diff mbox series

[v2,1/4] KVM: Reject overly excessive IDs in KVM_CREATE_VCPU

Message ID 20240612215415.3450952-2-minipli@grsecurity.net (mailing list archive)
State New, archived
Headers show
Series KVM: Reject vCPU IDs above 2^32 | expand

Commit Message

Mathias Krause June 12, 2024, 9:54 p.m. UTC
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>
- add comment and build bug to make truncation check more obvious (Sean)
 virt/kvm/kvm_main.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)
diff mbox series


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.
+	 */
 	if (id >= KVM_MAX_VCPU_IDS)
 		return -EINVAL;