Re: [PATCH] KVM: Fix error path in kvm_vm_ioctl_create_vcpu() on xa_store() failure

[Sean Christopherson <seanjc@google.com>]

On Tue, Jul 30, 2024, Michal Luczaj wrote:
> On 7/30/24 17:56, Will Deacon wrote:
> > If the xa_store() fails in kvm_vm_ioctl_create_vcpu() then we shouldn't
> > drop the reference to the 'struct kvm' because the vCPU fd has been
> > installed and will take care of the refcounting.
> > 
> > This was found by inspection, but forcing the xa_store() to fail
> > confirms the problem:
> > 
> >  | Unable to handle kernel paging request at virtual address ffff800080ecd960
> >  | Call trace:
> >  |  _raw_spin_lock_irq+0x2c/0x70
> >  |  kvm_irqfd_release+0x24/0xa0
> >  |  kvm_vm_release+0x1c/0x38
> >  |  __fput+0x88/0x2ec
> >  |  ____fput+0x10/0x1c
> >  |  task_work_run+0xb0/0xd4
> >  |  do_exit+0x210/0x854
> >  |  do_group_exit+0x70/0x98
> >  |  get_signal+0x6b0/0x73c
> >  |  do_signal+0xa4/0x11e8
> >  |  do_notify_resume+0x60/0x12c
> >  |  el0_svc+0x64/0x68
> >  |  el0t_64_sync_handler+0x84/0xfc
> >  |  el0t_64_sync+0x190/0x194
> >  | Code: b9000909 d503201f 2a1f03e1 52800028 (88e17c08)
> > 
> > Add a new label to the error path so that we can branch directly to the
> > xa_release() if the xa_store() fails.
> > 
> > Cc: Paolo Bonzini <pbonzini@redhat.com>
> > Cc: Michal Luczaj <mhal@rbox.co>
> > Cc: Alexander Potapenko <glider@google.com>
> > Cc: Marc Zyngier <maz@kernel.org>
> > Signed-off-by: Will Deacon <will@kernel.org>
> > ---
> >  virt/kvm/kvm_main.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index d0788d0a72cc..b80dd8cead8c 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -4293,7 +4293,7 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
> >  
> >  	if (KVM_BUG_ON(xa_store(&kvm->vcpu_array, vcpu->vcpu_idx, vcpu, 0), kvm)) {
> >  		r = -EINVAL;
> > -		goto kvm_put_xa_release;
> > +		goto err_xa_release;
> >  	}
> >  
> >  	/*
> > @@ -4310,6 +4310,7 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
> >  
> >  kvm_put_xa_release:
> >  	kvm_put_kvm_no_destroy(kvm);
> > +err_xa_release:
> >  	xa_release(&kvm->vcpu_array, vcpu->vcpu_idx);
> >  unlock_vcpu_destroy:
> >  	mutex_unlock(&kvm->lock);
> 
> My bad for neglecting the "impossible" path. Thanks for the fix.
> 
> I wonder if it's complete. If we really want to consider the possibility of
> this xa_store() failing, then keeping vCPU fd installed and calling
> kmem_cache_free(kvm_vcpu_cache, vcpu) on the error path looks wrong.

Yeah, the vCPU is exposed to userspace, freeing its assets will just cause
different problems.  KVM_BUG_ON() will prevent _new_ vCPU ioctl() calls (and kick
running vCPUs out of the guest), but it doesn't interrupt other CPUs, e.g. if
userspace is being sneaking and has already invoked a vCPU ioctl(), KVM will hit
a use-after-free (several of them).

As Michal alluded to, it should be impossible for xa_store() to fail since KVM
pre-allocates/reserves memory.  Given that, deliberately leaking the vCPU seems
like the least awful "solution".