On Thu, Apr 03, 2025 at 03:28:43PM +0800, Xiaoyao Li wrote: > On 4/2/2025 11:49 PM, Daniel P. Berrangé wrote: > > On Wed, Apr 02, 2025 at 11:26:11PM +0800, Xiaoyao Li wrote: > > > > > > I guess the raw mode was introduced due to the design was changed to let > > > guest kernel to forward to TD report to host QGS via TDVMCALL instead of > > > guest application communicates with host QGS via vsock, and Linux TD guest > > > driver doesn't integrate any QGS protocol but just forward the raw TD > > > report > > > data to KVM. > > > > > > > IMHO, QEMU should be made to pack & unpack the TDX report from > > > > the guest into the GET_QUOTE_REQ / GET_QUOTE_RESP messages, and > > > > this "raw" mode should be removed to QGS as it is inherantly > > > > dangerous to have this magic protocol overloading. > > > > > > There is no enforcement that the input data of TDVMCALL.GetQuote is the > > > raw > > > data of TD report. It is just the current Linux tdx-guest driver of tsm > > > implementation send the raw data. For other TDX OS, or third-party driver, > > > they might encapsulate the raw TD report data with QGS message header. For > > > such cases, if QEMU adds another layer of package, it leads to the wrong > > > result. > > > > If I look at the GHCI spec > > > > > > https://cdrdv2-public.intel.com/726790/TDX%20Guest-Hypervisor%20Communication%20Interface_1.0_344426_006%20-%2020230311.pdf > > > > In "3.3 TDG.VP.VMCALL<GetQuote>", it indicates the parameter is a > > "TDREPORT_STRUCT". IOW, it doesn't look valid to allow the guest to > > send arbitrary other data as QGS protocol messages. > > In table 3-7, the description of R12 is > > Shared GPA as input - the memory contains a TDREPORT_STRUCT. > The same buffer is used as output - the memory contains a TD Quote. > > table 3-10, describes the detailed format of the shared GPA: > > starting from offset 24 bytes, it is the "Data" > > On input, the data filled by TD with input length. The data should > include TDREPORT_STRUCT. TD should zeroize the remaining buffer to > avoid information leak if size of shared GPA (R13) > Input Length. > > It uses the word "contains" and "include", but without "only". So it is not > clear to me. > > I will work with internal attestation folks to make it clearer that who (TD > guest or host VMM) is responsible to encapsulate the raw TDERPORT_STRCUT > with QGS MSG protocol, and update the spec accordingly.
To be clear, my strong preference is that the spec be updated to only permit the raw TDREPORT_STRUCT. IMHO allowing arbitrary QGS MSGs would be a significant host security weakness, as it exposes a huge amount of the QGS codebase to direct attack from the guest. QEMU needs to be able to block that attack vector. Without that, the benefit/value of shuffling of TDREPORTs via the GetQuote hypercall is largely eliminated, and might as well have just exposed QGS over VSOCK. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
