On Sun, Mar 27, 2022, Mingwei Zhang wrote: > Add a lockdep check before invoking lookup_address_in_mm(). > lookup_address_in_mm() walks all levels of host page table without > accquiring any lock. This is usually unsafe unless we are walking the > kernel addresses (check other usage cases of lookup_address_in_mm and > lookup_address_in_pgd). > > Walking host page table (especially guest addresses) usually requires > holding two types of locks: 1) mmu_lock in mm or the lock that protects > the reverse maps of host memory in range; 2) lock for the leaf paging > structures. > > One exception case is when we take the mmu_lock of the secondary mmu. > Holding mmu_lock of KVM MMU in either read mode or write mode prevents host > level entities from modifying the host page table concurrently. This is > because all of them will have to invoke KVM mmu_notifier first before doing > the actual work. Since KVM mmu_notifier invalidation operations always take > the mmu write lock, we are safe if we hold the mmu lock here. > > Note: this means that KVM cannot allow concurrent multiple mmu_notifier > invalidation callbacks by using KVM mmu read lock. Since, otherwise, any > host level entity can cause race conditions with this one. Walking host > page table here may get us stale information or may trigger NULL ptr > dereference that is hard to reproduce. > > Having a lockdep check here will prevent or at least warn future > development that directly walks host page table simply in a KVM ioctl > function. In addition, it provides a record for any future development on > KVM mmu_notifier. > > Cc: Sean Christopherson <seanjc@xxxxxxxxxx> > Cc: Ben Gardon <bgardon@xxxxxxxxxx> > Cc: David Matlack <dmatlack@xxxxxxxxxx> > > Signed-off-by: Mingwei Zhang <mizhang@xxxxxxxxxx> > --- > arch/x86/kvm/mmu/mmu.c | 18 ++++++++++++++++++ > 1 file changed, 18 insertions(+) > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c > index 1361eb4599b4..066bb5435156 100644 > --- a/arch/x86/kvm/mmu/mmu.c > +++ b/arch/x86/kvm/mmu/mmu.c > @@ -2820,6 +2820,24 @@ static int host_pfn_mapping_level(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn, > */ > hva = __gfn_to_hva_memslot(slot, gfn); > > + /* > + * lookup_address_in_mm() walks all levels of host page table without > + * accquiring any lock. This is not safe when KVM does not take the > + * mmu_lock. Holding mmu_lock in either read mode or write mode prevents > + * host level entities from modifying the host page table. This is > + * because all of them will have to invoke KVM mmu_notifier first before > + * doing the actual work. Since KVM mmu_notifier invalidation operations > + * always take the mmu write lock, we are safe if we hold the mmu lock > + * here. > + * > + * Note: this means that KVM cannot allow concurrent multiple > + * mmu_notifier invalidation callbacks by using KVM mmu read lock. > + * Otherwise, any host level entity can cause race conditions with this > + * one. Walking host page table here may get us stale information or may > + * trigger NULL ptr dereference that is hard to reproduce. > + */ > + lockdep_assert_held(&kvm->mmu_lock); Holding mmu_lock isn't strictly required. It would also be safe to use this helper if mmu_notifier_retry_hva() were checked after grabbing the mapping level, before consuming it. E.g. we could theoretically move this to kvm_faultin_pfn(). And simply holding the lock isn't sufficient, i.e. the lockdep gives a false sense of security. E.g. calling this while holding mmu_lock but without first checking mmu_notifier_count would let it run concurrently with host PTE modifications. I'm definitely in favor of adding a comment to document the mmu_notifier interactions, but I don't like adding a lockdep.
