[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <5592d8ff-e2c3-6474-4a10-96abe1962d6f@amd.com>
Date: Mon, 12 Jul 2021 11:11:54 -0500
From: Brijesh Singh <brijesh.singh@....com>
To: Dave Hansen <dave.hansen@...el.com>, x86@...nel.org,
linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
linux-coco@...ts.linux.dev, linux-mm@...ck.org,
linux-crypto@...r.kernel.org
Cc: brijesh.singh@....com, Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
Tom Lendacky <thomas.lendacky@....com>,
"H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
Paolo Bonzini <pbonzini@...hat.com>,
Sean Christopherson <seanjc@...gle.com>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Wanpeng Li <wanpengli@...cent.com>,
Jim Mattson <jmattson@...gle.com>,
Andy Lutomirski <luto@...nel.org>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
Peter Zijlstra <peterz@...radead.org>,
Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
David Rientjes <rientjes@...gle.com>,
Dov Murik <dovmurik@...ux.ibm.com>,
Tobin Feldman-Fitzthum <tobin@....com>,
Borislav Petkov <bp@...en8.de>,
Michael Roth <michael.roth@....com>,
Vlastimil Babka <vbabka@...e.cz>, tony.luck@...el.com,
npmccallum@...hat.com, brijesh.ksingh@...il.com
Subject: Re: [PATCH Part2 RFC v4 10/40] x86/fault: Add support to handle the
RMP fault for user address
On 7/12/21 11:00 AM, Dave Hansen wrote:
> On 7/12/21 8:43 AM, Brijesh Singh wrote:
>>>> + /*
>>>> + * The backing page level is higher than the RMP page level,
>>>> request
>>>> + * to split the page.
>>>> + */
>>>> + if (level > rmp_level)
>>>> + return RMP_FAULT_PAGE_SPLIT;
>>>
>>> This can theoretically trigger on a hugetlbfs page. 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.
>
> "The VMM"?
I was meaning a userspace qemu.
>
> We try to write kernel code so that it "works" and doesn't do unexpected
> 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.
>
> 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.
>
> 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.
>
>>> I also suspect you can just set VM_FAULT_SIGBUS and let the do_sigbus()
>>> call later on in the function do its work.
>>>> +static int handle_split_page_fault(struct vm_fault *vmf)
>>>> +{
>>>> + if (!IS_ENABLED(CONFIG_AMD_MEM_ENCRYPT))
>>>> + return VM_FAULT_SIGBUS;
>>>> +
>>>> + __split_huge_pmd(vmf->vma, vmf->pmd, vmf->address, false, NULL);
>>>> + 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.
>
> 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 make
> the VM work, just fail safely, ideally as early as possible.
>
> To me, this is a fundamental requirement before this code can be accepted.
Understood, if userspace decided to use the hugetlbfs backing pages then
I believe earliest we can detect is when we go about adding the pages in
the RMP table. I'll add a check, and fail the page state change.
-Brijesh
Powered by blists - more mailing lists