From: Christian Borntraeger <borntraeger@de.ibm.com>
Date: Tue, 21 Nov 2017 16:02:52 +0100
Subject: KVM: s390: mark irq_state.flags as non-usable
Patch-mainline: v4.15-rc3
Git-commit: bb64da9aba89765fee74b395967b18a7d6c364e9
References: bsc#1077761

Old kernels did not check for zero in the irq_state.flags field and old
QEMUs did not zero the flag/reserved fields when calling
KVM_S390_*_IRQ_STATE.  Let's add comments to prevent future uses of
these fields.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Alexander Graf <agraf@suse.de>
---
 Documentation/virtual/kvm/api.txt |   15 ++++++++++++---
 arch/s390/kvm/kvm-s390.c          |    6 ++++--
 include/uapi/linux/kvm.h          |    4 ++--
 3 files changed, 18 insertions(+), 7 deletions(-)

--- a/Documentation/virtual/kvm/api.txt
+++ b/Documentation/virtual/kvm/api.txt
@@ -2898,14 +2898,19 @@
 
 struct kvm_s390_irq_state {
 	__u64 buf;
-	__u32 flags;
+	__u32 flags;        /* will stay unused for compatibility reasons */
 	__u32 len;
-	__u32 reserved[4];
+	__u32 reserved[4];  /* will stay unused for compatibility reasons */
 };
 
 Userspace passes in the above struct and for each pending interrupt a
 struct kvm_s390_irq is copied to the provided buffer.
 
+The structure contains a flags and a reserved field for future extensions. As
+the kernel never checked for flags == 0 and QEMU never pre-zeroed flags and
+reserved, these fields can not be used in the future without breaking
+compatibility.
+
 If -ENOBUFS is returned the buffer provided was too small and userspace
 may retry with a bigger buffer.
 
@@ -2929,10 +2934,14 @@
 
 struct kvm_s390_irq_state {
 	__u64 buf;
+	__u32 flags;        /* will stay unused for compatibility reasons */
 	__u32 len;
-	__u32 pad;
+	__u32 reserved[4];  /* will stay unused for compatibility reasons */
 };
 
+The restrictions for flags and reserved apply as well.
+(see KVM_S390_GET_IRQ_STATE)
+
 The userspace memory referenced by buf contains a struct kvm_s390_irq
 for each interrupt to be injected into the guest.
 If one of the interrupts could not be injected for some reason the
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -1,8 +1,8 @@
 // SPDX-License-Identifier: GPL-2.0
 /*
- * hosting zSeries kernel virtual machines
+ * hosting IBM Z kernel virtual machines (s390x)
  *
- * Copyright IBM Corp. 2008, 2009
+ * Copyright IBM Corp. 2008, 2017
  *
  *    Author(s): Carsten Otte <cotte@de.ibm.com>
  *               Christian Borntraeger <borntraeger@de.ibm.com>
@@ -3805,6 +3805,7 @@
 			r = -EINVAL;
 			break;
 		}
+		/* do not use irq_state.flags, it will break old QEMUs */
 		r = kvm_s390_set_irq_state(vcpu,
 					   (void __user *) irq_state.buf,
 					   irq_state.len);
@@ -3820,6 +3821,7 @@
 			r = -EINVAL;
 			break;
 		}
+		/* do not use irq_state.flags, it will break old QEMUs */
 		r = kvm_s390_get_irq_state(vcpu,
 					   (__u8 __user *)  irq_state.buf,
 					   irq_state.len);
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -629,9 +629,9 @@
 
 struct kvm_s390_irq_state {
 	__u64 buf;
-	__u32 flags;
+	__u32 flags;        /* will stay unused for compatibility reasons */
 	__u32 len;
-	__u32 reserved[4];
+	__u32 reserved[4];  /* will stay unused for compatibility reasons */
 };
 
 /* for KVM_SET_GUEST_DEBUG */