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[2003:cb:c707:fa00:74f2:89da:ed65:8b50]) by smtp.gmail.com with ESMTPSA id cl5-20020a5d5f05000000b0033d1b760125sm1322830wrb.92.2024.02.29.01.51.09 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 29 Feb 2024 01:51:10 -0800 (PST) Message-ID: <1c284bac-a1cb-4726-be71-fb32d480e224@redhat.com> Date: Thu, 29 Feb 2024 10:51:08 +0100 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v1 00/26] KVM: Restricted mapping of guest_memfd at the host and pKVM/arm64 support Content-Language: en-US To: Quentin Perret Cc: Fuad Tabba , kvm@vger.kernel.org, kvmarm@lists.linux.dev, pbonzini@redhat.com, chenhuacai@kernel.org, mpe@ellerman.id.au, anup@brainfault.org, paul.walmsley@sifive.com, palmer@dabbelt.com, aou@eecs.berkeley.edu, seanjc@google.com, brauner@kernel.org, willy@infradead.org, akpm@linux-foundation.org, xiaoyao.li@intel.com, yilun.xu@intel.com, chao.p.peng@linux.intel.com, jarkko@kernel.org, amoorthy@google.com, dmatlack@google.com, yu.c.zhang@linux.intel.com, isaku.yamahata@intel.com, mic@digikod.net, vbabka@suse.cz, vannapurve@google.com, ackerleytng@google.com, mail@maciej.szmigiero.name, michael.roth@amd.com, wei.w.wang@intel.com, liam.merwick@oracle.com, isaku.yamahata@gmail.com, kirill.shutemov@linux.intel.com, suzuki.poulose@arm.com, steven.price@arm.com, quic_eberman@quicinc.com, quic_mnalajal@quicinc.com, quic_tsoni@quicinc.com, quic_svaddagi@quicinc.com, quic_cvanscha@quicinc.com, quic_pderrin@quicinc.com, quic_pheragu@quicinc.com, catalin.marinas@arm.com, james.morse@arm.com, yuzenghui@huawei.com, oliver.upton@linux.dev, maz@kernel.org, will@kernel.org, keirf@google.com References: <20240222161047.402609-1-tabba@google.com> <9e983090-f336-43b9-8f2b-5dabe7e73b72@redhat.com> <0951868c-911d-4879-bc79-14b5d3959462@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 28.02.24 15:01, Quentin Perret wrote: > On Wednesday 28 Feb 2024 at 11:12:18 (+0100), David Hildenbrand wrote: >>>> So you don't need any guest_memfd games to protect from that -- and one >>>> doesn't have to travel back in time to have memory that isn't >>>> swappable/migratable and only comes in one page size. >>>> >>>> [I'm not up-to-date which obscure corner-cases CCA requirement the s390x >>>> implementation cannot fulfill -- like replacing pages in page tables and >>>> such; I suspect pKVM also cannot cover all these corner-cases] >>> >>> Thanks for this. I'll do some more reading on how things work with s390x. >>> >>> Right, and of course, one key difference of course is that pKVM >>> doesn't encrypt anything, and only relies on stage-2 protection to >>> protect the guest. >> >> I don't remember what exactly s390x does, but I recall that it might only >> encrypt the memory content as it transitions a page from secure to >> non-secure. >> >> Something like that could also be implemented using pKVM (unless I am >> missing something), but it might not be that trivial, of course :) > > One of the complicated aspects of having the host migrate pages like so > is for the hypervisor to make sure the content of the page has not been > tempered with when the new page is re-mapped in the guest. That means > having additional tracking in the hypervisor of pages that have been > encrypted and returned to the host, indexed by IPA, with something like > a 'checksum' of some sort, which is non-trivial to do securely from a > cryptographic PoV. > I don't know what exactly s390x does, and how it does it -- and which CCA cases they can handle. Details are scarce, for example: https://www.ibm.com/docs/en/linux-on-systems?topic=virtualization-introducing-secure-execution-linux I suspect they do it in some way you describe, and I fully agree with the "non-trivial" aspect :) > A simpler and secure way to do this is (I think) is to do > hypervisor-assisted migration. IOW, pKVM exposes a new migrate_page(ipa, > old_pa, new_pa) hypercall which Linux can call to migrate a page. > pKVM unmaps the new page from the host stage-2, unmap the old page from > guest stage-2, does the copy, wipes the old page, maps the pages in the > respective page-tables, and off we go. That way the content is never > visible to Linux and that avoids the problems I highlighted above by > construction. The downside is that it doesn't work for swapping, but > that is quite hard to do in general... The main "problem" with that is that you still end up with these inaccessible pages, that require the use of guest_memfd for your in-place conversion requirement in the first place. I'm sure at some point we'll see migration support also for TDX and friends. For pKVM it might be even easier to support. -- Cheers, David / dhildenb