Re: WARNING: CPU: 0 PID: 3031 at ./arch/x86/include/asm/fpu/internal.h:530 fpu__restore+0x90/0x130()

[Andy Lutomirski]

On Feb 17, 2016 1:29 AM, "Borislav Petkov" <bp@xxxxxxxxx> wrote:
>
> On Wed, Feb 17, 2016 at 09:16:46AM +0100, Ingo Molnar wrote:
> > So I'm wondering why this started triggering only now. Is this a pre-existing bug
> > that somehow got triggered via:
> >
> >   58122bf1d856 x86/fpu: Default eagerfpu=on on all CPUs
> >
> > ?
>
> Well, that's an interesting question. See, the thing is, I triggered
> this only *once* by accident and I haven't seen it ever since.
>
> The "reliable" "reproducer" I used to debug this was Andy's suggestion
> to stick a schedule() in __fpu__restore_sig().
>
> So the answer to that question is not easy.
>
> BUT(!), regardless, the bug still needs to be fixed because my tracing
> here
>
> https://lkml.kernel.org/r/20160215191422.GB32716@xxxxxxx
>
> showed that getting preempted after setting
>
>         fpu->fpstate_active = 1;
>
> leads to the WARN. Because - and please doublecheck me on that - when
> we're in __switch_to() and the task which already has ->fpstate_active
> set and it is the next task to which we're going to switch to, when it
> enters switch_fpu_prepare(), it does:
>
>         fpu.preload = static_cpu_has(X86_FEATURE_FPU) &&
>                       new_fpu->fpstate_active &&
>                       ^^^^^^^^^^^^^^^^^^^^^^^
>
> so that fpu.preload is set now.
>
> A bit later in that same function:
>
>                 /* Don't change CR0.TS if we just switch! */
>                 if (fpu.preload) {
>                         new_fpu->counter++;
>                         __fpregs_activate(new_fpu);
>                         ^^^^^^^^^^^^^^^^^
>
> ->fpregs_active gets set here and when the task returns to
> __fpu__restore_sig(), fpu__restore() sets it again, leading to the WARN.
>
> Mind you, this happens on 32-bit only because there we sigreturn with
> irqs enabled. Look at the call trace.

Not quite.  IRQs are on in the 64-bit case, too, but the problematic
code doesn't run because the 64-bit sigreturn case doesn't need to
fiddle with the fpu layout.

--Andy