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

[Peter Xu]

On Fri, Jun 09, 2023 at 08:04:20PM +1000, 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>

Reviewed-by: Peter Xu <peterx@xxxxxxxxxx>

-- 
Peter Xu