On 21/06/2016 15:18, Dmitry Vyukov wrote:
> [<ffffffff811269be>] kvm_lapic_set_tpr+0x5e/0xe0 arch/x86/kvm/lapic.c:1648
> [<ffffffff8109496c>] kvm_set_cr8.part.135+0x2c/0xb0 arch/x86/kvm/x86.c:809
> [<ffffffff81094a22>] kvm_set_cr8+0x32/0x50 arch/x86/kvm/x86.c:806
> [<ffffffff81178dd4>] vmx_vcpu_reset+0x114/0xb60 arch/x86/kvm/vmx.c:4996
> [<ffffffff810b91b0>] kvm_vcpu_reset+0x410/0x580 arch/x86/kvm/x86.c:7470
> [<ffffffff810b9355>] kvm_arch_vcpu_setup+0x35/0x60 arch/x86/kvm/x86.c:7393
> [< inline >] kvm_vm_ioctl_create_vcpu virt/kvm/kvm_main.c:2355
> [<ffffffff8106b9f2>] kvm_vm_ioctl+0x582/0x10d0 virt/kvm/kvm_main.c:2839
> [< inline >] vfs_ioctl fs/ioctl.c:43
> [<ffffffff818510bc>] do_vfs_ioctl+0x18c/0xff0 fs/ioctl.c:674
> [< inline >] SYSC_ioctl fs/ioctl.c:689
> [<ffffffff81851faf>] SyS_ioctl+0x8f/0xc0 fs/ioctl.c:680
> [<ffffffff86a96fc0>] entry_SYSCALL_64_fastpath+0x23/0xc1
The bug is not in KVM. Modulo the shuffling of system calls from
threading, the program is equivalent to this:
long r[7];
int main(void)
{
struct kvm_vcpu_events ve;
r[0] = open("/dev/kvm", O_RDONLY);
r[1] = ioctl(r[0], KVM_CREATE_VM, 0x0ul, 0, 0, 0);
r[2] = ioctl(r[1], KVM_CREATE_VCPU, 0x1ul, 0, 0, 0);
r[3] = ioctl(r[2], KVM_RUN, 0, 0, 0, 0);
r[4] = ioctl(r[2], KVM_RUN, 0, 0, 0, 0);
r[5] = ioctl(r[2], KVM_RUN, 0, 0, 0, 0);
r[6] = ioctl(r[2], KVM_GET_VCPU_EVENTS, &ve, 0, 0, 0);
return 0;
}
However, it is easy to see that there is a clear critical path in the
bug, so that the threading must be completely irrelevant:
1) r[1] was valid in KVM_CREATE_VCPU, so KVM_CREATE_VCPU started after
the assignment of KVM_CREATE_VM
2) r[0] was valid in KVM_CREATE_VM, so KVM_CREATE_VM started after the
assignment of open("/dev/kvm")
3) the assignment of r[2] must not have happened because of the
backtrace, so KVM_CREATE_VCPU has not finished and the other ioctls will
not get to KVM.
The GP _could_ happen because of a bug, because lapic_in_kernel attempts
to skip a NULL check using a static key. But the static key is
incremented before the point that is in the backtrace:
kvm_vm_ioctl_create_vcpu (virt/kvm/kvm_main.c)
kvm_arch_vcpu_create (arch/x86/kvm/x86.c)
vmx_create_vcpu (arch/x86/kvm/vmx.c)
kvm_vcpu_init (virt/kvm/kvm_main.c)
kvm_arch_vcpu_init (arch/x86/kvm/x86.c)
static_key_slow_inc
<-
<-
<-
<-
kvm_arch_vcpu_setup (arch/x86/kvm/x86.c)
kvm_vcpu_reset (arch/x86/kvm/x86.c)
vmx_vcpu_reset (arch/x86/kvm/vmx.c)
kvm_set_cr8 (arch/x86/kvm/x86.c)
*mumble mumble*
Ok, the bug shows up when you do two concurrent calls to
static_key_slow_inc and is caused by commit 706249c22. Patch on its way.
Paolo
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