On Mon, Feb 28, 2022, Ben Gardon wrote: > On Fri, Feb 25, 2022 at 10:23 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > > > WARN and bail if is_unsync_root() is passed a root for which there is no > > shadow page, i.e. is passed the physical address of one of the special > > roots, which do not have an associated shadow page. The current usage > > squeaks by without bug reports because neither kvm_mmu_sync_roots() nor > > kvm_mmu_sync_prev_roots() calls the helper with pae_root or pml4_root, > > and 5-level AMD CPUs are not generally available, i.e. no one can coerce > > KVM into calling is_unsync_root() on pml5_root. > > > > Note, this doesn't fix the mess with 5-level nNPT, it just (hopefully) > > prevents KVM from crashing. > > > > Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx> > > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx> > > --- > > arch/x86/kvm/mmu/mmu.c | 8 ++++++++ > > 1 file changed, 8 insertions(+) > > > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c > > index 825996408465..3e7c8ad5bed9 100644 > > --- a/arch/x86/kvm/mmu/mmu.c > > +++ b/arch/x86/kvm/mmu/mmu.c > > @@ -3634,6 +3634,14 @@ static bool is_unsync_root(hpa_t root) > > */ > > smp_rmb(); > > sp = to_shadow_page(root); > > + > > + /* > > + * PAE roots (somewhat arbitrarily) aren't backed by shadow pages, the > > + * PDPTEs for a given PAE root need to be synchronized individually. > > + */ > > + if (WARN_ON_ONCE(!sp)) > > + return false; > > + > > I was trying to figure out if this should be returning true or false, > but neither really seems correct. Since we never expect this to fire, > perhaps it doesn't matter and it's easier to just return false so the > callers don't need to be changed. Yep, neither is correct. > If this did fire in a production scenario, I'd want it to terminate the VM > too. Me too, but practically speaking this should never get anywhere near production. IMO, it's not worth plumbing in @kvm just to be able to do KVM_BUG_ON.
