Re: [PATCH v10 12/26] gunyah: vm_mgr: Add/remove user memory regions

[Fuad Tabba]

Hi,

On Fri, Feb 24, 2023 at 6:08 PM Elliot Berman <quic_eberman@xxxxxxxxxxx> wrote:
>
>
>
> On 2/24/2023 2:19 AM, Fuad Tabba wrote:
> > Hi,
> >
> > On Tue, Feb 14, 2023 at 9:26 PM Elliot Berman <quic_eberman@xxxxxxxxxxx> wrote:
> >>
> >>
> >> When launching a virtual machine, Gunyah userspace allocates memory for
> >> the guest and informs Gunyah about these memory regions through
> >> SET_USER_MEMORY_REGION ioctl.
> >
> > I'm working on pKVM [1], and regarding the problem of donating private
> > memory to a guest, we and others working on confidential computing
> > have faced a similar issue that this patch is trying to address. In
> > pKVM, we've initially taken an approach similar to the one here by
> > pinning the pages being donated to prevent swapping or migration [2].
> > However, we've encountered issues with this approach since the memory
> > is still mapped by the host, which could cause the system to crash on
> > an errant access.
> >
> > Instead, we've been working on adopting an fd-based restricted memory
> > approach that was initially proposed for TDX [3] and is now being
> > considered by others in the confidential computing space as well
> > (e.g., Arm CCA [4]). The basic idea is that the host manages the guest
> > memory via a file descriptor instead of a userspace address. It cannot
> > map that memory (unless explicitly shared by the guest [5]),
> > eliminating the possibility of the host trying to access private
> > memory accidentally or being tricked by a malicious actor. This is
> > based on memfd with some restrictions. It handles swapping and
> > migration by disallowing them (for now [6]), and adds a new type of
> > memory region to KVM to accommodate having an fd representing guest
> > memory.
> >
> > Although the fd-based restricted memory isn't upstream yet, we've
> > ported the latest patches to arm64 and made changes and additions to
> > make it work with pKVM, to test it and see if the solution is feasible
> > for us (it is). I wanted to mention this work in case you find it
> > useful, and in the hopes that we can all work on confidential
> > computing using the same interfaces as much as possible.
>
> Thanks for highlighting the memfd_restricted changes to us! We'll
> investigate how/if it can suit Gunyah usecases. It sounds like you
> might've made memfd_restricted changes as well? Are those posted on the
> mailing lists? Also, are example userspace (crosvm?) changes posted?

I have posted kvmtool changes to make it work with memfd_restricted
and pKVM as an RFC [1] (git [2]). I haven't posted the arm64 port, but
it's in a git repo [3]. Chao has a repository with qemu support (TDX)
as well [4].

Eventually, we're likely to have crosvm support as well. If you're
interested, I can keep you CCed on anything we post upstream.

Cheers,
/fuad

[1] https://lore.kernel.org/all/20221202174417.1310826-1-tabba@xxxxxxxxxx/
[2] https://android-kvm.googlesource.com/kvmtool/+/refs/heads/tabba/fdmem-v10-core
[3] https://android-kvm.googlesource.com/linux/+/refs/heads/tabba/fdmem-v10-core
[4] https://github.com/chao-p/qemu/tree/privmem-v10

>
> Thanks,
> Elliot
>
> >
> > Some comments inline below...
> >
> > Cheers,
> > /fuad
> >
> > [1] https://lore.kernel.org/kvmarm/20220519134204.5379-1-will@xxxxxxxxxx/
> > [2] https://lore.kernel.org/kvmarm/20220519134204.5379-34-will@xxxxxxxxxx/
> > [3] https://lore.kernel.org/all/20221202061347.1070246-1-chao.p.peng@xxxxxxxxxxxxxxx/
> > [4] https://lore.kernel.org/lkml/20230127112932.38045-1-steven.price@xxxxxxx/
> > [5] This is a modification we've done for the arm64 port, after
> > discussing it with the original authors.
> > [6] Nothing inherent in the proposal to stop migration and swapping.
> > There are some technical issues that need to be resolved.
> >
> > <snip>
> <snip, looking at comments in parallel>