If, on a 64 bit system, a vCPU ID is provided that has the upper 32 bits
set to a non-zero value, it may get accepted if the truncated to 32 bits
integer value is below KVM_MAX_VCPU_IDS and 'max_vcpus'. This feels very
wrong and triggered the reporting logic of PaX's SIZE_OVERFLOW plugin.
Instead of silently truncating and accepting such values, pass the full
value to kvm_vm_ioctl_create_vcpu() and make the existing limit checks
return an error.
Even if this is a userland ABI breaking change, no sane userland could
have ever relied on that behaviour.
Reported-by: PaX's SIZE_OVERFLOW plugin running on grsecurity's syzkaller
Fixes: 6aa8b732ca01 ("[PATCH] kvm: userspace interface")
Cc: Emese Revfy <re.emese@xxxxxxxxx>
Cc: PaX Team <pageexec@xxxxxxxxxxx>
Signed-off-by: Mathias Krause <minipli@xxxxxxxxxxxxxx>
---
virt/kvm/kvm_main.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 14841acb8b95..b04e87f6568f 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4200,12 +4200,20 @@ static void kvm_create_vcpu_debugfs(struct kvm_vcpu *vcpu)
/*
* Creates some virtual cpus. Good luck creating more than one.
*/
-static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, u32 id)
+static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
{
int r;
struct kvm_vcpu *vcpu;
struct page *page;
+ /*
+ * KVM tracks vCPU IDs as 'int', be kind to userspace and reject
+ * too-large values instead of silently truncating.
+ *
+ * Also ensure we're not breaking this assumption by accidentally
+ * pushing KVM_MAX_VCPU_IDS above INT_MAX.
+ */
+ BUILD_BUG_ON(KVM_MAX_VCPU_IDS > INT_MAX);
if (id >= KVM_MAX_VCPU_IDS)
return -EINVAL;
--
2.30.2
