Re: [PATCH v3] KVM: Avoid illegal stage2 mapping on invalid memory slot

[Peter Xu]

On Thu, Jun 15, 2023 at 03:42:59PM +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
>     kvm_set_spte_gfn            (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.
> 
> We tried a git-bisect and the first problematic commit is cd4c71835228 ("
> KVM: arm64: Convert to the gfn-based MMU notifier callbacks"). With this,
> clean_dcache_guest_page() is called after the memory slots are iterated
> in kvm_mmu_notifier_change_pte(). clean_dcache_guest_page() is called
> before the iteration on the memory slots before this commit. This change
> literally enlarges the racy window between kvm_mmu_notifier_change_pte()
> and memory slot removal so that we're able to reproduce the issue in a
> practical test case. However, the issue exists since commit d5d8184d35c9
> ("KVM: ARM: Memory virtualization setup").
> 
> Cc: stable@xxxxxxxxxxxxxxx # v3.9+
> Fixes: d5d8184d35c9 ("KVM: ARM: Memory virtualization setup")
> Reported-by: Shuai Hu <hshuai@xxxxxxxxxx>
> Reported-by: Zhenyu Zhang <zhenyzha@xxxxxxxxxx>
> Signed-off-by: Gavin Shan <gshan@xxxxxxxxxx>
> ---
> v3: Skip the invalid memory slots in change_pte() MMU notifier only,
>     suggested by Sean. Improved changelog to describe how the fixes
>     tag is given.

True.. FWIW:

Reviewed-by: Peter Xu <peterx@xxxxxxxxxx>

-- 
Peter Xu