From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 40EF12459EC for ; Wed, 26 Feb 2025 09:08:53 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1740560935; cv=none; b=PctJVzLROx8JL7KZ8tbxLh6nrpof/Tw13IcpaWwNAuwXLwpVs0t5vyuKdSAyKW1s3MTKMSartPJ52l3R7yQm+pMTk0yCPGVENzMgtUbbG7zcxLIag346dkK1dsTJZIrRCrb5s9oYCtfQq3EYFrzsdW1l3s6jSEaIHNoSGgjzlTY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1740560935; c=relaxed/simple; bh=bJLY51LMKbmoPhaSC4QSOluS/yMA3KmLEs2qtXmVmaw=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=A6w9NnTulsc5Z4btAM7dadbzjZUz+qxttNWWx+JB2HNZd07qlkweEwSdVi9UFMhESXznQMUEGHwY/SX+KnrxD+X1Q0fiELvnXkinmOl3OgwVfF1q3AcH+c75OC7g/5PrhJMNIeW9qRuepnJbCLvaHAcjCyRbDgu4gphiM/zAMAk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=QQDKAvVk; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="QQDKAvVk" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1740560933; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:autocrypt:autocrypt; bh=O8TE1MQFrJ10owitDy4Sd/FdbR2Se527RsQmy+KP7OE=; b=QQDKAvVkRPy/FgpauTP8kOcUh29sOAxy9IF06nfm+W8GYrWrwOw+Nk24Hc400GH4DyUgXo UYk14DGmZUgzUxch22V4JmrMYytRCnilWDltM7trHs2QdukD1qZXfBbQ1Q3bGpamuNkLuw ZehmCdoJ2cs6hhv3rbpXOnLgHpqdI2c= Received: from mail-wm1-f70.google.com (mail-wm1-f70.google.com [209.85.128.70]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-558-RgWxstO_NQKAhi7wFTvwmg-1; Wed, 26 Feb 2025 04:08:47 -0500 X-MC-Unique: RgWxstO_NQKAhi7wFTvwmg-1 X-Mimecast-MFC-AGG-ID: RgWxstO_NQKAhi7wFTvwmg_1740560924 Received: by mail-wm1-f70.google.com with SMTP id 5b1f17b1804b1-43947a0919aso60022185e9.0 for ; Wed, 26 Feb 2025 01:08:45 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1740560924; x=1741165724; h=content-transfer-encoding:in-reply-to:organization:autocrypt :content-language:from:references:cc:to:subject:user-agent :mime-version:date:message-id:x-gm-message-state:from:to:cc:subject :date:message-id:reply-to; bh=O8TE1MQFrJ10owitDy4Sd/FdbR2Se527RsQmy+KP7OE=; b=X7QaZsjzU8PxBclYo3MfugOTzermknYfumF/vEp/pJQb/JJ/SuyfHBb4MHtA2X890k LU/FTI4cE6QATzY5aIRjf8zqweOer8KFHOx1wLZbgkjUMuPKIPvqXYzPQUu7zGeB1PHy dsCO2bXYuF2a5kROAmvcfEU5fL5zWZroNbTJFOLmdfqZyFlDSbSXLQArGKHR2za3OYu2 QQoMfUsrCbNAnHoj+idEWrH+hjiCPAB8sM4BgjNreC8cfto9Q3rVbANo1aeUpwFqkIRO 9but2WT7kE3V5y0CNWuPm61KJgRm+gnA24396c/3Cnsqseo9wEenRLzoTxPmJiopy5oV LDqA== X-Forwarded-Encrypted: i=1; AJvYcCXGzr3cthokqWwZ14E11lxx2j5t1OhlmYQxPfyaCctOTvnlB6KZANbQ4ovtShj0kloP9Q0BaSmPVUk=@vger.kernel.org X-Gm-Message-State: AOJu0Yxxb4qzfrjNKYyRVjZawWK/B+n+XKam8wdMCCndGv7sWSZb3Caa /gxXvxMWFnW8UU9E8ItrQH8LnNFHbtUIn4cZj4xTV+ksxPnsypasc7xgj8Nbk/ao4KmxlaHfJyE p+Mg9xbkpDOWUtsMKRN8j3+LZODlxuhnNoYBnrUO4eRI6ssHSe6kdYjNO3g== X-Gm-Gg: ASbGncsDJnLmHEWkhAihDDUhUmtF1Tk7CZzniflQ/ePsp0GCAXk5SjVOfGHwAwfD03R MPQP+6LirRAJUf7/qUGd7JK4AbrCNrezpVJVRcnFyvyi0/tPxMUDHr8YSMKqhMM6xkcTHYXyeN3 /T6G79g3pku4rEjSpcWSiD6Qhl9OmILgGBXcgYV6DYrXSsvhEJk0yWeknTaMtvnegJ0ZlOnhILN iw9j+ItZoH4YkwCzcZnUYKBbt/mDBuLGfYvc/yTYsDuz+ZEuk7td/XJw7bg61XojIxmdoqHrg2u uA+b3ppBI5OnoHxO23o1a0lSQc2kNkJGJ0rT23I0vFaP8J7p7Wib6GoyF+0ksOtoDGepu0JBm8F dwSG1wIfFDDWkWOn5VLZhX4HVFHnL806GrD93e4haoHA= X-Received: by 2002:a05:600c:1d15:b0:439:88bb:d023 with SMTP id 5b1f17b1804b1-43abcdbc490mr5762105e9.11.1740560924389; Wed, 26 Feb 2025 01:08:44 -0800 (PST) X-Google-Smtp-Source: AGHT+IGzCgV6uzLeA5R9rGGNcz1AqCTL7vjwN5EkwLdrel31QMyKpsknZsMvxZpzMpDk7Hfkg937Gw== X-Received: by 2002:a05:600c:1d15:b0:439:88bb:d023 with SMTP id 5b1f17b1804b1-43abcdbc490mr5761555e9.11.1740560923961; Wed, 26 Feb 2025 01:08:43 -0800 (PST) Received: from ?IPV6:2003:cb:c747:ff00:9d85:4afb:a7df:6c45? (p200300cbc747ff009d854afba7df6c45.dip0.t-ipconnect.de. [2003:cb:c747:ff00:9d85:4afb:a7df:6c45]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43aba543fdfsm14027565e9.31.2025.02.26.01.08.41 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 26 Feb 2025 01:08:42 -0800 (PST) Message-ID: Date: Wed, 26 Feb 2025 10:08:40 +0100 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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> <8642de57-553a-47ec-81af-803280a360ec@amazon.co.uk> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: <8642de57-553a-47ec-81af-803280a360ec@amazon.co.uk> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 26.02.25 09:48, Patrick Roy wrote: > > > On Tue, 2025-02-25 at 16:54 +0000, David Hildenbrand wrote:> 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? > > mh, I'm admittedly not too familiar with the differences that would come > from building KVM as a module vs not. I do remember something about the > direct map accessors not being available for modules, so this would > indeed not work. Does that mean moving gmem into core-mm will be a > pre-requisite for the direct map removal stuff? Likely, we'd need some shim. Maybe for the time being it could be fenced using #if IS_BUILTIN() ... but that sure won't win in a beauty contest. -- Cheers, David / dhildenb