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[35.185.214.157]) by smtp.gmail.com with ESMTPSA id f91-20020a17090a706400b001c7858a6879sm80756pjk.12.2022.03.28.10.13.13 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 28 Mar 2022 10:13:13 -0700 (PDT) Date: Mon, 28 Mar 2022 17:13:10 +0000 From: Sean Christopherson To: Quentin Perret Cc: Chao Peng , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org, qemu-devel@nongnu.org, Paolo Bonzini , Jonathan Corbet , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , Thomas Gleixner , Ingo Molnar , Borislav Petkov , x86@kernel.org, "H . Peter Anvin" , Hugh Dickins , Jeff Layton , "J . Bruce Fields" , Andrew Morton , Mike Rapoport , Steven Price , "Maciej S . Szmigiero" , Vlastimil Babka , Vishal Annapurve , Yu Zhang , "Kirill A . Shutemov" , luto@kernel.org, jun.nakajima@intel.com, dave.hansen@intel.com, ak@linux.intel.com, david@redhat.com, maz@kernel.org, will@kernel.org Subject: Re: [PATCH v5 00/13] KVM: mm: fd-based approach for supporting KVM guest private memory Message-ID: References: <20220310140911.50924-1-chao.p.peng@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Thu, Mar 24, 2022, Quentin Perret wrote: > For Protected KVM (and I suspect most other confidential computing > solutions), guests have the ability to share some of their pages back > with the host kernel using a dedicated hypercall. This is necessary > for e.g. virtio communications, so these shared pages need to be mapped > back into the VMM's address space. I'm a bit confused about how that > would work with the approach proposed here. What is going to be the > approach for TDX? > > It feels like the most 'natural' thing would be to have a KVM exit > reason describing which pages have been shared back by the guest, and to > then allow the VMM to mmap those specific pages in response in the > memfd. Is this something that has been discussed or considered? The proposed solution is to exit to userspace with a new exit reason, KVM_EXIT_MEMORY_ERROR, when the guest makes the hypercall to request conversion[1]. The private fd itself will never allow mapping memory into userspace, instead userspace will need to punch a hole in the private fd backing store. The absense of a valid mapping in the private fd is how KVM detects that a pfn is "shared" (memslots without a private fd are always shared)[2]. The key point is that KVM never decides to convert between shared and private, it's always a userspace decision. Like normal memslots, where userspace has full control over what gfns are a valid, this gives userspace full control over whether a gfn is shared or private at any given time. Another important detail is that this approach means the kernel and KVM treat the shared backing store and private backing store as independent, albeit related, entities. This is very deliberate as it makes it easier to reason about what is and isn't allowed/required. E.g. the kernel only needs to handle freeing private memory, there is no special handling for conversion to shared because no such path exists as far as host pfns are concerned. And userspace doesn't need any new "rules" for protecting itself against a malicious guest, e.g. userspace already needs to ensure that it has a valid mapping prior to accessing guest memory (or be able to handle any resulting signals). A malicious guest can DoS itself by instructing userspace to communicate over memory that is currently mapped private, but there are no new novel attack vectors from the host's perspective as coercing the host into accessing an invalid mapping after shared=>private conversion is just a variant of a use-after-free. One potential conversions that's TBD (at least, I think it is, I haven't read through this most recent version) is how to support populating guest private memory with non-zero data, e.g. to allow in-place conversion of the initial guest firmware instead of having to an extra memcpy(). [1] KVM will also exit to userspace with the same info on "implicit" conversions, i.e. if the guest accesses the "wrong" GPA. Neither SEV-SNP nor TDX mandate explicit conversions in their guest<->host ABIs, so KVM has to support implicit conversions :-/ [2] Ideally (IMO), KVM would require userspace to completely remove the private memslot, but that's too slow due to use of SRCU in both KVM and userspace (QEMU at least uses SRCU for memslot changes).