[GIT PULL 1/1] KVM: s390: Fix lockdep issue in vm memop

Christian Borntraeger
2022-05-02T15:31:10Z

From: Janis Schoetterl-Glausch <scgl@linux.ibm.com>

Issuing a vm 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: ef11c9463ae0 ("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>
---
arch/s390/kvm/kvm-s390.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index b53ff693b66e..7240a781ea82 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -2385,7 +2385,16 @@ 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))
--
2.35.1