Reject kvm_gpc_check() and kvm_gpc_refresh() if the cache is inactive.
Not checking the active flag during refresh is particularly egregious, as
KVM can end up with a valid, inactive cache, which can lead to a variety
of use-after-free bugs, e.g. consuming a NULL kernel pointer or missing
an mmu_notifier invalidation due to the cache not being on the list of
gfns to invalidate.
Note, "active" needs to be set if and only if the cache is on the list
of caches, i.e. is reachable via mmu_notifier events. If a relevant
mmu_notifier event occurs while the cache is "active" but not on the
list, KVM will not acquire the cache's lock and so will not serailize
the mmu_notifier event with active users and/or kvm_gpc_refresh().
A race between KVM_XEN_ATTR_TYPE_SHARED_INFO and KVM_XEN_HVM_EVTCHN_SEND
can be exploited to trigger the bug.
1. Deactivate shinfo cache:
kvm_xen_hvm_set_attr
case KVM_XEN_ATTR_TYPE_SHARED_INFO
kvm_gpc_deactivate
kvm_gpc_unmap
gpc->valid = false
gpc->khva = NULL
gpc->active = false
Result: active = false, valid = false
2. Cause cache refresh:
kvm_arch_vm_ioctl
case KVM_XEN_HVM_EVTCHN_SEND
kvm_xen_hvm_evtchn_send
kvm_xen_set_evtchn
kvm_xen_set_evtchn_fast
kvm_gpc_check
return -EWOULDBLOCK because !gpc->valid
kvm_xen_set_evtchn_fast
return -EWOULDBLOCK
kvm_gpc_refresh
hva_to_pfn_retry
gpc->valid = true
gpc->khva = not NULL
Result: active = false, valid = true
3. Race ioctl KVM_XEN_HVM_EVTCHN_SEND against ioctl
KVM_XEN_ATTR_TYPE_SHARED_INFO:
kvm_arch_vm_ioctl
case KVM_XEN_HVM_EVTCHN_SEND
kvm_xen_hvm_evtchn_send
kvm_xen_set_evtchn
kvm_xen_set_evtchn_fast
read_lock gpc->lock
kvm_xen_hvm_set_attr case
KVM_XEN_ATTR_TYPE_SHARED_INFO
mutex_lock kvm->lock
kvm_xen_shared_info_init
kvm_gpc_activate
gpc->khva = NULL
kvm_gpc_check
[ Check passes because gpc->valid is
still true, even though gpc->khva
is already NULL. ]
shinfo = gpc->khva
pending_bits = shinfo->evtchn_pending
CRASH: test_and_set_bit(..., pending_bits)
Fixes: 982ed0de4753 ("KVM: Reinstate gfn_to_pfn_cache with invalidation support")
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: : Michal Luczaj <mhal@xxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
virt/kvm/pfncache.c | 41 ++++++++++++++++++++++++++++++++++-------
1 file changed, 34 insertions(+), 7 deletions(-)
diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
index 08f97cf97264..346e47f15572 100644
--- a/virt/kvm/pfncache.c
+++ b/virt/kvm/pfncache.c
@@ -81,6 +81,9 @@ bool kvm_gfn_to_pfn_cache_check(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
{
struct kvm_memslots *slots = kvm_memslots(kvm);
+ if (!gpc->active)
+ return false;
+
if ((gpa & ~PAGE_MASK) + len > PAGE_SIZE)
return false;
@@ -240,10 +243,11 @@ int kvm_gfn_to_pfn_cache_refresh(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
{
struct kvm_memslots *slots = kvm_memslots(kvm);
unsigned long page_offset = gpa & ~PAGE_MASK;
- kvm_pfn_t old_pfn, new_pfn;
+ bool unmap_old = false;
unsigned long old_uhva;
+ kvm_pfn_t old_pfn;
void *old_khva;
- int ret = 0;
+ int ret;
/*
* If must fit within a single page. The 'len' argument is
@@ -261,6 +265,11 @@ int kvm_gfn_to_pfn_cache_refresh(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
write_lock_irq(&gpc->lock);
+ if (!gpc->active) {
+ ret = -EINVAL;
+ goto out_unlock;
+ }
+
old_pfn = gpc->pfn;
old_khva = gpc->khva - offset_in_page(gpc->khva);
old_uhva = gpc->uhva;
@@ -291,6 +300,7 @@ int kvm_gfn_to_pfn_cache_refresh(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
/* If the HVA→PFN mapping was already valid, don't unmap it. */
old_pfn = KVM_PFN_ERR_FAULT;
old_khva = NULL;
+ ret = 0;
}
out:
@@ -305,14 +315,15 @@ int kvm_gfn_to_pfn_cache_refresh(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
gpc->khva = NULL;
}
- /* Snapshot the new pfn before dropping the lock! */
- new_pfn = gpc->pfn;
+ /* Detect a pfn change before dropping the lock! */
+ unmap_old = (old_pfn != gpc->pfn);
+out_unlock:
write_unlock_irq(&gpc->lock);
mutex_unlock(&gpc->refresh_lock);
- if (old_pfn != new_pfn)
+ if (unmap_old)
gpc_unmap_khva(kvm, old_pfn, old_khva);
return ret;
@@ -366,11 +377,19 @@ int kvm_gpc_activate(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
gpc->vcpu = vcpu;
gpc->usage = usage;
gpc->valid = false;
- gpc->active = true;
spin_lock(&kvm->gpc_lock);
list_add(&gpc->list, &kvm->gpc_list);
spin_unlock(&kvm->gpc_lock);
+
+ /*
+ * Activate the cache after adding it to the list, a concurrent
+ * refresh must not establish a mapping until the cache is
+ * reachable by mmu_notifier events.
+ */
+ write_lock_irq(&gpc->lock);
+ gpc->active = true;
+ write_unlock_irq(&gpc->lock);
}
return kvm_gfn_to_pfn_cache_refresh(kvm, gpc, gpa, len);
}
@@ -379,12 +398,20 @@ EXPORT_SYMBOL_GPL(kvm_gpc_activate);
void kvm_gpc_deactivate(struct kvm *kvm, struct gfn_to_pfn_cache *gpc)
{
if (gpc->active) {
+ /*
+ * Deactivate the cache before removing it from the list, KVM
+ * must stall mmu_notifier events until all users go away, i.e.
+ * until gpc->lock is dropped and refresh is guaranteed to fail.
+ */
+ write_lock_irq(&gpc->lock);
+ gpc->active = false;
+ write_unlock_irq(&gpc->lock);
+
spin_lock(&kvm->gpc_lock);
list_del(&gpc->list);
spin_unlock(&kvm->gpc_lock);
kvm_gfn_to_pfn_cache_unmap(kvm, gpc);
- gpc->active = false;
}
}
EXPORT_SYMBOL_GPL(kvm_gpc_deactivate);
--
2.38.0.413.g74048e4d9e-goog
