On Tue, Jun 21, 2016 at 06:52:17PM +0200, Paolo Bonzini wrote:
> The following scenario is possible:
>
> CPU 1 CPU 2
> static_key_slow_inc
> atomic_inc_not_zero
> -> key.enabled == 0, no increment
> jump_label_lock
> atomic_inc_return
> -> key.enabled == 1 now
> static_key_slow_inc
> atomic_inc_not_zero
> -> key.enabled == 1, inc to 2
> return
> ** static key is wrong!
> jump_label_update
> jump_label_unlock
>
> Testing the static key at the point marked by (**) will follow the wrong
> path for jumps that have not been patched yet. This can actually happen
> when creating many KVM virtual machines with userspace LAPIC emulation;
> just run several copies of the following program:
>
> #include <fcntl.h>
> #include <unistd.h>
> #include <sys/ioctl.h>
> #include <linux/kvm.h>
>
> int main(void)
> {
> for (;;) {
> int kvmfd = open("/dev/kvm", O_RDONLY);
> int vmfd = ioctl(kvmfd, KVM_CREATE_VM, 0);
> close(ioctl(vmfd, KVM_CREATE_VCPU, 1));
> close(vmfd);
> close(kvmfd);
> }
> return 0;
> }
>
> Every KVM_CREATE_VCPU ioctl will attempt a static_key_slow_inc. The
> static key's purpose is to skip NULL pointer checks and indeed one of
> the processes eventually dereferences NULL.
>
> As explained in the commit that introduced the bug (which is 706249c222f6,
> "locking/static_keys: Rework update logic", 2015-07-24), jump_label_update
> needs key.enabled to be true. The solution adopted here is to temporarily
> make key.enabled == -1, and use go down the slow path when key.enabled
> <= 0.
Thanks!
(I frobbed a whitespace fail and fixed the Fixes line).
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