Re: [PATCH v2] KVM: Avoid illegal stage2 mapping on invalid memory slot

[David Hildenbrand]

On 09.06.23 12:04, Gavin Shan 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 of 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 edk2
> firmware runs into an infinite loop with the wrong status.
>
> The illegal stage2 mapping is populated due to same page sharing by KSM
> at (C) even the associated memory slot has been marked as invalid at (B)
> when the memory slot is requested to be deleted. It's notable that the
> active and inactive memory slots can't be swapped when we're in the middle
> of kvm_mmu_notifier_change_pte() because kvm->mn_active_invalidate_count
> is elevated, and kvm_swap_active_memslots() will busy loop until it reaches
> to zero again. Besides, the swapping from the active to the inactive memory
> slots is also avoided by holding &kvm->srcu in __kvm_handle_hva_range(),
> corresponding to synchronize_srcu_expedited() in kvm_swap_active_memslots().
>
>    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 sharing by KSM
>    kvm_mmu_notifier_invalidate_range_start
>          :
>    kvm_mmu_notifier_change_pte
>      kvm_handle_hva_range
>      __kvm_handle_hva_range       (C)
>          :
>    kvm_mmu_notifier_invalidate_range_end
>
> Fix the issue by skipping the invalid memory slot at (C) to avoid the
> illegal stage2 mapping so that the read request for the pflash's status
> is forwarded to QEMU and emulated by it. In this way, the correct pflash's
> status can be returned from QEMU to break the infinite loop in the edk2
> firmware.
>
> Cc: stable@xxxxxxxxxxxxxxx # v5.13+
> Fixes: 3039bcc74498 ("KVM: Move x86's MMU notifier memslot walkers to generic code")
> Reported-by: Shuai Hu <hshuai@xxxxxxxxxx>
> Reported-by: Zhenyu Zhang <zhenyzha@xxxxxxxxxx>
> Signed-off-by: Gavin Shan <gshan@xxxxxxxxxx>
> ---
> v2: Improved changelog suggested by Marc
> ---
>   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));

Nice debugging!

LGTM

Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>

--
Cheers,

David / dhildenb