Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Message-ID: <c09c961d-f433-4a68-0b38-208ffe8b36c7@linux.intel.com>
Date:   Tue, 12 Oct 2021 14:18:01 -0700
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101
 Thunderbird/91.2.0
Subject: Re: [PATCH v5 16/16] x86/tdx: Add cmdline option to force use of
 ioremap_host_shared
Content-Language: en-US
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Richard Henderson <rth@twiddle.net>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        James E J Bottomley <James.Bottomley@hansenpartnership.com>,
        Helge Deller <deller@gmx.de>,
        "David S . Miller" <davem@davemloft.net>,
        Arnd Bergmann <arnd@arndb.de>,
        Jonathan Corbet <corbet@lwn.net>,
        Paolo Bonzini <pbonzini@redhat.com>,
        David Hildenbrand <david@redhat.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter H Anvin <hpa@zytor.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Kirill Shutemov <kirill.shutemov@linux.intel.com>,
        Sean Christopherson <seanjc@google.com>,
        Kuppuswamy Sathyanarayanan <knsathya@kernel.org>,
        x86@kernel.org, linux-kernel@vger.kernel.org,
        linux-pci@vger.kernel.org, linux-alpha@vger.kernel.org,
        linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org,
        sparclinux@vger.kernel.org, linux-arch@vger.kernel.org,
        linux-doc@vger.kernel.org,
        virtualization@lists.linux-foundation.org
References: <20211009003711.1390019-1-sathyanarayanan.kuppuswamy@linux.intel.com>
 <20211009003711.1390019-17-sathyanarayanan.kuppuswamy@linux.intel.com>
 <20211009070132-mutt-send-email-mst@kernel.org>
 <8c906de6-5efa-b87a-c800-6f07b98339d0@linux.intel.com>
 <20211011075945-mutt-send-email-mst@kernel.org>
 <9d0ac556-6a06-0f2e-c4ff-0c3ce742a382@linux.intel.com>
 <20211011142330-mutt-send-email-mst@kernel.org>
 <4fe8d60a-2522-f111-995c-dcbefd0d5e31@linux.intel.com>
 <20211012165705-mutt-send-email-mst@kernel.org>
From:   Andi Kleen <ak@linux.intel.com>
In-Reply-To: <20211012165705-mutt-send-email-mst@kernel.org>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk


> 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.

> 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.

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

-Andi