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>
---
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
