Re: [edk2-devel] [PATCH V1 3/3] OvmfPkg/TdxDxe: Clear the registers before tdcall

["sunceping" <cepingx.sun@intel.com>]

On Tuesday, February 27, 2024 8:26 PM Yamahata, Isaku wrote:
> > +%macro tdcall_regs_preamble 2
> > +    mov rax, %1
> > +
> > +    xor rcx, rcx
> > +    mov ecx, %2
> > +
> > +    ; R10 = 0 (standard TDVMCALL)
> > +
> > +    xor r10d, r10d
> > +
> > +    ; Zero out unused (for standard TDVMCALL) registers to avoid leaking
> > +    ; secrets to the VMM.
> > +
> > +    xor esi, esi
> > +    xor edi, edi
> > +
> > +    xor edx, edx
> > +    xor ebp, ebp
> > +    xor r8d, r8d
> > +    xor r9d, r9d
> > +    xor r14, r14
> > +    xor r15, r15
> 
> We can just clear the corresponding bit of TDVMCALL_EXPOSE_REGS_MASK in
> addition to RBP.
> Same to 1/3 and 3/3. We can eliminate tdcall_regs_postamble.
> Any reason to bother to zero those registers and pass them to VMM?
>
Zero out these registers to avoid leaking secrets to the VMM.
There are also some registers (e.g., r10, r14...... etc.) are output operands and should be cleared.
The tdcall_regs_preamble  was already using in the TdVmcall.nasm and TdVmcallCpuid.nasm .
For the ApRunLoop.nasm , it is fixed now.

Thanks
Ceping



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