linux-kernel - Re: [RFC Part2 PATCH 04/30] x86/mm: split the physmap when adding the page in RMP table

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <D67BDFB6-84AA-4CA3-A951-7EEE0E4B4B26@amacapital.net>
Date:   Mon, 19 Apr 2021 11:37:19 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Brijesh Singh <brijesh.singh@....com>,
        Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org,
        x86@...nel.org, kvm@...r.kernel.org, linux-crypto@...r.kernel.org,
        ak@...ux.intel.com, herbert@...dor.apana.org.au,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        "H. Peter Anvin" <hpa@...or.com>, Tony Luck <tony.luck@...el.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        David Rientjes <rientjes@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [RFC Part2 PATCH 04/30] x86/mm: split the physmap when adding the page in RMP table



> On Apr 19, 2021, at 11:33 AM, Dave Hansen <dave.hansen@...el.com> wrote:
> 
> On 4/19/21 11:10 AM, Andy Lutomirski wrote:
>> I’m confused by this scenario. This should only affect physical pages
>> that are in the 2M area that contains guest memory. But, if we have a
>> 2M direct map PMD entry that contains kernel data and guest private
>> memory, we’re already in a situation in which the kernel touching
>> that memory would machine check, right?
> 
> Not machine check, but page fault.  Do machine checks even play a
> special role in SEV-SNP?  I thought that was only TDX?

Brain fart.

> 
> My point was just that you can't _easily_ do the 2M->4k kernel mapping
> demotion in a page fault handler, like I think Borislav was suggesting.

We are certainly toast if this hits the stack.  Or if it hits a page table or the GDT or IDT :). The latter delightful choices would be triple faults.

I sure hope the code we use to split a mapping is properly NMI safe.

> 
>> ISTM we should fully unmap any guest private page from the kernel and
>> all host user pagetables before actually making it be a guest private
>> page.
> 
> Yes, that sounds attractive.  Then, we'd actually know if the host
> kernel was doing stray reads somehow because we'd get a fault there too.