On Mon, Feb 08, 2021, Andy Lutomirski wrote: > On Mon, Feb 8, 2021 at 9:11 AM Sean Christopherson <[email protected]> wrote: > > > > On Sun, Feb 07, 2021, Andy Lutomirski wrote: > > > > > > > How much of the register state is revealed to the VMM when we do a > > > TDVMCALL? > > > Presumably we should fully sanitize all register state that shows up in > > > cleartext on the other end, and we should treat all regs that can be > > > modified > > > by the VMM as clobbered. > > > > The guest gets to choose, with a few restrictions. RSP cannot be exposed > > to the > > host. RAX, RCX, R10, and R11 are always exposed as they hold mandatory info > > about the TDVMCALL (TDCALL fn, GPR mask, GHCI vs. vendor, and TDVMCALL fn). > > All > > other GPRs are exposed and clobbered if their bit in RCX is set, otherwise > > they > > are saved/restored by the TDX-Module. > > > > I agree with Dave, pass everything required by the GHCI in the main > > routine, and > > sanitize and save/restore all such GPRs. > > Sounds okay to me.
One clarification: only non-volatile GPRs (R12-R15) need to be saved/restored. And I think it makes sense to sanitize any exposed GPRs (that don't hold an output value) after TDVMCALL to avoid speculating with a host-controlled value.
