From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-yw1-f177.google.com (mail-yw1-f177.google.com [209.85.128.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 02F123FF9 for ; Wed, 14 Sep 2022 16:39:29 +0000 (UTC) Received: by mail-yw1-f177.google.com with SMTP id 00721157ae682-3450a7358baso187146617b3.13 for ; Wed, 14 Sep 2022 09:39:29 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:from:to:cc:subject:date; bh=qw5c1rQO9auXlb1weto/2p+w4wwBPYYtzhiHNjVL//I=; b=OAZPO49RrJRJ1iDc42pcPY8eoQOr9U6LI/jzWl/t/qHJGAwMcmrdHEHcTpZAoEhple vmC5s4DomwuO1+aqLB8TMmpmXa4+GKx8HHg3AY/pPl/n26W8L7+GLOtWsIv2AVaWwP4Z ZQNaQeralnxC2np7lvsaONpxNi6tvum3IDZggIXPXTJp9vOigUhfQMvo1WFRSQ0ovgeY ysF+7Rn8+Te/7o66UPvqNQsZr8wJVkRGpZa/Wk186u5fYhn8lQ0jDLw/f7B7H6UMAQRs exEGbIfuANJwGK8IU+YFusPFqhFGXGudweKXCFkCPqfbQuIl49QspG/wobLqsnBjfOE1 UjPA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:x-gm-message-state:from:to:cc:subject:date; bh=qw5c1rQO9auXlb1weto/2p+w4wwBPYYtzhiHNjVL//I=; b=KXWJ/kdzYH/3OpoecWhV2ocws5EmHBpCMhb380+Z9a+UkWdfn+bhS8eP1nx8ypr4Ni tWdayc9F569gZvKeLrod+6ayrxCnGQJdmh7C5I2tj2TxpktcrPteHE4ZLbKp95JZrjTd wV8LxV0HplMCvOntlZHAiUNeI72Z/OHiBR9XpHL6IkwuC+qLo9edQbNZB6XWzNLkh8HX h0pNDYjVOBw1h9T/AJgxyP697N5MNLs22/DeuoU1oebTVCU7WRI0Onoc4MG7t/8FSdXv CFSegKTp8XtAiIlGlIUEj7u58VTKO3lXpqdvMOfVu66WtOVbOIssjwVRiusJTqt42Wnn YMEA== X-Gm-Message-State: ACgBeo3Jgk2EwrtNdz0gwKIsMkFa1Ab+4NbrxWRbTwlM9gvNa5EVTiJq NKiIsAkjCEXmTaGHRhaV3CI9LsFCxf/mTcyD95IxwA== X-Google-Smtp-Source: AA6agR4ZwapujsKhSGBJ8nLTDmAGCZkhuO0sc3uJlAZUs5ydbGgLq/Slr/TreGV6Rcw8FLrJapUcRgU+ddTrKrUYAUQ= X-Received: by 2002:a81:c30a:0:b0:328:4a6c:bc89 with SMTP id r10-20020a81c30a000000b003284a6cbc89mr31170890ywk.29.1663173568737; Wed, 14 Sep 2022 09:39:28 -0700 (PDT) Precedence: bulk X-Mailing-List: linux-coco@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 References: <20210820155918.7518-1-brijesh.singh@amd.com> <20210820155918.7518-40-brijesh.singh@amd.com> <4e41dcff-7c7b-cf36-434a-c7732e7e8ff2@amd.com> <20220908212114.sqne7awimfwfztq7@amd.com> In-Reply-To: From: Marc Orr Date: Wed, 14 Sep 2022 17:39:18 +0100 Message-ID: Subject: Re: [PATCH Part2 v5 39/45] KVM: SVM: Introduce ops for the post gfn map and unmap To: Sean Christopherson Cc: Michael Roth , Brijesh Singh , x86 , LKML , kvm list , linux-coco@lists.linux.dev, Linux Memory Management List , Linux Crypto Mailing List , Thomas Gleixner , Ingo Molnar , Joerg Roedel , Tom Lendacky , "H. Peter Anvin" , Ard Biesheuvel , Paolo Bonzini , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Andy Lutomirski , Dave Hansen , Sergio Lopez , Peter Gonda , Peter Zijlstra , Srinivas Pandruvada , David Rientjes , Dov Murik , Tobin Feldman-Fitzthum , Borislav Petkov , Vlastimil Babka , "Kirill A . Shutemov" , Andi Kleen , Tony Luck , Sathyanarayanan Kuppuswamy , jarkko@profian.com Content-Type: text/plain; charset="UTF-8" On Wed, Sep 14, 2022 at 5:32 PM Marc Orr wrote: > > On Wed, Sep 14, 2022 at 5:15 PM Sean Christopherson wrote: > > > > On Wed, Sep 14, 2022, Marc Orr wrote: > > > On Wed, Sep 14, 2022 at 9:05 AM Sean Christopherson wrote: > > > > > > > > On Thu, Sep 08, 2022, Michael Roth wrote: > > > > > On Fri, Oct 15, 2021 at 05:16:28PM +0000, Sean Christopherson wrote: > > > > > So in the context of this interim solution, we're trying to look for a > > > > > solution that's simple enough that it can be used reliably, without > > > > > introducing too much additional complexity into KVM. There is one > > > > > approach that seems to fit that bill, that Brijesh attempted in an > > > > > earlier version of this series (I'm not sure what exactly was the > > > > > catalyst to changing the approach, as I wasn't really in the loop at > > > > > the time, but AIUI there weren't any showstoppers there, but please > > > > > correct me if I'm missing anything): > > > > > > > > > > - if the host is writing to a page that it thinks is supposed to be > > > > > shared, and the guest switches it to private, we get an RMP fault > > > > > (actually, we will get a !PRESENT fault, since as of v5 we now > > > > > remove the mapping from the directmap as part of conversion) > > > > > - in the host #PF handler, if we see that the page is marked private > > > > > in the RMP table, simply switch it back to shared > > > > > - if this was a bug on the part of the host, then the guest will see > > > > > > > > As discussed off-list, attempting to fix up RMP violations in the host #PF handler > > > > is not a viable approach. There was also extensive discussion on-list a while back: > > > > > > > > https://lore.kernel.org/all/8a244d34-2b10-4cf8-894a-1bf12b59cf92@www.fastmail.com > > > > > > I mentioned this during Mike's talk at the micro-conference: For pages > > > mapped in by the kernel can we disallow them to be converted to > > > private? > > > > In theory, yes. Do we want to do something like this? No. kmap() does something > > vaguely similar for 32-bit PAE/PSE kernels, but that's a lot of complexity and > > overhead to take on. And this issue goes far beyond a kmap(); when the kernel gup()s > > a page, the kernel expects the pfn to be available, no exceptions (pun intended). > > > > > Note, userspace accesses are already handled by UPM. > > > > I'm confused by the UPM comment. Isn't the gist of this thread about the ability > > to merge SNP _without_ UPM? Or am I out in left field? > > I think that was the overall gist: yes. But it's not what I was trying > to comment on :-). > > HOWEVER, thinking about this more: I was confused when I wrote out my > last reply. I had thought that the issue that Michael brought up > applied even with UPM. That is, I was thinking it was still possibly > for a guest to maliciously convert a page to private mapped in by the > kernel and assumed to be shared. > > But I now realize that is not what will actually happen. To be > concrete, let's assume the GHCB page. What will happen is: > - KVM has GHCB page mapped in. GHCB is always assumed to be shared. So > far so good. > - Malicious guest converts GHCB page to private (e.g., via Page State > Change request) > - Guest exits to KVM > - KVM exits to userspace VMM > - Userspace VM allocates page in private FD. > > Now, what happens here depends on how UPM works. If we allow double > allocation then our host kernel is safe. However, now we have the > "double allocation problem". > > If on the other hand, we deallocate the page in the shared FD, the > host kernel can segfault. And now we actually do have essentially the > same problem Michael was describing that we have without UPM. Because > we'll end up in fault.c in the kernel context and likely panic the > host. Thinking about this even more... Even if we deallocate in the userspace VMM's shared FD, the kernel has its own page tables -- right? So maybe we are actually 100% OK under UPM then regardless of the userspace VMM's policy around managing the private and shared FDs.