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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 > Cc: Michal Luczaj > Signed-off-by: Sean Christopherson --- > 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 > + > i = array_index_nospec(i, num_vcpus); > > /* Pairs with smp_wmb() in kvm_vm_ioctl_create_vcpu. */