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[2003:cb:c73e:aa00:c9db:441d:a65e:6999]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-439b02ce685sm146345305e9.2.2025.02.25.08.54.57 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 25 Feb 2025 08:54:59 -0800 (PST) Message-ID: Date: Tue, 25 Feb 2025 17:54:57 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 03/12] KVM: guest_memfd: Add flag to remove from direct map To: Patrick Roy , rppt@kernel.org, seanjc@google.com Cc: pbonzini@redhat.com, corbet@lwn.net, willy@infradead.org, akpm@linux-foundation.org, song@kernel.org, jolsa@kernel.org, ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, martin.lau@linux.dev, eddyz87@gmail.com, yonghong.song@linux.dev, john.fastabend@gmail.com, kpsingh@kernel.org, sdf@fomichev.me, haoluo@google.com, Liam.Howlett@oracle.com, lorenzo.stoakes@oracle.com, vbabka@suse.cz, jannh@google.com, shuah@kernel.org, kvm@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, bpf@vger.kernel.org, linux-kselftest@vger.kernel.org, tabba@google.com, jgowans@amazon.com, graf@amazon.com, kalyazin@amazon.com, xmarcalx@amazon.com, derekmn@amazon.com, jthoughton@google.com References: <20250221160728.1584559-1-roypat@amazon.co.uk> <20250221160728.1584559-4-roypat@amazon.co.uk> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Queue-Id: 93E94160011 X-Stat-Signature: r8unfru7tm9qqs1xryfzihnhjn796d5o X-Rspamd-Server: rspam03 X-HE-Tag: 1740502503-274187 X-HE-Meta: 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 VLiN7NxR 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 21.02.25 17:07, Patrick Roy wrote: > Add KVM_GMEM_NO_DIRECT_MAP flag for KVM_CREATE_GUEST_MEMFD() ioctl. When > set, guest_memfd folios will be removed from the direct map after > preparation, with direct map entries only restored when the folios are > freed. > > To ensure these folios do not end up in places where the kernel cannot > deal with them, set AS_NO_DIRECT_MAP on the guest_memfd's struct > address_space if KVM_GMEM_NO_DIRECT_MAP is requested. > > Note that this flag causes removal of direct map entries for all > guest_memfd folios independent of whether they are "shared" or "private" > (although current guest_memfd only supports either all folios in the > "shared" state, or all folios in the "private" state if > !IS_ENABLED(CONFIG_KVM_GMEM_SHARED_MEM)). The usecase for removing > direct map entries of also the shared parts of guest_memfd are a special > type of non-CoCo VM where, host userspace is trusted to have access to > all of guest memory, but where Spectre-style transient execution attacks > through the host kernel's direct map should still be mitigated. > > Note that KVM retains access to guest memory via userspace > mappings of guest_memfd, which are reflected back into KVM's memslots > via userspace_addr. This is needed for things like MMIO emulation on > x86_64 to work. Previous iterations attempted to instead have KVM > temporarily restore direct map entries whenever such an access to guest > memory was needed, but this turned out to have a significant performance > impact, as well as additional complexity due to needing to refcount > direct map reinsertion operations and making them play nicely with gmem > truncations. > > This iteration also doesn't have KVM perform TLB flushes after direct > map manipulations. This is because TLB flushes resulted in a up to 40x > elongation of page faults in guest_memfd (scaling with the number of CPU > cores), or a 5x elongation of memory population. On the one hand, TLB > flushes are not needed for functional correctness (the virt->phys > mapping technically stays "correct", the kernel should simply to not it > for a while), so this is a correct optimization to make. On the other > hand, it means that the desired protection from Spectre-style attacks is > not perfect, as an attacker could try to prevent a stale TLB entry from > getting evicted, keeping it alive until the page it refers to is used by > the guest for some sensitive data, and then targeting it using a > spectre-gadget. > > Signed-off-by: Patrick Roy ... > > +static bool kvm_gmem_test_no_direct_map(struct inode *inode) > +{ > + return ((unsigned long) inode->i_private) & KVM_GMEM_NO_DIRECT_MAP; > +} > + > static inline void kvm_gmem_mark_prepared(struct folio *folio) > { > + struct inode *inode = folio_inode(folio); > + > + if (kvm_gmem_test_no_direct_map(inode)) { > + int r = set_direct_map_valid_noflush(folio_page(folio, 0), folio_nr_pages(folio), > + false); Will this work if KVM is built as a module, or is this another good reason why we might want guest_memfd core part of core-mm? -- Cheers, David / dhildenb