On Thu, Jul 27, 2023, Yikebaer Aizezi wrote: > Hello, I'm sorry for the mistake in my previous email. I forgot to add > a subject. This is my second attempt to send the message. > > When using Healer to fuzz the latest Linux kernel, the following crash > was triggered. > > HEAD commit: fdf0eaf11452d72945af31804e2a1048ee1b574c (tag: v6.5-rc2) > > git tree: upstream > > console output: > https://drive.google.com/file/d/1FiemC_AWRT-6EGscpQJZNzYhXZty6BVr/view?usp=drive_link > kernel config: https://drive.google.com/file/d/1fgPLKOw7QbKzhK6ya5KUyKyFhumQgunw/view?usp=drive_link > C reproducer: https://drive.google.com/file/d/1SiLpYTZ7Du39ubgf1k1BIPlu9ZvMjiWZ/view?usp=drive_link > Syzlang reproducer: > https://drive.google.com/file/d/1eWSmwvNGOlZNU-0-xsKhUgZ4WG2VLZL5/view?usp=drive_link > Similar report: > https://groups.google.com/g/syzkaller-bugs/c/C2ud-S1Thh0/m/z4iI7l_dAgAJ > > If you fix this issue, please add the following tag to the commit: > Reported-by: Yikebaer Aizezi <yikebaer61@xxxxxxxxx> > > kvm: vcpu 129: requested lapic timer restore with starting count > register 0x390=4241646265 (4241646265 ns) > initial count (296265111 > ns). Using initial count to start timer. > ------------[ cut here ]------------ > WARNING: CPU: 0 PID: 1977 at arch/x86/kvm/x86.c:11098 > kvm_arch_vcpu_ioctl_run+0x152f/0x1830 arch/x86/kvm/x86.c:11098 Well that's annoying. The WARN is a sanity check that KVM doesn't somehow put the guest into an uninitialized state while emulating the guest's APIC timer, but I completely overlooked the fact that userspace can simply stuff the should-be- impossible guest state. *sigh* Sadly, I think the most reasonable thing to do is to simply drop the sanity check :-( diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 0145d844283b..e9e262b244b8 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -11091,12 +11091,17 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu) r = -EINTR; goto out; } + /* - * It should be impossible for the hypervisor timer to be in - * use before KVM has ever run the vCPU. + * Don't bother switching APIC timer emulation from the + * hypervisor timer to the software timer, the only way for the + * APIC timer to be active is if userspace stuffed vCPU state, + * i.e. put the vCPU and into a nonsensical state. The only + * transition out of UNINITIALIZED (without more state stuffing + * from userspace) is an INIT, which will reset the local APIC + * and thus smother the timer anyways, i.e. APIC timer IRQs + * will be dropped no matter what. */ - WARN_ON_ONCE(kvm_lapic_hv_timer_in_use(vcpu)); - kvm_vcpu_srcu_read_unlock(vcpu); kvm_vcpu_block(vcpu); kvm_vcpu_srcu_read_lock(vcpu);
