From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id DA128C021BC for ; Wed, 26 Feb 2025 09:08:52 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 211CB6B007B; Wed, 26 Feb 2025 04:08:52 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 19B26280003; Wed, 26 Feb 2025 04:08:52 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id F082B6B008A; Wed, 26 Feb 2025 04:08:51 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0012.hostedemail.com [216.40.44.12]) by kanga.kvack.org (Postfix) with ESMTP id CF7C46B007B for ; Wed, 26 Feb 2025 04:08:51 -0500 (EST) Received: from smtpin21.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay03.hostedemail.com (Postfix) with ESMTP id 33976A2C1B for ; Wed, 26 Feb 2025 09:08:51 +0000 (UTC) X-FDA: 83161520862.21.821B75F Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by imf12.hostedemail.com (Postfix) with ESMTP id C98B940005 for ; Wed, 26 Feb 2025 09:08:47 +0000 (UTC) Authentication-Results: imf12.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=U4rYuDlX; spf=pass (imf12.hostedemail.com: domain of david@redhat.com designates 170.10.133.124 as permitted sender) smtp.mailfrom=david@redhat.com; dmarc=pass (policy=none) header.from=redhat.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1740560928; h=from:from:sender: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:dkim-signature; bh=O8TE1MQFrJ10owitDy4Sd/FdbR2Se527RsQmy+KP7OE=; b=ro9G03N/OIFbaMVN/EmJKG12ycMZxiD2ugQHltBR/8M4J95BdzdLw+MwsoQAA36a9X3rfm xcnI6DE28AOzSAbevM/WzQyTkI4/70SdQjBxfo4sm/qpLTaAjULcc87YfJKKf4CYIIEOVQ qGopVjEwSVytBPbu9ZGfRFtX/wYRib4= ARC-Authentication-Results: i=1; imf12.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=U4rYuDlX; spf=pass (imf12.hostedemail.com: domain of david@redhat.com designates 170.10.133.124 as permitted sender) smtp.mailfrom=david@redhat.com; dmarc=pass (policy=none) header.from=redhat.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1740560928; a=rsa-sha256; cv=none; b=CnNAu0dagzc3Y+ASZgGGz9DnO+0pW7QaR/IqlHmVCCRCTRi2dOamaOUPqqaeNeq1ybf5YK 2bfi4fR97VLrYUcIUYrilBUVkHeXERSOUUzruI8x/41KWzr30MmYZKBWLtHE26Ndfw7ru0 JonHCaIPlQZO3V5EKg9SU6ScLLWXMrU= DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1740560927; 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=U4rYuDlXsnHHz2L9ff/q90BXn2xtjiqAlUkBkUg9+wXmVx8zr2/t6+oTpza6yl/X97geys rHfXcsn0/FlnCH8oDnznctVoPxJOnClTdyHgjN/6jLlID6gOtyVCnM8jej2IlPM7ynCIt4 7LPe0oM0bZ9k7zCQZAC+CeFMWl199wg= Received: from mail-wm1-f71.google.com (mail-wm1-f71.google.com [209.85.128.71]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-22-NQtLsBZaOUaKp5DEd1Jtlg-1; Wed, 26 Feb 2025 04:08:45 -0500 X-MC-Unique: NQtLsBZaOUaKp5DEd1Jtlg-1 X-Mimecast-MFC-AGG-ID: NQtLsBZaOUaKp5DEd1Jtlg_1740560924 Received: by mail-wm1-f71.google.com with SMTP id 5b1f17b1804b1-43945f32e2dso59883625e9.2 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=BLzWbkaiQZ7NNpIawzgpEsxZZVFwwinxUy+KD/v9pfWXo02s1yqJ9FBqixPzGNOY9D XuMCZGi8q6KwAs70Z0zoVGw0AtofBL3bdI7v07D6jdxe6AA248LJaZVc+iu+o8zeZMV0 sFQeGYFY6QYxqGmsoiu9rsjcdOv1QDFK6EXLxIgrsc9Snbgajs25r6Oc0JdWNy6LFKwT 5zuwJvq+EGA55tSdrjZgTM7DRVqY9d/mhlSQ/LwMhisZub40KWmAYfJJpJVP0JsW4dtr Vn+dK9PGSOIIoRzqChUyuywZ2H8jzjUoYg7Obmbt+yao1DLS4yp/ozAKUl5GrbNvM2Lv +R5Q== X-Forwarded-Encrypted: i=1; AJvYcCXcz1HJiLmNWM9jav/WQfnv3kmludbe3134vYX6G3kZ7KhW8UiYavbhBNKmaqfs/PQb4Vd+2/Zx6w==@kvack.org X-Gm-Message-State: AOJu0Yx3BhyXJqmNIQp7/UNBxru4HaktKqQnNvGb3hgHuy1zc0Drfihs /re5EL2FAN529rJ4JfkSV5ZzrAz4qSa6axrGSIio4TqnRypdqx7l1j+cER+MsLSFZM3K94kKZLv W4MHC2g13jwnS6YpdZ5mlEJwbVK5y3VriPDBWbMI5e27t6hmb X-Gm-Gg: ASbGncuEbVx25uyjO63QaYYx3o8ydhVkSBmZiPgBseDl3QUJPpOWU+YpAmm9ydNkNtC zxCmkVYksqvcndpgj1weM3xDpZx/klRKMrb2pPDI7ZJEx7HIqyJHjvd+xYLRt/g/E+mkMDunECb hjBDwbNZyqYbIeTUDzvtHhDXdWbMte44dbnp61Y/nsEu8Hkm1yTDK6qL8IQHlwt4mm1d1AkddhT PVLtk4iETTgOL/IvklxVac8+M00nSIXAyVvGCIiFk/8me/tZsRJWxUdSnWWFE2paIBF48mpllae 30dr8VWhRKxbZkM1xV4Jt0RmYe9iUcMyBAfjzLaXmsgK9YoL/BCPRnuDP3m6BuxZ3BegcoAJaMH K6sD8YwhJ0BgkpeNDntEQ7wGbQE9/9O2ZoQ3W+cwhcF0= X-Received: by 2002:a05:600c:1d15:b0:439:88bb:d023 with SMTP id 5b1f17b1804b1-43abcdbc490mr5761935e9.11.1740560924380; 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 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 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> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: 8V3rEVSg8s89rzVaBqssxF91jpnccPG5JuX4H-VjfLI_1740560924 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: C98B940005 X-Stat-Signature: o7wbqzdt7bpsotgded48wjrz3jhx53za X-Rspam-User: X-HE-Tag: 1740560927-313733 X-HE-Meta: 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 y7nAlaY4 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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 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