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

[Arvind Sankar]

On Mon, Oct 19, 2020 at 10:39:35PM +0200, Joerg Roedel wrote:
> On Mon, Oct 19, 2020 at 01:54:47PM -0400, Arvind Sankar wrote:
> > Also, isn't it possible that the initial page tables we're running on
> > have already been messed with and have the C-bit in the wrong location,
> > so that this write happens decrypted?
> 
> The code assumes that the page-table it is running on has the correct C
> bit position set and that the code which set it up verified that it is
> correct. For the kernel itself this is true, at least, but when booting
> via UEFI the check also needs to happen in the firmware.
> 
> Note that the possibilies are limited when the hypervisor reports the
> wrong C bit position because code fetches always assume encryption, even
> when the C bit is cleared in the page-table. So a wrong C bit position
> in the decompression stub would write the kernel image to memory
> unencrypted and executing it would not be possible.

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.

> 
> Regards,
> 
> 	Joerg
>