KVM: SEV: Prohibit migration of a VM that has mirrors

VMs that mirror an encryption context rely on the owner to keep the
ASID allocated.  Performing a KVM_CAP_VM_MOVE_ENC_CONTEXT_FROM
would cause a dangling ASID:

1. copy context from A to B (gets ref to A)
2. move context from A to L (moves ASID from A to L)
3. close L (releases ASID from L, B still references it)

The right way to do the handoff instead is to create a fresh mirror VM
on the destination first:

1. copy context from A to B (gets ref to A)
[later] 2. close B (releases ref to A)
3. move context from A to L (moves ASID from A to L)
4. copy context from L to M

So, catch the situation by adding a count of how many VMs are
mirroring this one's encryption context.

Fixes: 0b020f5af092 ("KVM: SEV: Add support for SEV-ES intra host migration")
Message-Id: <20211123005036.2954379-11-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 025d9731b66c22d69e8b469dc7d55bc106834336..89a716290fac0b6efaf38a938dd17bdcad06a838 100644 (file)
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -1696,6 +1696,16 @@ int svm_vm_migrate_from(struct kvm *kvm, unsigned int source_fd)
        }
 
        src_sev = &to_kvm_svm(source_kvm)->sev_info;
+
+       /*
+        * VMs mirroring src's encryption context rely on it to keep the
+        * ASID allocated, but below we are clearing src_sev->asid.
+        */
+       if (src_sev->num_mirrored_vms) {
+               ret = -EBUSY;
+               goto out_unlock;
+       }
+
        dst_sev->misc_cg = get_current_misc_cg();
        cg_cleanup_sev = dst_sev;
        if (dst_sev->misc_cg != src_sev->misc_cg) {
@@ -1987,6 +1997,7 @@ int svm_vm_copy_asid_from(struct kvm *kvm, unsigned int source_fd)
         */
        source_sev = &to_kvm_svm(source_kvm)->sev_info;
        kvm_get_kvm(source_kvm);
+       source_sev->num_mirrored_vms++;
 
        /* Set enc_context_owner and copy its encryption context over */
        mirror_sev = &to_kvm_svm(kvm)->sev_info;
@@ -2019,12 +2030,21 @@ void sev_vm_destroy(struct kvm *kvm)
        struct list_head *head = &sev->regions_list;
        struct list_head *pos, *q;
 
+       WARN_ON(sev->num_mirrored_vms);
+
        if (!sev_guest(kvm))
                return;
 
        /* If this is a mirror_kvm release the enc_context_owner and skip sev cleanup */
        if (is_mirroring_enc_context(kvm)) {
-               kvm_put_kvm(sev->enc_context_owner);
+               struct kvm *owner_kvm = sev->enc_context_owner;
+               struct kvm_sev_info *owner_sev = &to_kvm_svm(owner_kvm)->sev_info;
+
+               mutex_lock(&owner_kvm->lock);
+               if (!WARN_ON(!owner_sev->num_mirrored_vms))
+                       owner_sev->num_mirrored_vms--;
+               mutex_unlock(&owner_kvm->lock);
+               kvm_put_kvm(owner_kvm);
                return;
        }
 

diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 5faad3dc10e27ac0dc987cd6c7041fd8b2ea4162..1c7306c370fa3c4924a83371c1a1b21da8f6c5b2 100644 (file)
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -79,6 +79,7 @@ struct kvm_sev_info {
        struct list_head regions_list;  /* List of registered regions */
        u64 ap_jump_table;      /* SEV-ES AP Jump Table address */
        struct kvm *enc_context_owner; /* Owner of copied encryption context */
+       unsigned long num_mirrored_vms; /* Number of VMs sharing this ASID */
        struct misc_cg *misc_cg; /* For misc cgroup accounting */
        atomic_t migration_in_progress;
 };

diff --git a/tools/testing/selftests/kvm/x86_64/sev_migrate_tests.c b/tools/testing/selftests/kvm/x86_64/sev_migrate_tests.c
index d265cea5de85ba8f8ceaa4010f1338b2ab5e9034..29b18d565cf4ce1cfbbce5b29e4298dc781e99c1 100644 (file)
--- a/tools/testing/selftests/kvm/x86_64/sev_migrate_tests.c
+++ b/tools/testing/selftests/kvm/x86_64/sev_migrate_tests.c
@@ -294,6 +294,41 @@ static void test_sev_mirror_parameters(void)
        kvm_vm_free(vm_no_vcpu);
 }
 
+static void test_sev_move_copy(void)
+{
+       struct kvm_vm *dst_vm, *sev_vm, *mirror_vm, *dst_mirror_vm;
+       int ret;
+
+       sev_vm = sev_vm_create(/* es= */ false);
+       dst_vm = aux_vm_create(true);
+       mirror_vm = aux_vm_create(false);
+       dst_mirror_vm = aux_vm_create(false);
+
+       sev_mirror_create(mirror_vm->fd, sev_vm->fd);
+       ret = __sev_migrate_from(dst_vm->fd, sev_vm->fd);
+       TEST_ASSERT(ret == -1 && errno == EBUSY,
+                   "Cannot migrate VM that has mirrors. ret %d, errno: %d\n", ret,
+                   errno);
+
+       /* The mirror itself can be migrated.  */
+       sev_migrate_from(dst_mirror_vm->fd, mirror_vm->fd);
+       ret = __sev_migrate_from(dst_vm->fd, sev_vm->fd);
+       TEST_ASSERT(ret == -1 && errno == EBUSY,
+                   "Cannot migrate VM that has mirrors. ret %d, errno: %d\n", ret,
+                   errno);
+
+       /*
+        * mirror_vm is not a mirror anymore, dst_mirror_vm is.  Thus,
+        * the owner can be copied as soon as dst_mirror_vm is gone.
+        */
+       kvm_vm_free(dst_mirror_vm);
+       sev_migrate_from(dst_vm->fd, sev_vm->fd);
+
+       kvm_vm_free(mirror_vm);
+       kvm_vm_free(dst_vm);
+       kvm_vm_free(sev_vm);
+}
+
 int main(int argc, char *argv[])
 {
        if (kvm_check_cap(KVM_CAP_VM_MOVE_ENC_CONTEXT_FROM)) {
@@ -301,6 +336,8 @@ int main(int argc, char *argv[])
                test_sev_migrate_from(/* es= */ true);
                test_sev_migrate_locking();
                test_sev_migrate_parameters();
+               if (kvm_check_cap(KVM_CAP_VM_COPY_ENC_CONTEXT_FROM))
+                       test_sev_move_copy();
        }
        if (kvm_check_cap(KVM_CAP_VM_COPY_ENC_CONTEXT_FROM)) {
                test_sev_mirror(/* es= */ false);