Re: [patch 3/3] x86/fpu: Make FPU protection more robust

[Jason A. Donenfeld]

Hey Thomas,

On Wed, May 04, 2022 at 06:45:45PM +0200, Thomas Gleixner wrote:
> add_disk_randomness() on !RT kernels. That's what made me look into this
> in the first place as it unearthed the long standing FPU protection
> bug. See the first patch in this thread.
> 
> Possibly add_device_randomness() too, but I haven't seen evidence so far.

It looks like it's being hit from add_input_randomness() via
input_handle_event() too. There are two positions we could take toward
this:

One stance to take would be that if an event happens in an interrupt,
add_interrupt_randomness() should in theory already be siphashing in a
cycle counter so additional calls to add_{input,disk}_randomness() don't
contribute substantially (unless you assume the num field has some
entropic value). If that's compelling, then the next thing to do would
be adding a `if (in_interrupt()) return;` early on in some function, and
then we forever after impose a rule, "never mix into the input pool
directly from an irq".

The other stance is that these input/disk events are relatively rare --
compared to, say, a storm of interrupts from a NIC -- so mixing into the
input pool from there isn't actually a problem, and we benefit from the
quasi domain-specific accounting and the superior mixing function,
there, so keep it around. And the non-raw spinlock on the input pool
won't negatively affect RT from this context, because all its callers on
RT should be threaded.

The second stance seems easier and more conservative from a certain
perspective -- we don't need to change anything -- so I'm more inclined
toward it. And given that you've fixed the bug now, it sounds like
that's fine with you too. But if you're thinking about it differently in
fact, let me know.

Jason