On Mon, Apr 24, 2017 at 11:10:23AM +0100, Suzuki K Poulose wrote: > The KVM uses mmu_notifier (wherever available) to keep track > of the changes to the mm of the guest. The guest shadow page > tables are released when the VM exits via mmu_notifier->ops.release(). > There is a rare chance that the mmu_notifier->release could be > called more than once via two different paths, which could end > up in use-after-free of kvm instance (such as [0]). > > e.g: > > thread A thread B > ------- -------------- > > get_signal-> kvm_destroy_vm()-> > do_exit-> mmu_notifier_unregister-> > exit_mm-> kvm_arch_flush_shadow_all()-> > exit_mmap-> spin_lock(&kvm->mmu_lock) > mmu_notifier_release-> .... > kvm_arch_flush_shadow_all()-> ..... > ... spin_lock(&kvm->mmu_lock) ..... > spin_unlock(&kvm->mmu_lock) > kvm_arch_free_kvm() > *** use after free of kvm *** > > This patch attempts to solve the problem by holding a reference to the KVM > for the mmu_notifier, which is dropped only from notifier->ops.release(). > This will ensure that the KVM struct is available till we reach the > kvm_mmu_notifier_release, and the kvm_destroy_vm is called only from/after > it. So, we can unregister the notifier with no_release option and hence > avoiding the race above. However, we need to make sure that the KVM is > freed only after the mmu_notifier has finished processing the notifier due to > the following possible path of execution : > > mmu_notifier_release -> kvm_mmu_notifier_release -> kvm_put_kvm -> > kvm_destroy_vm -> kvm_arch_free_kvm > > [0] http://lkml.kernel.org/r/CAAeHK+x8udHKq9xa1zkTO6ax5E8Dk32HYWfaT05FMchL2cr48g@xxxxxxxxxxxxxx > > Fixes: commit 85db06e514422 ("KVM: mmu_notifiers release method") > Reported-by: andreyknvl@xxxxxxxxxx > Cc: Mark Rutland <mark.rutland@xxxxxxx> > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx> > Cc: Radim Krčmář <rkrcmar@xxxxxxxxxx> > Cc: Marc Zyngier <marc.zyngier@xxxxxxx> > Cc: Christoffer Dall <christoffer.dall@xxxxxxxxxx> > Cc: andreyknvl@xxxxxxxxxx > Cc: Marc Zyngier <marc.zyngier@xxxxxxx> > Tested-by: Mark Rutland <mark.rutland@xxxxxxx> > Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx> This looks good to me, but we should have some KVM generic experts look at it as well. Reviewed-by: Christoffer Dall <cdall@xxxxxxxxxx> > --- > include/linux/kvm_host.h | 1 + > virt/kvm/kvm_main.c | 59 ++++++++++++++++++++++++++++++++++++++++++------ > 2 files changed, 53 insertions(+), 7 deletions(-) > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h > index d025074..561e968 100644 > --- a/include/linux/kvm_host.h > +++ b/include/linux/kvm_host.h > @@ -424,6 +424,7 @@ struct kvm { > struct mmu_notifier mmu_notifier; > unsigned long mmu_notifier_seq; > long mmu_notifier_count; > + struct rcu_head mmu_notifier_rcu; > #endif > long tlbs_dirty; > struct list_head devices; > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > index 88257b3..2c3fdd4 100644 > --- a/virt/kvm/kvm_main.c > +++ b/virt/kvm/kvm_main.c > @@ -471,6 +471,7 @@ static void kvm_mmu_notifier_release(struct mmu_notifier *mn, > idx = srcu_read_lock(&kvm->srcu); > kvm_arch_flush_shadow_all(kvm); > srcu_read_unlock(&kvm->srcu, idx); > + kvm_put_kvm(kvm); > } > > static const struct mmu_notifier_ops kvm_mmu_notifier_ops = { > @@ -486,8 +487,46 @@ static const struct mmu_notifier_ops kvm_mmu_notifier_ops = { > > static int kvm_init_mmu_notifier(struct kvm *kvm) > { > + int rc; > kvm->mmu_notifier.ops = &kvm_mmu_notifier_ops; > - return mmu_notifier_register(&kvm->mmu_notifier, current->mm); > + rc = mmu_notifier_register(&kvm->mmu_notifier, current->mm); > + /* > + * We hold a reference to KVM here to make sure that the KVM > + * doesn't get free'd before ops->release() completes. > + */ > + if (!rc) > + kvm_get_kvm(kvm); > + return rc; > +} > + > +static void kvm_free_vm_rcu(struct rcu_head *rcu) > +{ > + struct kvm *kvm = container_of(rcu, struct kvm, mmu_notifier_rcu); > + kvm_arch_free_vm(kvm); > +} > + > +static void kvm_flush_shadow_mmu(struct kvm *kvm) > +{ > + /* > + * We hold a reference to kvm instance for mmu_notifier and is > + * only released when ops->release() is called via exit_mmap path. > + * So, when we reach here ops->release() has been called already, which > + * flushes the shadow page tables. Hence there is no need to call the > + * release() again when we unregister the notifier. However, we need > + * to delay freeing up the kvm until the release() completes, since > + * we could reach here via : > + * kvm_mmu_notifier_release() -> kvm_put_kvm() -> kvm_destroy_vm() > + */ > + mmu_notifier_unregister_no_release(&kvm->mmu_notifier, kvm->mm); > +} > + > +static void kvm_free_vm(struct kvm *kvm) > +{ > + /* > + * Wait until the mmu_notifier has finished the release(). > + * See comments above in kvm_flush_shadow_mmu. > + */ > + mmu_notifier_call_srcu(&kvm->mmu_notifier_rcu, kvm_free_vm_rcu); > } > > #else /* !(CONFIG_MMU_NOTIFIER && KVM_ARCH_WANT_MMU_NOTIFIER) */ > @@ -497,6 +536,16 @@ static int kvm_init_mmu_notifier(struct kvm *kvm) > return 0; > } > > +static void kvm_flush_shadow_mmu(struct kvm *kvm) > +{ > + kvm_arch_flush_shadow_all(kvm); > +} > + > +static void kvm_free_vm(struct kvm *kvm) > +{ > + kvm_arch_free_vm(kvm); > +} > + > #endif /* CONFIG_MMU_NOTIFIER && KVM_ARCH_WANT_MMU_NOTIFIER */ > > static struct kvm_memslots *kvm_alloc_memslots(void) > @@ -733,18 +782,14 @@ static void kvm_destroy_vm(struct kvm *kvm) > kvm->buses[i] = NULL; > } > kvm_coalesced_mmio_free(kvm); > -#if defined(CONFIG_MMU_NOTIFIER) && defined(KVM_ARCH_WANT_MMU_NOTIFIER) > - mmu_notifier_unregister(&kvm->mmu_notifier, kvm->mm); > -#else > - kvm_arch_flush_shadow_all(kvm); > -#endif > + kvm_flush_shadow_mmu(kvm); > kvm_arch_destroy_vm(kvm); > kvm_destroy_devices(kvm); > for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) > kvm_free_memslots(kvm, kvm->memslots[i]); > cleanup_srcu_struct(&kvm->irq_srcu); > cleanup_srcu_struct(&kvm->srcu); > - kvm_arch_free_vm(kvm); > + kvm_free_vm(kvm); > preempt_notifier_dec(); > hardware_disable_all(); > mmdrop(mm); > -- > 2.7.4 >
