Re: [patch 0/3] x86/fpu: Prevent FPU state corruption

Thomas Gleixner
2022-05-18T13:10:00Z

On Wed, May 18 2022 at 03:02, Jason A. Donenfeld wrote:
> On Wed, May 04, 2022 at 05:40:26PM +0200, Jason A. Donenfeld wrote:
>> On Sun, May 01, 2022 at 09:31:42PM +0200, Thomas Gleixner wrote:
>> > The recent changes in the random code unearthed a long standing FPU state
>> > corruption due do a buggy condition for granting in-kernel FPU usage.
>>
>> Thanks for working that out. I've been banging my head over [1] for a
>> few days now trying to see if it's a mis-bisect or a real thing. I'll
>> ask Larry to retry with this patchset.
>
> So, Larry's debugging was inconsistent and didn't result in anything I
> could piece together into basic cause and effect. But luckily Vadim, who
> maintains the VirtualBox drivers for Oracle, was able to reproduce the
> issue and was able to conduct some real debugging. I've CC'd him here.
> From talking with Vadim, here are some findings thus far:
>
> - Certain Linux guest processes crash under high load.
> - Windows kernel guest panics.
>
> Observation: the Windows kernel uses SSSE3 in their kernel all over the
> place, generated by the compiler.
>
> - Moving the mouse around helps induce the crash.
>
> Observation: add_input_randomness() -> .. -> kernel_fpu_begin() -> blake2s_compress().
>
> - The problem exhibits itself in rc7, so this patchset does not fix
> the issue.
> - Applying https://xn--4db.cc/ttEUSvdC fixes the issue.
>
> Observation: the problem is definitely related to using the FPU in a
> hard IRQ.
>
> I went reading KVM to get some idea of why KVM does *not* have this
> problem, and it looks like there's some careful code there about doing
> xsave and such around IRQs. So my current theory is that VirtualBox's
> VMM just forgot to do this, and until now this bug went unnoticed.

That's a very valid assumption. I audited all places which fiddle with
FPU in Linus tree and with the fix applied they're all safe.

> Since VirtualBox is out of tree (and extremely messy of a codebase), and
> this appears to be an out of tree module problem rather than a kernel
> problem, I'm inclined to think that there's not much for us to do, at
> least until we receive information to the contrary of this presumption.

Agreed in all points.

> But in case you do want to do something proactively, I don't have any
> objections to just disabling the FPU in hard IRQ for 5.18. And in 5.19,
> add_input_randomness() isn't going to hit that path anyway. But also,
> doing nothing and letting the VirtualBox people figure out their bug
> would be fine with me too. Either way, just wanted to give you a heads
> up.

That virtualborx bug has to be fixed in any case as this problem exists
forever and there have been drivers using FPU in hard interrupt context
in the past sporadically, so it's sheer luck that this didn't explode
before. AFAICT all of this has been moved to softirq context over the
years, so the random code is probably the sole in hard interrupt user in
mainline today.

In the interest of users we should probably bite the bullet and just
disable hard interrupt FPU usage upstream and Cc stable. The stable
kernel updates probably reach users faster.

Thanks,

tglx