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Peter Anvin" , Ard Biesheuvel , Paolo Bonzini , Sean Christopherson , 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 , Michael Roth , Vlastimil Babka , tony.luck@intel.com, npmccallum@redhat.com, brijesh.ksingh@gmail.com Subject: Re: [PATCH Part2 RFC v4 10/40] x86/fault: Add support to handle the RMP fault for user address To: Dave Hansen , x86@kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, linux-efi@vger.kernel.org, platform-driver-x86@vger.kernel.org, linux-coco@lists.linux.dev, linux-mm@kvack.org, linux-crypto@vger.kernel.org References: <20210707183616.5620-1-brijesh.singh@amd.com> <20210707183616.5620-11-brijesh.singh@amd.com> <3c6b6fc4-05b2-8d18-2eb8-1bd1a965c632@intel.com> <2b4accb6-b68e-02d3-6fed-975f90558099@amd.com> From: Brijesh Singh Message-ID: <5592d8ff-e2c3-6474-4a10-96abe1962d6f@amd.com> Date: Mon, 12 Jul 2021 11:11:54 -0500 User-Agent: Mozilla/5.0 (X11; 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dkim=pass header.d=amd.com header.s=selector1 header.b=a2oHQqJj; spf=pass (imf30.hostedemail.com: domain of brijesh.singh@amd.com designates 40.107.223.50 as permitted sender) smtp.mailfrom=brijesh.singh@amd.com; dmarc=pass (policy=quarantine) header.from=amd.com X-HE-Tag: 1626106319-863431 Content-Transfer-Encoding: quoted-printable 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: On 7/12/21 11:00 AM, Dave Hansen wrote: > On 7/12/21 8:43 AM, Brijesh Singh wrote: >>>> +=C2=A0=C2=A0=C2=A0 /* >>>> +=C2=A0=C2=A0=C2=A0=C2=A0 * The backing page level is higher than th= e RMP page level, >>>> request >>>> +=C2=A0=C2=A0=C2=A0=C2=A0 * to split the page. >>>> +=C2=A0=C2=A0=C2=A0=C2=A0 */ >>>> +=C2=A0=C2=A0=C2=A0 if (level > rmp_level) >>>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 return RMP_FAULT_PAGE_SP= LIT; >>> >>> This can theoretically trigger on a hugetlbfs page.=C2=A0 Right? >> >> Yes, theoretically. >> >> In the current implementation, the VMM is enlightened to not use the >> hugetlbfs for backing page when creating the SEV-SNP guests. >=20 > "The VMM"? I was meaning a userspace qemu. >=20 > We try to write kernel code so that it "works" and doesn't do unexpecte= d > things with whatever userspace might throw at it. This seems to be > written with an assumption that no VMM will ever use hugetlbfs with SEV= -SNP. >=20 > That worries me. Not only because someone is sure to try it, but it's > the kind of assumption that an attacker or a fuzzer might try. >=20 > Could you please test this kernel code in practice with hugetblfs? Yes, I will make sure that hugetlbfs path is tested in non-RFC version. >=20 >>> I also suspect you can just set VM_FAULT_SIGBUS and let the do_sigbus= () >>> call later on in the function do its work. >>>> =C2=A0 +static int handle_split_page_fault(struct vm_fault *vmf) >>>> +{ >>>> +=C2=A0=C2=A0=C2=A0 if (!IS_ENABLED(CONFIG_AMD_MEM_ENCRYPT)) >>>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 return VM_FAULT_SIGBUS; >>>> + >>>> +=C2=A0=C2=A0=C2=A0 __split_huge_pmd(vmf->vma, vmf->pmd, vmf->addres= s, false, NULL); >>>> +=C2=A0=C2=A0=C2=A0 return 0; >>>> +} >>> >>> What will this do when you hand it a hugetlbfs page? >> >> VMM is updated to not use the hugetlbfs when creating SEV-SNP guests. >> So, we should not run into it. >=20 > Please fix this code to handle hugetlbfs along with any other non-THP > source of level>0 mappings. DAX comes to mind. "Handle" can mean > rejecting these. You don't have to find some way to split them and mak= e > the VM work, just fail safely, ideally as early as possible. >=20 > To me, this is a fundamental requirement before this code can be accept= ed. Understood, if userspace decided to use the hugetlbfs backing pages then=20 I believe earliest we can detect is when we go about adding the pages in=20 the RMP table. I'll add a check, and fail the page state change. -Brijesh