[PATCH 3/3] KVM: Add helpers to wrap vcpu->srcu_idx and yell if it's abused

[Sean Christopherson <seanjc@google.com>]

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.