rcu. Also the container_of is necessary to avoid some race where you could be doing: CPU0 CPU1 CPU2 hlist_for_each () mmu_notifier_unregister_no_release(hmm1) spin_lock(&mm->page_table_lock); mm->hmm = NULL spin_unlock(&mm->page_table_lock); hmm2 = hmm_get_or_create() mn == hmm1->mn mn->ops->invalidate_range_start(mn, range) mm_get_mm() == hmm2 hist_for_each con't mn == hmm2->mn mn->ops->invalidate_range_start(mn, range) mm_get_mm() == hmm2 Now we called the same notifier twice on hmm2. Ooops. There is no reason to risk this confusion just to avoid container_of. So we agree this patch is necessary? Can you test it an ack it please? Jason