[Qemu-devel] [PATCH v2] kvm: Fix overlapping check for memory slots

[Jan Kiszka <jan.kiszka@web.de>]

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When checking for overlapping slots on registration of a new one, kvm
currently also considers zero-length (ie. deleted) slots and rejects
requests incorrectly. This finally denies user space from joining slots.
Fix the check by skipping deleted slots and advertise this via a
KVM_CAP_JOIN_MEMORY_REGIONS_WORKS.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
---

 include/linux/kvm.h |    2 ++
 virt/kvm/kvm_main.c |    3 ++-
 2 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/include/linux/kvm.h b/include/linux/kvm.h
index ee755e2..644e3a9 100644
--- a/include/linux/kvm.h
+++ b/include/linux/kvm.h
@@ -413,6 +413,8 @@ struct kvm_trace_rec {
 #define KVM_CAP_DEVICE_MSIX 28
 #endif
 #define KVM_CAP_ASSIGN_DEV_IRQ 29
+/* Another bug in KVM_SET_USER_MEMORY_REGION fixed: */
+#define KVM_CAP_JOIN_MEMORY_REGIONS_WORKS 30
 
 #ifdef KVM_CAP_IRQ_ROUTING
 
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 363af32..3265566 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1117,7 +1117,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
 	for (i = 0; i < KVM_MEMORY_SLOTS; ++i) {
 		struct kvm_memory_slot *s = &kvm->memslots[i];
 
-		if (s == memslot)
+		if (s == memslot || !s->npages)
 			continue;
 		if (!((base_gfn + npages <= s->base_gfn) ||
 		      (base_gfn >= s->base_gfn + s->npages)))
@@ -2262,6 +2262,7 @@ static long kvm_dev_ioctl_check_extension_generic(long arg)
 	switch (arg) {
 	case KVM_CAP_USER_MEMORY:
 	case KVM_CAP_DESTROY_MEMORY_REGION_WORKS:
+	case KVM_CAP_JOIN_MEMORY_REGIONS_WORKS:
 		return 1;
 #ifdef CONFIG_HAVE_KVM_IRQCHIP
 	case KVM_CAP_IRQ_ROUTING:


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