linux-kernel - [PATCH] KVM: s390: Fix lockdep issue in vm memop

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Message-Id: <20220322153204.2637400-1-scgl@linux.ibm.com>
Date:   Tue, 22 Mar 2022 16:32:04 +0100
From:   Janis Schoetterl-Glausch <scgl@...ux.ibm.com>
To:     Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Janis Schoetterl-Glausch <scgl@...ux.ibm.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Sven Schnelle <svens@...ux.ibm.com>, kvm@...r.kernel.org,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] 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: ef11c9463ae0 ("KVM: s390: Add vm IOCTL for key checked guest absolute memory access")
Signed-off-by: Janis Schoetterl-Glausch <scgl@...ux.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 ca96f84db2cc..53adbe86a68f 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))

base-commit: c9b8fecddb5bb4b67e351bbaeaa648a6f7456912
-- 
2.32.0