Re: [PATCH 3/5] x86/boot/compressed/64: Check SEV encryption in 64-bit boot-path

[Joerg Roedel]

On Mon, Oct 19, 2020 at 05:31:06PM -0400, Arvind Sankar wrote:
> Is it possible to take advantage of this to make the check independent
> of the original page tables? i.e. switch to the new pagetables, then
> write into .data or .bss the opcodes for a function that does
> 	movabs	$imm64, %rax
> 	jmp	*%rdi	// avoid using stack for the return
> filling in the imm64 with the RDRAND value, and then try to execute it.
> If the C-bit value is wrong, this will probably crash, and at any rate
> shouldn't return with the correct value in %rax.

That could work, but is not reliable. When the C bit is wrong the CPU
would essentially execute random data, which could also be a valid
instruction stream. A crash is not guaranteed.

Regards,

	Joerg