Re: [PATCH 1/6] KVM: Explicitly verify target vCPU is online in kvm_get_vcpu()

["Gupta, Pankaj" <pankaj.gupta@amd.com>]

On 10/9/2024 5:04 PM, Sean Christopherson wrote:
> Explicitly verify the target vCPU is fully online _prior_ to clamping the
> index in kvm_get_vcpu().  If the index is "bad", the nospec clamping will
> generate '0', i.e. KVM will return vCPU0 instead of NULL.
> 
> In practice, the bug is unlikely to cause problems, as it will only come
> into play if userspace or the guest is buggy or misbehaving, e.g. KVM may
> send interrupts to vCPU0 instead of dropping them on the floor.
> 
> However, returning vCPU0 when it shouldn't exist per online_vcpus is
> problematic now that KVM uses an xarray for the vCPUs array, as KVM needs
> to insert into the xarray before publishing the vCPU to userspace (see
> commit c5b077549136 ("KVM: Convert the kvm->vcpus array to a xarray")),
> i.e. before vCPU creation is guaranteed to succeed.
> 
> As a result, incorrectly providing access to vCPU0 will trigger a
> use-after-free if vCPU0 is dereferenced and kvm_vm_ioctl_create_vcpu()
> bails out of vCPU creation due to an error and frees vCPU0.  Commit
> afb2acb2e3a3 ("KVM: Fix vcpu_array[0] races") papered over that issue, but
> in doing so introduced an unsolvable teardown conundrum.  Preventing
> accesses to vCPU0 before it's fully online will allow reverting commit
> afb2acb2e3a3, without re-introducing the vcpu_array[0] UAF race.

I think I have observed this (the cause, not the effect on teardown) 
accidentally when creating a vCPU for an overflowing vcpu_id.

> 
> Fixes: 1d487e9bf8ba ("KVM: fix spectrev1 gadgets")
> Cc: stable@vger.kernel.org
> Cc: Will Deacon <will@kernel.org>
> Cc: Michal Luczaj <mhal@rbox.co>
> Signed-off-by: Sean Christopherson <seanjc@google.com > ---
>   include/linux/kvm_host.h | 9 +++++++++
>   1 file changed, 9 insertions(+)
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index db567d26f7b9..450dd0444a92 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -969,6 +969,15 @@ static inline struct kvm_io_bus *kvm_get_bus(struct kvm *kvm, enum kvm_bus idx)
>   static inline struct kvm_vcpu *kvm_get_vcpu(struct kvm *kvm, int i)
>   {
>   	int num_vcpus = atomic_read(&kvm->online_vcpus);
> +
> +	/*
> +	 * Explicitly verify the target vCPU is online, as the anti-speculation
> +	 * logic only limits the CPU's ability to speculate, e.g. given a "bad"
> +	 * index, clamping the index to 0 would return vCPU0, not NULL.
> +	 */
> +	if (i >= num_vcpus)
> +		return NULL;

Would sev.c needs a NULL check for?

sev_migrate_from()
...
src_vcpu = kvm_get_vcpu(src_kvm, i);
src_svm = to_svm(src_vcpu);
...

Apart from the above comment, this looks good to me:

Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>


> +
>   	i = array_index_nospec(i, num_vcpus);
>   
>   	/* Pairs with smp_wmb() in kvm_vm_ioctl_create_vcpu.  */