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

Brijesh Singh
2021-11-29T14:45:01Z

On 11/25/21 4:05 AM, Joerg Roedel wrote:
> On Wed, Nov 24, 2021 at 09:48:14AM -0800, Dave Hansen wrote:
>> 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.
>
> The kmap case is guarded by KVM code, which locks the page first so that
> the guest can't change the page state, then checks the page state, and
> if it is shared does the kmap and the access.

The KVM use-case is well covered in the series, but I believe Dave is
highlighting what if the access happens outside of the KVM driver (such
as a ptrace() or others).

One possible approach to fix this is to enlighten the kmap/unmap().
Basically, move the per page locking mechanism used by the KVM in the
arch-specific code and have kmap/kunmap() call the arch hooks. The arch
hooks will do this:

Before the map, check whether the page is added as a shared in the RMP
table. If not shared, then error.
Acquire a per-page map_lock.
Release the per-page map_lock on the kunmap().

The current patch set provides helpers to change the page from private
to shared. Enhance the helpers to check for the per-page map_lock, if
the map_lock is held then do not allow changing the page from shared to
private.

Thoughts ?

>
> This should turn an RMP fault in the kernel which is not covered in the
> uaccess exception table into a fatal error.
>
> Regards,
>