Merge: CVE-2024-58083: KVM: Explicitly verify target vCPU is online in kvm_get_vcpu()

MR: https://gitlab.com/redhat/centos-stream/src/kernel/centos-stream-9/-/merge_requests/6541

JIRA: https://issues.redhat.com/browse/RHEL-82650
CVE: CVE-2024-58083

```
commit 1e7381f
Author: Sean Christopherson <seanjc@google.com>
Date:   Wed Oct 9 08:04:50 2024 -0700

    KVM: Explicitly verify target vCPU is online in kvm_get_vcpu()

    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 c5b0775 ("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
    afb2acb ("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
    afb2acb, without re-introducing the vcpu_array[0] UAF race.

    Fixes: 1d487e9 ("KVM: fix spectrev1 gadgets")
    Cc: stable@vger.kernel.org
    Cc: Will Deacon <will@kernel.org>
    Cc: Michal Luczaj <mhal@rbox.co>
    Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
    Acked-by: Will Deacon <will@kernel.org>
    Link: https://lore.kernel.org/r/20241009150455.1057573-2-seanjc@google.com
    Signed-off-by: Sean Christopherson <seanjc@google.com>```

Signed-off-by: CKI Backport Bot <cki-ci-bot+cki-gitlab-backport-bot@redhat.com>

---

<small>Created 2025-03-07 11:50 UTC by backporter - [KWF FAQ](https://red.ht/kernel_workflow_doc) - [Slack #team-kernel-workflow](https://redhat-internal.slack.com/archives/C04LRUPMJQ5) - [Source](https://gitlab.com/cki-project/kernel-workflow/-/blob/main/webhook/utils/backporter.py) - [Documentation](https://gitlab.com/cki-project/kernel-workflow/-/blob/main/docs/README.backporter.md) - [Report an issue](https://gitlab.com/cki-project/kernel-workflow/-/issues/new?issue%5Btitle%5D=backporter%20webhook%20issue)</small>

Approved-by: Jon Maloy <jmaloy@redhat.com>
Approved-by: Leonardo Brás <leobras@redhat.com>
Approved-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Approved-by: CKI KWF Bot <cki-ci-bot+kwf-gitlab-com@redhat.com>

Merged-by: Augusto Caringi <acaringi@redhat.com>

Author: caringi

include/linux/kvm_host.h

@@ -955,6 +955,15 @@ 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;
+
 	i = array_index_nospec(i, num_vcpus);
 
 	/* Pairs with smp_wmb() in kvm_vm_ioctl_create_vcpu.  */