KVM: s390: Fix lockdep issue in vm memop

Issuing a memop on a protected vm does not make sense,
neither is the memory readable/writable, nor does it make sense to check
storage keys. This is why the ioctl will return -EINVAL when it detects
the vm to be protected. However, in order to ensure that the vm cannot
become protected during the memop, the kvm->lock would need to be taken
for the duration of the ioctl. This is also required because
kvm_s390_pv_is_protected asserts that the lock must be held.
Instead, don't try to prevent this. If user space enables secure
execution concurrently with a memop it must accecpt the possibility of
the memop failing.
Still check if the vm is currently protected, but without locking and
consider it a heuristic.

Fixes: 4f43b1a785ce ("KVM: s390: Add vm IOCTL for key checked guest absolute memory access")
Signed-off-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Link: https://lore.kernel.org/r/20220322153204.2637400-1-scgl@linux.ibm.com
Signed-off-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index da3dabda1a12623a2ee09bb7c1e7abc855793fc0..76ad6408cb2cd0c2a4f5e91cbc3f735a7721e416 100644 (file)
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -2384,7 +2384,16 @@ static int kvm_s390_vm_mem_op(struct kvm *kvm, struct kvm_s390_mem_op *mop)
                return -EINVAL;
        if (mop->size > MEM_OP_MAX_SIZE)
                return -E2BIG;
-       if (kvm_s390_pv_is_protected(kvm))
+       /*
+        * This is technically a heuristic only, if the kvm->lock is not
+        * taken, it is not guaranteed that the vm is/remains non-protected.
+        * This is ok from a kernel perspective, wrongdoing is detected
+        * on the access, -EFAULT is returned and the vm may crash the
+        * next time it accesses the memory in question.
+        * There is no sane usecase to do switching and a memop on two
+        * different CPUs at the same time.
+        */
+       if (kvm_s390_pv_get_handle(kvm))
                return -EINVAL;
        if (mop->flags & KVM_S390_MEMOP_F_SKEY_PROTECTION) {
                if (access_key_invalid(mop->key))