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

From: Joerg Roedel
Date: Tue Nov 16 2021 - 08:02:22 EST

On Mon, Nov 15, 2021 at 06:26:16PM +0000, Sean Christopherson wrote:
> No, because as Andy pointed out, host userspace must already guard against a bad
> GPA, i.e. this is just a variant of the guest telling the host to DMA to a GPA
> that is completely bogus. The shared vs. private behavior just means that when
> host userspace is doing a GPA=>HVA lookup, it needs to incorporate the "shared"
> state of the GPA. If the host goes and DMAs into the completely wrong HVA=>PFN,
> then that is a host bug; that the bug happened to be exploited by a buggy/malicious
> guest doesn't change the fact that the host messed up.

The thing is that the usual checking mechanisms can't be applied to
guest-private pages. For user-space the GPA is valid if it fits into the
guest memory layout user-space set up before. But whether a page is
shared or private is the guests business. And without an expensive
reporting/query mechanism user-space doesn't have the information to do
the check.

A mechanism to lock pages to shared is also needed, and that creates the
next problems:

* Who can release the lock, only the process which created it or
anyone who has the memory mapped?

* What happens when a process has locked guest regions and then
dies with SIGSEGV, will its locks on guest memory be released
stay around forever?

And this is only what comes to mind immediatly, I sure there are more
problematic details in such an interface.


Jörg Rödel

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