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 <[email protected]>
> Cc: Radim Krčmář <[email protected]>
> Signed-off-by: Christoffer Dall <[email protected]>
> ---
> 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.
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,
thanks.
> kfree(fpu);
> kfree(kvm_sregs);
> return r;
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