[RFC PATCH 3/4] KVM: x86/xen: Disallow gpc locks reinitialization

[Michal Luczaj <mhal@rbox.co>]

There are race conditions possible due to kvm_gfn_to_pfn_cache_init()'s
ability to _re_initialize gfn_to_pfn_cache.lock.

For example: a race between ioctl(KVM_XEN_HVM_EVTCHN_SEND) and
kvm_gfn_to_pfn_cache_init() leads to a corrupted shinfo gpc lock.

                (thread 1)                |           (thread 2)
                                          |
 kvm_xen_set_evtchn_fast                  |
  read_lock_irqsave(&gpc->lock, ...)      |
                                          | kvm_gfn_to_pfn_cache_init
                                          |  rwlock_init(&gpc->lock)
  read_unlock_irqrestore(&gpc->lock, ...) |

Introduce bool locks_initialized.

Signed-off-by: Michal Luczaj <mhal@rbox.co>
---
 include/linux/kvm_types.h | 1 +
 virt/kvm/pfncache.c       | 7 +++++--
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/include/linux/kvm_types.h b/include/linux/kvm_types.h
index 3ca3db020e0e..7e7b7667cd9e 100644
--- a/include/linux/kvm_types.h
+++ b/include/linux/kvm_types.h
@@ -74,6 +74,7 @@ struct gfn_to_pfn_cache {
 	void *khva;
 	kvm_pfn_t pfn;
 	enum pfn_cache_usage usage;
+	bool locks_initialized;
 	bool active;
 	bool valid;
 };
diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
index 68ff41d39545..564607e10586 100644
--- a/virt/kvm/pfncache.c
+++ b/virt/kvm/pfncache.c
@@ -354,8 +354,11 @@ int kvm_gfn_to_pfn_cache_init(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
 	WARN_ON_ONCE(!usage || (usage & KVM_GUEST_AND_HOST_USE_PFN) != usage);
 
 	if (!gpc->active) {
-		rwlock_init(&gpc->lock);
-		mutex_init(&gpc->refresh_lock);
+		if (!gpc->locks_initialized) {
+			rwlock_init(&gpc->lock);
+			mutex_init(&gpc->refresh_lock);
+			gpc->locks_initialized = true;
+		}
 
 		gpc->khva = NULL;
 		gpc->pfn = KVM_PFN_ERR_FAULT;
-- 
2.37.2