linux-kernel - Re: [PATCH v5 16/16] x86/tdx: Add cmdline option to force use of ioremap_host

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Date:   Tue, 12 Oct 2021 17:30:36 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Richard Henderson <rth@...ddle.net>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        James E J Bottomley <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>,
        "David S . Miller" <davem@...emloft.net>,
        Arnd Bergmann <arnd@...db.de>,
        Jonathan Corbet <corbet@....net>,
        Paolo Bonzini <pbonzini@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter H Anvin <hpa@...or.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        linux-pci@...r.kernel.org, linux-alpha@...r.kernel.org,
        linux-mips@...r.kernel.org, linux-parisc@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-doc@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v5 16/16] x86/tdx: Add cmdline option to force use of
 ioremap_host_shared

On Tue, Oct 12, 2021 at 02:18:01PM -0700, Andi Kleen wrote:
> 
> > Interesting. VT-d tradeoffs ... what are they?
> 
> The connection to the device is not encrypted and also not authenticated.
> 
> This is different that even talking to the (untrusted) host through shared
> memory where you at least still have a common key.

Well it's different sure enough but how is talking to host less secure?
Cold boot attacks and such?

> > Allowing hypervisor to write into BIOS looks like it will
> > trivially lead to code execution, won't it?
> 
> This is not about BIOS code executing. While the guest firmware runs it is
> protected of course. This is for BIOS structures like ACPI tables that are
> mapped by Linux. While AML can run byte code it can normally not write to
> arbitrary memory.

I thought you basically create an OperationRegion of SystemMemory type,
and off you go. Maybe the OSPM in Linux is clever and protects
some memory, I wouldn't know.

> The risk is more that all the Linux code dealing with this hasn't been
> hardened to deal with malicious input.
> 
> -Andi


-- 
MST