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

From: Dave Hansen
Date: Mon Nov 22 2021 - 14:14:26 EST

On 11/22/21 11:06 AM, Brijesh Singh wrote:
>> 3. Kernel accesses guest private memory via a kernel mapping. This one
>> is tricky. These probably *do* result in a panic() today, but
>> ideally shouldn't.
> KVM has defined some helper functions to maps and unmap the guest pages.
> Those helper functions do the GPA to PFN lookup before calling the
> kmap(). Those helpers are enhanced such that it check the RMP table
> before the kmap() and acquire a lock to prevent a page state change
> until the kunmap() is called. So, in the current implementation, we
> should *not* see a panic() unless there is a KVM driver bug that didn't
> use the helper functions or a bug in the helper function itself.

I don't think this is really KVM specific.

Think of a remote process doing ptrace(PTRACE_POKEUSER) or pretty much
any generic get_user_pages() instance. As long as the memory is mapped
into the page tables, you're exposed to users that walk the page tables.

How do we, for example, prevent ptrace() from inducing a panic()?