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Date:   Tue, 19 Apr 2022 15:45:52 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Maxim Levitsky <mlevitsk@redhat.com>
Cc:     Anup Patel <anup@brainfault.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Christian Borntraeger <borntraeger@linux.ibm.com>,
        Janosch Frank <frankja@linux.ibm.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Atish Patra <atishp@atishpatra.org>,
        David Hildenbrand <david@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, linuxppc-dev@lists.ozlabs.org,
        kvm@vger.kernel.org, kvm-riscv@lists.infradead.org,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/3] KVM: Add helpers to wrap vcpu->srcu_idx and yell if
 it's abused
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On Tue, Apr 19, 2022, Maxim Levitsky wrote:
> On Fri, 2022-04-15 at 00:43 +0000, Sean Christopherson wrote:
> > Add wrappers to acquire/release KVM's SRCU lock when stashing the index
> > in vcpu->src_idx, along with rudimentary detection of illegal usage,
> > e.g. re-acquiring SRCU and thus overwriting vcpu->src_idx.  Because the
> > SRCU index is (currently) either 0 or 1, illegal nesting bugs can go
> > unnoticed for quite some time and only cause problems when the nested
> > lock happens to get a different index.
> > 
> > Wrap the WARNs in PROVE_RCU=y, and make them ONCE, otherwise KVM will
> > likely yell so loudly that it will bring the kernel to its knees.
> > 
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---

...

> Looks good to me overall.
> 
> Note that there are still places that acquire the lock and store the idx into
> a local variable, for example kvm_xen_vcpu_set_attr and such.
> I didn't check yet if these should be converted as well.

Using a local variable is ok, even desirable.  Nested/multiple readers is not an
issue, the bug fixed by patch 1 is purely that kvm_vcpu.srcu_idx gets corrupted.

In an ideal world, KVM would _only_ track the SRCU index in local variables, but
that would require plumbing the local variable down into vcpu_enter_guest() and
kvm_vcpu_block() so that SRCU can be unlocked prior to entering the guest or
scheduling out the vCPU.