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Message-ID: <ea6777df-7bb6-eeb9-645e-548bcbd6c2f6@redhat.com>
Date: Fri, 16 Jun 2023 16:35:18 +0800
From: Shaoqin Huang <shahuang@...hat.com>
To: Gavin Shan <gshan@...hat.com>, kvmarm@...ts.linux.dev
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
pbonzini@...hat.com, seanjc@...gle.com, mtosatti@...hat.com,
maz@...nel.org, will@...nel.org, c.dall@...tualopensystems.com,
peterx@...hat.com, david@...hat.com, aarcange@...hat.com,
hshuai@...hat.com, zhenyzha@...hat.com, shan.gavin@...il.com
Subject: Re: [PATCH v3] KVM: Avoid illegal stage2 mapping on invalid memory
slot
On 6/15/23 13:42, 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@...r.kernel.org # v3.9+
> Fixes: d5d8184d35c9 ("KVM: ARM: Memory virtualization setup")
> Reported-by: Shuai Hu <hshuai@...hat.com>
> Reported-by: Zhenyu Zhang <zhenyzha@...hat.com>
> Signed-off-by: Gavin Shan <gshan@...hat.com>
Reviewed-by: Shaoqin Huang <shahuang@...hat.com>
> ---
> 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.
> ---
> virt/kvm/kvm_main.c | 20 +++++++++++++++++++-
> 1 file changed, 19 insertions(+), 1 deletion(-)
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 479802a892d4..65f94f592ff8 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -686,6 +686,24 @@ static __always_inline int kvm_handle_hva_range_no_flush(struct mmu_notifier *mn
>
> return __kvm_handle_hva_range(kvm, &range);
> }
> +
> +static bool kvm_change_spte_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
> +{
> + /*
> + * Skipping invalid memslots is correct if and only change_pte() is
> + * surrounded by invalidate_range_{start,end}(), which is currently
> + * guaranteed by the primary MMU. If that ever changes, KVM needs to
> + * unmap the memslot instead of skipping the memslot to ensure that KVM
> + * doesn't hold references to the old PFN.
> + */
> + WARN_ON_ONCE(!READ_ONCE(kvm->mn_active_invalidate_count));
> +
> + if (range->slot->flags & KVM_MEMSLOT_INVALID)
> + return false;
> +
> + return kvm_set_spte_gfn(kvm, range);
> +}
> +
> static void kvm_mmu_notifier_change_pte(struct mmu_notifier *mn,
> struct mm_struct *mm,
> unsigned long address,
> @@ -707,7 +725,7 @@ static void kvm_mmu_notifier_change_pte(struct mmu_notifier *mn,
> if (!READ_ONCE(kvm->mmu_invalidate_in_progress))
> return;
>
> - kvm_handle_hva_range(mn, address, address + 1, pte, kvm_set_spte_gfn);
> + kvm_handle_hva_range(mn, address, address + 1, pte, kvm_change_spte_gfn);
> }
>
> void kvm_mmu_invalidate_begin(struct kvm *kvm, unsigned long start,
--
Shaoqin
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