Re: [PATCH] KVM: Avoid illegal stage2 mapping on invalid memory slot

Marc Zyngier
2023-06-08T14:31:23Z

Hi Gavin,

On Thu, 08 Jun 2023 10:03:48 +0100,
Gavin Shan <gshan@redhat.com> wrote:
>
> We run into guest hang in edk2 firmware when KSM is kept as running
> on the host. The edk2 firmware is waiting for status 0x80 from QEMU's
> pflash device (TYPE_PFLASH_CFI01) during the operation for sector
> erasing or buffered write. The status is returned by reading the
> memory region of the pflash device and the read request should
> have been forwarded to QEMU and emulated by it. Unfortunately, the
> read request is covered by an illegal stage2 mapping when the guest
> hang issue occurs. The read request is completed with QEMU bypassed and
> wrong status is fetched.
>
> The illegal stage2 mapping is populated due to same page mering by
> KSM at (C) even the associated memory slot has been marked as invalid
> at (B).
>
> CPU-A CPU-B
> ----- -----
> ioctl(kvm_fd, KVM_SET_USER_MEMORY_REGION)
> kvm_vm_ioctl_set_memory_region
> kvm_set_memory_region
> __kvm_set_memory_region
> kvm_set_memslot(kvm, old, NULL, KVM_MR_DELETE)
> kvm_invalidate_memslot
> kvm_copy_memslot
> kvm_replace_memslot
> kvm_swap_active_memslots (A)
> kvm_arch_flush_shadow_memslot (B)
> same page merging by KSM
> kvm_mmu_notifier_change_pte
> kvm_handle_hva_range
> __kvm_handle_hva_range (C)
>
> Fix the issue by skipping the invalid memory slot at (C) to avoid the
> illegal stage2 mapping. Without the illegal stage2 mapping, the read
> request for the pflash's status is forwarded to QEMU and emulated by
> it. The correct pflash's status can be returned from QEMU to break
> the infinite wait in edk2 firmware.

Huh, nice one :-(.

>
> Cc: stable@vger.kernel.org # v5.13+
> Fixes: 3039bcc74498 ("KVM: Move x86's MMU notifier memslot walkers to generic code")
> Reported-by: Shuai Hu <hshuai@redhat.com>
> Reported-by: Zhenyu Zhang <zhenyzha@redhat.com>
> Signed-off-by: Gavin Shan <gshan@redhat.com>
> ---
> virt/kvm/kvm_main.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 479802a892d4..7f81a3a209b6 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -598,6 +598,9 @@ static __always_inline int __kvm_handle_hva_range(struct kvm *kvm,
> unsigned long hva_start, hva_end;
>
> slot = container_of(node, struct kvm_memory_slot, hva_node[slots->node_idx]);
> + if (slot->flags & KVM_MEMSLOT_INVALID)
> + continue;
> +
> hva_start = max(range->start, slot->userspace_addr);
> hva_end = min(range->end, slot->userspace_addr +
> (slot->npages << PAGE_SHIFT));

I don't immediately see what makes it safer. If we're not holding one
of slots_{,arch_}lock in the notifier, we can still race against the
update, can't we? I don't think holding the srcu lock helps us here.

Thanks,

M.

--
Without deviation from the norm, progress is not possible.