On Mon, May 9, 2022 at 9:09 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>
> On Mon, May 09 2022 at 18:50, Alexander Potapenko wrote:
> > Indeed, calling kmsan_unpoison_memory() in irqentry_enter() was
> > supposed to be enough, but we have code in kmsan_unpoison_memory() (as
> > well as other runtime functions) that checks for kmsan_in_runtime()
> > and bails out to prevent potential recursion if KMSAN code starts
> > calling itself.
> >
> > kmsan_in_runtime() is implemented as follows:
> >
> > ==============================================
> > static __always_inline bool kmsan_in_runtime(void)
> > {
> > if ((hardirq_count() >> HARDIRQ_SHIFT) > 1)
> > return true;
> > return kmsan_get_context()->kmsan_in_runtime;
> > }
> > ==============================================
> > (see the code here:
> > https://lore.kernel.org/lkml/20220426164315.625149-13-glider@xxxxxxxxxx/#Z31mm:kmsan:kmsan.h)
> >
> > If we are running in the task context (in_task()==true),
> > kmsan_get_context() returns a per-task `struct *kmsan_ctx`.
> > If `in_task()==false` and `hardirq_count()>>HARDIRQ_SHIFT==1`, it
> > returns a per-CPU one.
> > Otherwise kmsan_in_runtime() is considered true to avoid dealing with
> > nested interrupts.
> >
> > So in the case when `hardirq_count()>>HARDIRQ_SHIFT` is greater than
> > 1, kmsan_in_runtime() becomes a no-op, which leads to false positives.
>
> But, that'd only > 1 when there is a nested interrupt, which is not the
> case. Interrupt handlers keep interrupts disabled. The last exception from
> that rule was some legacy IDE driver which is gone by now.
That's good to know, then we probably don't need this hardirq_count()
check anymore.
> So no, not a good explanation either.
After looking deeper I see that unpoisoning was indeed skipped because
kmsan_in_runtime() returned true, but I was wrong about the root
cause.
The problem was not caused by a nested hardirq, but rather by the fact
that the KMSAN hook in irqentry_enter() was called with in_task()==1.
Roughly said, T0 was running some code in the task context, then it
started executing KMSAN instrumentation and entered the runtime by
setting current->kmsan_ctx.kmsan_in_runtime.
Then an IRQ kicked in and started calling
asm_sysvec_apic_timer_interrupt() => sysvec_apic_timer_interrupt(regs)
=> irqentry_enter(regs) - but at that point in_task() was still true,
therefore kmsan_unpoison_memory() became a no-op.
As far as I can see, it is irq_enter_rcu() that makes in_task() return
0 by incrementing the preempt count in __irq_enter_raw(), so our
unpoisoning can only work if we perform it after we enter the IRQ
context.
I think the best that can be done here is (as suggested above) to
provide some kmsan_unpoison_pt_regs() function that will only be
called from the entry points and won't be doing reentrancy checks.
It should be safe, because unpoisoning boils down to calculating
shadow/origin addresses and calling memset() on them, no instrumented
code will be involved.
We could try to figure out the places in idtentry code where normal
kmsan_unpoison_memory() can be called in IRQ context, but as far as I
can see it will depend on the type of the entry point.
Another way to deal with the problem is to not rely on in_task(), but
rather use some per-cpu counter in irqentry_enter()/irqentry_exit() to
figure out whether we are in IRQ code already.
However this is only possible irqentry_enter() itself guarantees that
the execution cannot be rescheduled to another CPU - is that the case?
> Thanks,
>
> tglx
>
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--
Alexander Potapenko
Software Engineer
Google Germany GmbH
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