Commit 93984f19 authored by Sean Christopherson's avatar Sean Christopherson Committed by Paolo Bonzini
Browse files

KVM: Fully serialize gfn=>pfn cache refresh via mutex 



Protect gfn=>pfn cache refresh with a mutex to fully serialize refreshes.
The refresh logic doesn't protect against

- concurrent unmaps, or refreshes with different GPAs (which may or may not
  happen in practice, for example if a cache is only used under vcpu->mutex;
  but it's allowed in the code)

- a false negative on the memslot generation.  If the first refresh sees
  a stale memslot generation, it will refresh the hva and generation before
  moving on to the hva=>pfn translation.  If it then drops gpc->lock, a
  different user of the cache can come along, acquire gpc->lock, see that
  the memslot generation is fresh, and skip the hva=>pfn update due to the
  userspace address also matching (because it too was updated).

The refresh path can already sleep during hva=>pfn resolution, so wrap
the refresh with a mutex to ensure that any given refresh runs to
completion before other callers can start their refresh.

Cc: stable@vger.kernel.org
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
Message-Id: <20220429210025.3293691-7-seanjc@google.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 3ba2c95e

include/linux/kvm_types.h

+2 −0
Original line number Diff line number Diff line
@@ -19,6 +19,7 @@ struct kvm_memslots; 
enum kvm_mr_change;



#include <linux/bits.h>

#include <linux/mutex.h>

#include <linux/types.h>

#include <linux/spinlock_types.h>
@@ -69,6 +70,7 @@ struct gfn_to_pfn_cache { 
	struct kvm_vcpu *vcpu;

	struct list_head list;

	rwlock_t lock;

	struct mutex refresh_lock;

	void *khva;

	kvm_pfn_t pfn;

	enum pfn_cache_usage usage;

virt/kvm/pfncache.c

+12 −0
Original line number Diff line number Diff line
@@ -157,6 +157,13 @@ int kvm_gfn_to_pfn_cache_refresh(struct kvm *kvm, struct gfn_to_pfn_cache *gpc, 
	if (page_offset + len > PAGE_SIZE)

		return -EINVAL;



	/*

	 * If another task is refreshing the cache, wait for it to complete.

	 * There is no guarantee that concurrent refreshes will see the same

	 * gpa, memslots generation, etc..., so they must be fully serialized.

	 */

	mutex_lock(&gpc->refresh_lock);



	write_lock_irq(&gpc->lock);



	old_pfn = gpc->pfn;
@@ -248,6 +255,8 @@ int kvm_gfn_to_pfn_cache_refresh(struct kvm *kvm, struct gfn_to_pfn_cache *gpc, 
 out:

	write_unlock_irq(&gpc->lock);



	mutex_unlock(&gpc->refresh_lock);



	gpc_release_pfn_and_khva(kvm, old_pfn, old_khva);



	return ret;
@@ -259,6 +268,7 @@ void kvm_gfn_to_pfn_cache_unmap(struct kvm *kvm, struct gfn_to_pfn_cache *gpc) 
	void *old_khva;

	kvm_pfn_t old_pfn;



	mutex_lock(&gpc->refresh_lock);

	write_lock_irq(&gpc->lock);



	gpc->valid = false;
@@ -274,6 +284,7 @@ void kvm_gfn_to_pfn_cache_unmap(struct kvm *kvm, struct gfn_to_pfn_cache *gpc) 
	gpc->pfn = KVM_PFN_ERR_FAULT;



	write_unlock_irq(&gpc->lock);

	mutex_unlock(&gpc->refresh_lock);



	gpc_release_pfn_and_khva(kvm, old_pfn, old_khva);

}
@@ -288,6 +299,7 @@ int kvm_gfn_to_pfn_cache_init(struct kvm *kvm, struct gfn_to_pfn_cache *gpc, 


	if (!gpc->active) {

		rwlock_init(&gpc->lock);

		mutex_init(&gpc->refresh_lock);



		gpc->khva = NULL;

		gpc->pfn = KVM_PFN_ERR_FAULT;