Re: [PATCH v3 1/2] KVM: s390: Don't indicate suppression on dirtying, failing memop

[David Hildenbrand <david@redhat.com>]

On 12.05.22 15:10, Janis Schoetterl-Glausch wrote:
> If user space uses a memop to emulate an instruction and that
> memop fails, the execution of the instruction ends.
> Instruction execution can end in different ways, one of which is
> suppression, which requires that the instruction execute like a no-op.
> A writing memop that spans multiple pages and fails due to key
> protection may have modified guest memory, as a result, the likely
> correct ending is termination. Therefore, do not indicate a
> suppressing instruction ending in this case.

I think that is possibly problematic handling.

In TCG we stumbled in similar issues in the past for MVC when crossing
page boundaries. Failing after modifying the first page already
seriously broke some user space, because the guest would retry the
instruction after fixing up the fault reason on the second page: if
source and destination operands overlap, you'll be in trouble because
the input parameters already changed.

For this reason, in TCG we make sure that all accesses are valid before
starting modifications.

See target/s390x/tcg/mem_helper.c:do_helper_mvc with access_prepare()
and friends as an example.

Now, I don't know how to tackle that for KVM, I just wanted to raise
awareness that injecting an interrupt after modifying page content is
possible dodgy and dangerous.

-- 
Thanks,

David / dhildenb