KVM: Reject overly excessive IDs in KVM_CREATE_VCPU

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@gmail.com>
Cc: PaX Team <pageexec@freemail.hu>
Signed-off-by: Mathias Krause <minipli@grsecurity.net>
Link: https://lore.kernel.org/r/20240614202859.3597745-2-minipli@grsecurity.net
[sean: tweak comment about INT_MAX assertion]
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 436ca41f61e50cbfd1c2b42505cd5134e472e65b..fed80bfbe99bbbc96007d88096fb09f5c6102768 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4207,12 +4207,21 @@ 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.
+        *
+        * Ensure KVM_MAX_VCPU_IDS isn't pushed above INT_MAX without first
+        * changing the storage type (at the very least, IDs should be tracked
+        * as unsigned ints).
+        */
+       BUILD_BUG_ON(KVM_MAX_VCPU_IDS > INT_MAX);
        if (id >= KVM_MAX_VCPU_IDS)
                return -EINVAL;