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Re: [PATCH v2 0/2] MTE support for KVM guest

From: Steven Price
Subject: Re: [PATCH v2 0/2] MTE support for KVM guest
Date: Thu, 10 Sep 2020 15:14:47 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

On 10/09/2020 14:56, Andrew Jones wrote:
On Thu, Sep 10, 2020 at 10:21:04AM +0100, Steven Price wrote:
On 10/09/2020 07:29, Andrew Jones wrote:
But if userspace created the memslots with memory already set with
PROT_MTE, then this wouldn't be necessary, right? And, as long as
there's still a way to access the memory with tag checking disabled,
then it shouldn't be a problem.

Yes, so one option would be to attempt to validate that the VMM has provided
memory pages with the PG_mte_tagged bit set (e.g. by mapping with PROT_MTE).
The tricky part here is that we support KVM_CAP_SYNC_MMU which means that
the VMM can change the memory backing at any time - so we could end up in
user_mem_abort() discovering that a page doesn't have PG_mte_tagged set - at
that point there's no nice way of handling it (other than silently upgrading
the page) so the VM is dead.

So since enforcing that PG_mte_tagged is set isn't easy and provides a
hard-to-debug foot gun to the VMM I decided the better option was to let the
kernel set the bit automatically.

The foot gun still exists when migration is considered, no? If userspace
is telling a guest it can use MTE on its normal memory, but then doesn't
prepare that memory correctly, or remember to migrate the tags correctly
(which requires knowing the memory has tags and knowing how to get them),
then I guess the VM is in trouble one way or another.

Well not all VMMs support migration, and it's only migration that is affected by this for a simple VMM (e.g. the changes to kvmtool are minimal for MTE). But yes fundamentally if a VMM enables MTE it needs to know how to deal with the extra tags everywhere.

I feel like we should trust the VMM to ensure MTE will work on any memory
the guest could use it on, and change the action in user_mem_abort() to
abort the guest with a big error message if it sees the flag is missing.

I'm happy to change it, if you feel this is easier to debug.

If userspace needs to write to guest memory then it should be due to
a device DMA or other specific hardware emulation. Those accesses can
be done with tag checking disabled.

Yes, the question is can the VMM (sensibly) wrap the accesses with a
disable/renable tag checking for the process sequence. The alternative at
the moment is to maintain a separate (untagged) mapping for the purpose
which might present it's own problems.

Hmm, so there's no easy way to disable tag checking when necessary? If we
don't map the guest ram with PROT_MTE and continue setting the attribute
in KVM, as this series does, then we don't need to worry about it tag
checking when accessing the memory, but then we can't access the tags for

There's a "TCO" (Tag Check Override) bit in PSTATE which allows disabling
tag checking, so if it's reasonable to wrap accesses to the memory you can
simply set the TCO bit, perform the memory access and then unset TCO. That
would mean a single mapping with MTE enabled would work fine. What I don't
have a clue about is whether it's practical in the VMM to wrap guest
accesses like this.

At least QEMU goes through many abstractions to get to memory already.
There may already be a hook we could use, if not, it probably wouldn't
be too hard to add one (famous last words).

Sounds good. My hope was that the abstractions were already in there.



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