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Thu, 3 Mar 2022 14:40:43 +0000 (GMT) Received: from [9.171.88.22] (unknown [9.171.88.22]) by d06av23.portsmouth.uk.ibm.com (Postfix) with ESMTP; Thu, 3 Mar 2022 14:40:43 +0000 (GMT) Message-ID: Date: Thu, 3 Mar 2022 15:40:42 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.5.0 Subject: Re: [PATCH v8 01/17] KVM: s390: pv: leak the topmost page table when destroy fails Content-Language: en-US To: Claudio Imbrenda , kvm@vger.kernel.org Cc: borntraeger@de.ibm.com, frankja@linux.ibm.com, thuth@redhat.com, pasic@linux.ibm.com, david@redhat.com, linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org, mimu@linux.ibm.com, nrb@linux.ibm.com References: <20220302181143.188283-1-imbrenda@linux.ibm.com> <20220302181143.188283-2-imbrenda@linux.ibm.com> From: Janis Schoetterl-Glausch In-Reply-To: <20220302181143.188283-2-imbrenda@linux.ibm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: jjLJm-JJJzjaQphRJB-X2Tv_HlEpXB00 X-Proofpoint-ORIG-GUID: _RZzT-NDxqHr4wj7P21quRCt1O7s5pFE X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.205,Aquarius:18.0.816,Hydra:6.0.425,FMLib:17.11.64.514 definitions=2022-03-03_07,2022-02-26_01,2022-02-23_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 suspectscore=0 priorityscore=1501 spamscore=0 impostorscore=0 malwarescore=0 mlxscore=0 clxscore=1015 mlxlogscore=999 lowpriorityscore=0 adultscore=0 phishscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2201110000 definitions=main-2203030069 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 3/2/22 19:11, 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. > > Signed-off-by: Claudio Imbrenda > Fixes: 29b40f105ec8d55 ("KVM: s390: protvirt: Add initial vm and cpu lifecycle handling") > --- > arch/s390/include/asm/gmap.h | 2 + > arch/s390/kvm/pv.c | 9 +++-- > arch/s390/mm/gmap.c | 71 ++++++++++++++++++++++++++++++++++++ > 3 files changed, 79 insertions(+), 3 deletions(-) > [...] > +/** > + * s390_replace_asce - Try to replace the current ASCE of a gmap with > + * another equivalent one. > + * @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 > + * 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)); Is concurrent modification of *gmap->table possible during the copy? > + > + /* > + * 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);