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

["Isaku Yamahata" <isaku.yamahata@intel.com>]

> REF: https://bugzilla.tianocore.org/show_bug.cgi?id=4696
> 
> Refer to the [GHCI] spec, TDVF should clear the BIT5 for RBP in the mask.
> And TDVF should clear the regitsers to avoid leaking secrets to VMM.
> 
> Reference:
> [GHCI]: TDX Guest-Host-Communication Interface v1.5
> https://cdrdv2.intel.com/v1/dl/getContent/726792
> 
> Cc: Erdem Aktas <erdemaktas@google.com>
> Cc: James Bottomley <jejb@linux.ibm.com>
> Cc: Jiewen Yao <jiewen.yao@intel.com>
> Cc: Min Xu <min.m.xu@intel.com>
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
> Cc: Michael Roth <michael.roth@amd.com>
> Cc: Gerd Hoffmann <kraxel@redhat.com>
> Cc: Erdem Aktas <erdemaktas@google.com>
> Cc: Isaku Yamahata <isaku.yamahata@intel.com>
> Signed-off-by: Ceping Sun <cepingx.sun@intel.com>
> ---
>  OvmfPkg/TdxDxe/X64/ApRunLoop.nasm | 30 ++++++++++++++++++++++++++----
>  1 file changed, 26 insertions(+), 4 deletions(-)
> 
> diff --git a/OvmfPkg/TdxDxe/X64/ApRunLoop.nasm b/OvmfPkg/TdxDxe/X64/ApRunLoop.nasm
> index 0bef89c48552..57560015f491 100644
> --- a/OvmfPkg/TdxDxe/X64/ApRunLoop.nasm
> +++ b/OvmfPkg/TdxDxe/X64/ApRunLoop.nasm
> @@ -20,7 +20,7 @@ SECTION .text
> 
>  BITS 64
> 
> -%define TDVMCALL_EXPOSE_REGS_MASK       0xffec
> +%define TDVMCALL_EXPOSE_REGS_MASK       0xffcc
>  %define TDVMCALL                        0x0
>  %define EXIT_REASON_CPUID               0xa
> 
> @@ -28,6 +28,30 @@ BITS 64
>    db  0x66, 0x0f, 0x01, 0xcc
>  %endmacro
> 
> +%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?

Thanks,


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