[PATCH v4] arm64: fpsimd: improve stacking logic in non-interruptible context

[Ard Biesheuvel]

On 9 December 2016 at 19:29, Dave Martin <Dave.Martin at arm.com> wrote:
> On Fri, Dec 09, 2016 at 06:21:55PM +0000, Catalin Marinas wrote:
>> On Fri, Dec 09, 2016 at 04:46:32PM +0000, Ard Biesheuvel wrote:
>> >  void kernel_neon_begin_partial(u32 num_regs)
>> >  {
>> > -   if (in_interrupt()) {
>> > -           struct fpsimd_partial_state *s = this_cpu_ptr(
>> > -                   in_irq() ? &hardirq_fpsimdstate : &softirq_fpsimdstate);
>> > +   struct fpsimd_partial_state *s;
>> > +   int level;
>> > +
>> > +   preempt_disable();
>> > +
>> > +   level = this_cpu_inc_return(kernel_neon_nesting_level);
>> > +   BUG_ON(level > 3);
>> > +
>> > +   if (level > 1) {
>> > +           s = this_cpu_ptr(nested_fpsimdstate);
>> >
>> > -           BUG_ON(num_regs > 32);
>> > -           fpsimd_save_partial_state(s, roundup(num_regs, 2));
>> > +           WARN_ON_ONCE(num_regs > 32);
>> > +           num_regs = min(roundup(num_regs, 2), 32U);
>> > +
>> > +           fpsimd_save_partial_state(&s[level - 2], num_regs);
>> >     } else {
>> >             /*
>> >              * Save the userland FPSIMD state if we have one and if we
>> > @@ -241,7 +256,6 @@ void kernel_neon_begin_partial(u32 num_regs)
>> >              * that there is no longer userland FPSIMD state in the
>> >              * registers.
>> >              */
>> > -           preempt_disable();
>> >             if (current->mm &&
>> >                 !test_and_set_thread_flag(TIF_FOREIGN_FPSTATE))
>> >                     fpsimd_save_state(&current->thread.fpsimd_state);
>>
>> I wonder whether we could actually do this saving and flag/level setting
>> in reverse to simplify the races. Something like your previous patch but
>> only set TIF_FOREIGN_FPSTATE after saving:
>>
>>       level = this_cpu_read(kernel_neon_nesting_level);
>>       if (level > 0) {
>>               ...
>>               fpsimd_save_partial_state();
>>       } else {
>>               if (!test_thread_flag(TIF_FOREIGN_FPSTATE))
>>                       fpsimd_save_state();
>>               set_thread_flag(TIF_FOREIGN_FPSTATE);
>>       }
>>       this_cpu_inc(kernel_neon_nesting_level);
>>
>> There is a risk of extra saving if we get an interrupt after
>> test_thread_flag() and before set_thread_flag() but I don't think this
>> would corrupt any state, just writing things twice.
>
> I would worry that we can save two states over the same buffer and then
> restore an uninitialised buffer in this case unless we are careful.
> Because the level-dependent code is now misbracketed by the inc/dec,
> a preempting call races with the outer call and use the same value.
>
> I guess we could do
>
> if (!test_thread_flag(TIF_FOREIGN_FPSTATE))
>         fpsimd_save_state();
> clear_thread_flag(TIF_FOREIGN_FPSTATE);
>
> at the start unconditionally, before the _inc_return().
>
> The task state may then get saved in the middle of being saved, but
> as you say it shouldn't have changed in the meantime.

It /will/ have changed in the meantime: when the interrupted context
is resumed, it will happily proceed with saving the state where it
left off, but now the register file contains whatever was left after
the interrupt handler is done with the NEON.

> The nested
> save code may then do a partial save of the same state on top of that
> which could get restored at the inner kernel_neon_end() call.
>

I'm afraid the only way to deal with this correctly is to treat the
whole sequence as a critical section, which means execute it with
interrupts disabled.