Commit 8b8e57e5 authored by Mathias Krause's avatar Mathias Krause Committed by Sean Christopherson
Browse files

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: 6aa8b732 ("[PATCH] kvm: userspace interface")
Cc: Emese Revfy <re.emese@gmail.com>
Cc: PaX Team <pageexec@freemail.hu>
Signed-off-by: default avatarMathias 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: default avatarSean Christopherson <seanjc@google.com>
parent 5c1f50ab

virt/kvm/kvm_main.c

+10 −1
Original line number Diff line number Diff line
@@ -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;