On Fri, Oct 13, 2017 at 07:13:07PM +0200, Radim Krčmář wrote:
> 2017-10-12 12:41+0200, Christoffer Dall:
> > Some architectures may decide to do different things during
> > kvm_arch_vcpu_load depending on the ioctl being executed. For example,
> > arm64 is about to do significant work in vcpu load/put when running a
> > vcpu, but not when doing things like KVM_SET_ONE_REG or
> > KVM_SET_MP_STATE.
> >
> > Therefore, store the ioctl number that we are executing on the VCPU
> > during the first vcpu_load() which succeeds in getting the vcpu->mutex
> > and set the ioctl number to 0 when exiting kvm_vcpu_ioctl() after
> > successfully loading the vcpu.
> >
> > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > Cc: Radim Krčmář <rkrcmar@xxxxxxxxxx>
> > Signed-off-by: Christoffer Dall <christoffer.dall@xxxxxxxxxx>
> > ---
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > @@ -147,12 +147,13 @@ bool kvm_is_reserved_pfn(kvm_pfn_t pfn)
> > /*
> > * Switches to specified vcpu, until a matching vcpu_put()
> > */
> > -int vcpu_load(struct kvm_vcpu *vcpu)
> > +int vcpu_load(struct kvm_vcpu *vcpu, unsigned int ioctl)
> > {
> > int cpu;
> >
> > if (mutex_lock_killable(&vcpu->mutex))
> > return -EINTR;
> > + vcpu->ioctl = ioctl;
>
> This seems to prevent races by protecting the store by a mutex, but
>
> > cpu = get_cpu();
> > preempt_notifier_register(&vcpu->preempt_notifier);
> > kvm_arch_vcpu_load(vcpu, cpu);
> > @@ -2529,7 +2530,7 @@ static long kvm_vcpu_ioctl(struct file *filp,
> > #endif
> >
> >
> > - r = vcpu_load(vcpu);
> > + r = vcpu_load(vcpu, ioctl);
> > if (r)
> > return r;
> > switch (ioctl) {
> > @@ -2704,6 +2705,7 @@ static long kvm_vcpu_ioctl(struct file *filp,
> > }
> > out:
> > vcpu_put(vcpu);
> > + vcpu->ioctl = 0;
>
> we should still have a race as we clear ioctl only after releasing the
> lock. For example malicious userspace could break KVM terms of use and
> issue !KVM_RUN followed by KVM_RUN, so we would have these races:
>
> !KVM_RUN | KVM_RUN
>
> mutex_lock_killable(&vcpu->mutex); |
> vcpu->ioctl = !KVM_RUN; |
> ... | mutex_lock_killable(&vcpu->mutex);
> mutex_unlock(&vcpu->mutex); |
> | vcpu->ioctl = KVM_RUN;
> | kvm_arch_vcpu_load() // variant 1
> vcpu->ioctl = 0; | ...
> | kvm_arch_vcpu_load() // variant 2
> | vcpu_put()
>
> where the observed value of vcpu->ioctl in vcpu_put() would not
> correctly pair with vcpu_load() or worse, kvm_arch_arch_load() in
> KVM_RUN would execute with vcpu->ioctl = 0.
Yeah, this is super racy, thanks for pointing that out.
>
> I think that other (special) callsites of vcpu_load()/vcpu_put() have a
> well defined IOCTL that can be used instead of vcpu->ioctl, so we could
> just pass the ioctl value all the way to arch code and never save it
> anywhere,
I don't think that works; what would you do with preempt notifier calls?
One solution is to add a parameter to vcpu_put, lie for vcpu_load, which
also sets the ioctl, and other callers than the final vcpu_put in
kvm_vcpu_ioctl() just pass the existing value, where the kvm_vcpu_ioctl
call can pass 0 which gets set before releasing the mutex.
Can you think of a more elegant solution?
Thanks,
-Christoffer
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