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

[Arvind Sankar]

On Tue, Oct 20, 2020 at 10:59:57AM +0200, Joerg Roedel wrote:
> 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.
> 

That doesn't feel like a big loss: if a malicious hypervisor wanted to
induce completely random code execution, it can do that anyway by just
messing with the guest-to-host translation, no?

We would need to avoid calling this in the secondary cpu startup, I guess.

I was hoping to be able to clean up the identity mapping in
__startup_64(), which currently maps the entire kernel using wraparound
entries, to just map the head page of the kernel, since AFAICT nothing
else is actually used from the identity mapping after switching to the
new page tables. But we'd need to keep it to support this check.