From: Sean Christopherson seanjc@google.com

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@vger.kernel.org Reported-by: : Michal Luczaj mhal@rbox.co Signed-off-by: Sean Christopherson seanjc@google.com Message-Id: 20221013211234.1318131-3-seanjc@google.com Signed-off-by: Paolo Bonzini pbonzini@redhat.com --- 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);