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Thu, 14 Apr 2022 12:19:10 +0000 (GMT) Received: from p-imbrenda (unknown [9.145.1.140]) by d06av25.portsmouth.uk.ibm.com (Postfix) with ESMTP; Thu, 14 Apr 2022 12:19:10 +0000 (GMT) Date: Thu, 14 Apr 2022 14:19:08 +0200 From: Claudio Imbrenda To: Janosch Frank Cc: kvm@vger.kernel.org, borntraeger@de.ibm.com, thuth@redhat.com, pasic@linux.ibm.com, david@redhat.com, linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org, scgl@linux.ibm.com, mimu@linux.ibm.com, nrb@linux.ibm.com Subject: Re: [PATCH v10 01/19] KVM: s390: pv: leak the topmost page table when destroy fails Message-ID: <20220414141908.1ba04a38@p-imbrenda> In-Reply-To: References: <20220414080311.1084834-1-imbrenda@linux.ibm.com> <20220414080311.1084834-2-imbrenda@linux.ibm.com> Organization: IBM X-Mailer: Claws Mail 3.18.0 (GTK+ 2.24.33; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: lTYcqtpuQn1xSTVnPB5J3BzhCbusdvOx X-Proofpoint-ORIG-GUID: JN7UPdRLJZ5OKFrmqRYZxaY8kozqKHqP X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.205,Aquarius:18.0.858,Hydra:6.0.486,FMLib:17.11.64.514 definitions=2022-04-14_04,2022-04-14_01,2022-02-23_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 bulkscore=0 spamscore=0 malwarescore=0 priorityscore=1501 phishscore=0 lowpriorityscore=0 adultscore=0 mlxscore=0 suspectscore=0 impostorscore=0 clxscore=1015 mlxlogscore=999 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2202240000 definitions=main-2204140066 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 14 Apr 2022 13:30:33 +0200 Janosch Frank wrote: > On 4/14/22 10:02, Claudio Imbrenda wrote: > > Each secure guest must have a unique ASCE (address space control > > element); we must avoid that new guests use the same page for their > > ASCE, to avoid errors. > > > > Since the ASCE mostly consists of the address of the topmost page table > > (plus some flags), we must not return that memory to the pool unless > > the ASCE is no longer in use. > > > > Only a successful Destroy Secure Configuration UVC will make the ASCE > > reusable again. > > > > If the Destroy Configuration UVC fails, the ASCE cannot be reused for a > > secure guest (either for the ASCE or for other memory areas). To avoid > > a collision, it must not be used again. This is a permanent error and > > the page becomes in practice unusable, so we set it aside and leak it. > > On failure we already leak other memory that belongs to the ultravisor > > (i.e. the variable and base storage for a guest) and not leaking the > > topmost page table was an oversight. > > > > This error (and thus the leakage) should not happen unless the hardware > > is broken or KVM has some unknown serious bug. > > > > Reviewed-by: Janosch Frank > > > + /* > > + * In case the ASCE needs to be "removed" multiple times, for example > > + * if the VM is rebooted into secure mode several times > > + * concurrently, or if s390_replace_asce fails after calling > > + * s390_remove_old_asce and is attempted again later. In that case > > + * the old asce has been removed from the list, and therefore it > > + * will not be freed when the VM terminates, but the ASCE is still > > + * in use and still pointed to. > > + * A subsequent call to replace_asce will follow the pointer and try > > + * to remove the same page from the list again. > > + * Therefore it's necessary that the page of the ASCE has valid > > + * pointers, so list_del can work (and do nothing) without > > + * dereferencing stale or invalid pointers. > > + */ > > + INIT_LIST_HEAD(&old->lru); > > + spin_unlock(&gmap->guest_table_lock); > > +} > > +EXPORT_SYMBOL_GPL(s390_remove_old_asce); > > + > > +/** > > + * s390_replace_asce - Try to replace the current ASCE of a gmap with > > + * another equivalent one. > > with a copy? will fix > > > + * @gmap the gmap > > + * > > + * If the allocation of the new top level page table fails, the ASCE is not > > + * replaced. > > + * In any case, the old ASCE is always removed from the list. Therefore the > > removed from the gmap crst list will fix > > > + * caller has to make sure to save a pointer to it beforehands, unless an > > + * intentional leak is intended. > > + */ > > +int s390_replace_asce(struct gmap *gmap) > > +{ > > + unsigned long asce; > > + struct page *page; > > + void *table; > > + > > + s390_remove_old_asce(gmap); > > + > > + page = alloc_pages(GFP_KERNEL_ACCOUNT, CRST_ALLOC_ORDER); > > + if (!page) > > + return -ENOMEM; > > + table = page_to_virt(page); > > + memcpy(table, gmap->table, 1UL << (CRST_ALLOC_ORDER + PAGE_SHIFT)); > > + > > + /* > > + * The caller has to deal with the old ASCE, but here we make sure > > + * the new one is properly added to the list of page tables, so that > > + * it will be freed when the VM is torn down. > > + */ > > + spin_lock(&gmap->guest_table_lock); > > + list_add(&page->lru, &gmap->crst_list); > > + spin_unlock(&gmap->guest_table_lock); > > + > > + /* Set new table origin while preserving existing ASCE control bits */ > > + asce = (gmap->asce & ~_ASCE_ORIGIN) | __pa(table); > > + WRITE_ONCE(gmap->asce, asce); > > + WRITE_ONCE(gmap->mm->context.gmap_asce, asce); > > + WRITE_ONCE(gmap->table, table); > > + > > + return 0; > > +} > > +EXPORT_SYMBOL_GPL(s390_replace_asce); >