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Date:   Mon, 30 Oct 2023 17:27:47 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Marc Zyngier <maz@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Huacai Chen <chenhuacai@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Anup Patel <anup@...infault.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     kvm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        kvmarm@...ts.linux.dev, linux-mips@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, kvm-riscv@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        Xu Yilun <yilun.xu@...el.com>,
        Chao Peng <chao.p.peng@...ux.intel.com>,
        Fuad Tabba <tabba@...gle.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Anish Moorthy <amoorthy@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        Isaku Yamahata <isaku.yamahata@...el.com>,
        Mickaël Salaün <mic@...ikod.net>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Ackerley Tng <ackerleytng@...gle.com>,
        Maciej Szmigiero <mail@...iej.szmigiero.name>,
        David Hildenbrand <david@...hat.com>,
        Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>,
        Wang <wei.w.wang@...el.com>,
        Liam Merwick <liam.merwick@...cle.com>,
        Isaku Yamahata <isaku.yamahata@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v13 02/35] KVM: Assert that mmu_invalidate_in_progress
 *never* goes negative

On 10/27/23 20:21, Sean Christopherson wrote:
> Move the assertion on the in-progress invalidation count from the primary
> MMU's notifier path to KVM's common notification path, i.e. assert that
> the count doesn't go negative even when the invalidation is coming from
> KVM itself.
> 
> Opportunistically convert the assertion to a KVM_BUG_ON(), i.e. kill only
> the affected VM, not the entire kernel.  A corrupted count is fatal to the
> VM, e.g. the non-zero (negative) count will cause mmu_invalidate_retry()
> to block any and all attempts to install new mappings.  But it's far from
> guaranteed that an end() without a start() is fatal or even problematic to
> anything other than the target VM, e.g. the underlying bug could simply be
> a duplicate call to end().  And it's much more likely that a missed
> invalidation, i.e. a potential use-after-free, would manifest as no
> notification whatsoever, not an end() without a start().

Reviewed-by: Paolo Bonzini <pbonzini@...hat.com>

> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   virt/kvm/kvm_main.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 0524933856d4..5a97e6c7d9c2 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -833,6 +833,7 @@ void kvm_mmu_invalidate_end(struct kvm *kvm, unsigned long start,
>   	 * in conjunction with the smp_rmb in mmu_invalidate_retry().
>   	 */
>   	kvm->mmu_invalidate_in_progress--;
> +	KVM_BUG_ON(kvm->mmu_invalidate_in_progress < 0, kvm);
>   }
>   
>   static void kvm_mmu_notifier_invalidate_range_end(struct mmu_notifier *mn,
> @@ -863,8 +864,6 @@ static void kvm_mmu_notifier_invalidate_range_end(struct mmu_notifier *mn,
>   	 */
>   	if (wake)
>   		rcuwait_wake_up(&kvm->mn_memslots_update_rcuwait);
> -
> -	BUG_ON(kvm->mmu_invalidate_in_progress < 0);
>   }
>   
>   static int kvm_mmu_notifier_clear_flush_young(struct mmu_notifier *mn,

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