Re: [PATCH Part2 v5 00/45] Add AMD Secure Nested Paging (SEV-SNP) Hypervisor Support

On 11/24/21 18:48, Dave Hansen wrote:

On 11/24/21 8:03 AM, Joerg Roedel wrote:

quoted

On Mon, Nov 22, 2021 at 02:51:35PM -0800, Dave Hansen wrote:

quoted

My preference would be that we never have SEV-SNP code in the kernel
that can panic() the host from guest userspace.  If that means waiting
until there's common guest unmapping infrastructure around, then I think
we should wait.

Can you elaborate how to crash host kernel from guest user-space? If I
understood correctly it was about crashing host kernel from _host_
user-space.

Sorry, I misspoke there.

My concern is about crashing the host kernel.  It appears that *host*
userspace can do that quite easily by inducing the host kernel to access
some guest private memory via a kernel mapping.

I thought some of the scenarios discussed here also went along "guest
(doesn't matter if userspace or kernel) shares a page with host, invokes
some host kernel operation and in parallel makes the page private again".

quoted

I think the RMP-fault path in the page-fault handler needs to take the
uaccess exception tables into account before actually causing a panic.
This should solve most of the problems discussed here.

That covers things like copy_from_user().  It does not account for
things where kernel mappings are used, like where a
get_user_pages()/kmap() is in play.