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Peter Anvin" , x86@kernel.org References: <20190220201609.28290-1-joao.m.martins@oracle.com> <20190220201609.28290-4-joao.m.martins@oracle.com> From: Ankur Arora Message-ID: <2d4df59d-f945-32dc-6999-a6f711e972ea@oracle.com> Date: Tue, 1 Dec 2020 16:40:07 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.11.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9822 signatures=668682 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=0 bulkscore=0 malwarescore=0 mlxscore=0 mlxlogscore=999 phishscore=0 spamscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2012020001 X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9822 signatures=668682 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 malwarescore=0 bulkscore=0 clxscore=1011 mlxscore=0 spamscore=0 priorityscore=1501 mlxlogscore=999 suspectscore=0 lowpriorityscore=0 phishscore=0 adultscore=0 impostorscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2012020001 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2020-12-01 5:07 a.m., David Woodhouse wrote: > On Wed, 2019-02-20 at 20:15 +0000, Joao Martins wrote: >> +static int kvm_xen_shared_info_init(struct kvm *kvm, gfn_t gfn) >> +{ >> + struct shared_info *shared_info; >> + struct page *page; >> + >> + page = gfn_to_page(kvm, gfn); >> + if (is_error_page(page)) >> + return -EINVAL; >> + >> + kvm->arch.xen.shinfo_addr = gfn; >> + >> + shared_info = page_to_virt(page); >> + memset(shared_info, 0, sizeof(struct shared_info)); >> + kvm->arch.xen.shinfo = shared_info; >> + return 0; >> +} >> + > > Hm. > > How come we get to pin the page and directly dereference it every time, > while kvm_setup_pvclock_page() has to use kvm_write_guest_cached() > instead? So looking at my WIP trees from the time, this is something that we went back and forth on as well with using just a pinned page or a persistent kvm_vcpu_map(). I remember distinguishing shared_info/vcpu_info from kvm_setup_pvclock_page() as shared_info is created early and is not expected to change during the lifetime of the guest which didn't seem true for MSR_KVM_SYSTEM_TIME (or MSR_KVM_STEAL_TIME) so that would either need to do a kvm_vcpu_map() kvm_vcpu_unmap() dance or do some kind of synchronization. That said, I don't think this code explicitly disallows any updates to shared_info. > > If that was allowed, wouldn't it have been a much simpler fix for > CVE-2019-3016? What am I missing? Agreed. Perhaps, Paolo can chime in with why KVM never uses pinned page and always prefers to do cached mappings instead? > > Should I rework these to use kvm_write_guest_cached()? kvm_vcpu_map() would be better. The event channel logic does RMW operations on shared_info->vcpu_info. Ankur > >