Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Thu, 08 Jun 2023 15:31:09 +0100
Message-ID: <87bkhqnhzm.wl-maz@kernel.org>
From:   Marc Zyngier <maz@kernel.org>
To:     Gavin Shan <gshan@redhat.com>
Cc:     kvmarm@lists.linux.dev, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, pbonzini@redhat.com,
        seanjc@google.com, oliver.upton@linux.dev, hshuai@redhat.com,
        zhenyzha@redhat.com, shan.gavin@gmail.com
Subject: Re: [PATCH] KVM: Avoid illegal stage2 mapping on invalid memory slot
In-Reply-To: <20230608090348.414990-1-gshan@redhat.com>
References: <20230608090348.414990-1-gshan@redhat.com>
User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue)
 FLIM-LB/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL-LB/10.8 EasyPG/1.0.0 Emacs/28.2
 (x86_64-pc-linux-gnu) MULE/6.0 (HANACHIRUSATO)
MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue")
Content-Type: text/plain; charset=US-ASCII
Precedence: bulk

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.