linux-kernel - Re: [PATCH v2 2/4] kcov: add interrupt handling self test

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Date: Wed, 19 Jun 2024 13:18:52 +0200
From: Dmitry Vyukov <dvyukov@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, 
	dave.hansen@...ux.intel.com, x86@...nel.org, linux-kernel@...r.kernel.org, 
	syzkaller@...glegroups.com, elver@...gle.com, glider@...gle.com, 
	nogikh@...gle.com, tarasmadan@...gle.com
Subject: Re: [PATCH v2 2/4] kcov: add interrupt handling self test

On Wed, 19 Jun 2024 at 13:13, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Tue, Jun 11, 2024 at 09:50:31AM +0200, Dmitry Vyukov wrote:
> > Add a boot self test that can catch sprious coverage from interrupts.
> > The coverage callback filters out interrupt code, but only after the
> > handler updates preempt count. Some code periodically leaks out
> > of that section and leads to spurious coverage.
> > Add a best-effort (but simple) test that is likely to catch such bugs.
> > If the test is enabled on CI systems that use KCOV, they should catch
> > any issues fast.
> >
> > Signed-off-by: Dmitry Vyukov <dvyukov@...gle.com>
> > Reviewed-by: Alexander Potapenko <glider@...gle.com>
> > Cc: x86@...nel.org
> > Cc: linux-kernel@...r.kernel.org
> > Cc: syzkaller@...glegroups.com
> >
> > ---
> >
> > Changed since v1:
> >  - renamed KCOV_TEST to KCOV_SELFTEST
> >  - improved the config description
> >  - loop for exactly 300ms in the test
> >
> > In my local testing w/o the previous fix,
> > it immidiatly produced the following splat:
> >
> > kcov: running selftest
> > BUG: TASK stack guard page was hit at ffffc90000147ff8
> > Oops: stack guard page: 0000 [#1] PREEMPT SMP KASAN PTI
> > ...
> >  kvm_set_cpu_l1tf_flush_l1d+0x5/0x20
> >  sysvec_call_function+0x15/0xb0
> >  asm_sysvec_call_function+0x1a/0x20
> >  kcov_init+0xe4/0x130
> >  do_one_initcall+0xbc/0x470
> >  kernel_init_freeable+0x4fc/0x930
> >  kernel_init+0x1c/0x2b0
>
> So I'm not entirely sure how the above BUG comes about, nor how this
> selftest tickles it. Could you elaborate?
>
> I've found check_kcov_mode() which has this !in_task() clause, but I'm
> not entirely sure how failing that leads to the above mentioned failure.

I've tried to explain it in the test comment, maybe I need to improve it:

+        * We set kcov_mode to enable tracing, but don't setup the area,
+        * so any attempt to trace will crash. Note: we must not call any
+        * potentially traced functions in this region.

Basically, we setup current task kcov in a way that any attempt to
trace in __sanitizer_cov_trace_pc() will crash, and then just loop
waiting for interrupts.

A more legit way to achieve the same would be to properly setup kcov
for tracing from within the kernel, then call outermost interrupt
entry function, then check we traced nothing. But that would require a
non-trivial amount of new complex code, and e.g. the top-most
interrupt entry function is not exported and is arch-specific.



> > ---
> >  kernel/kcov.c     | 31 +++++++++++++++++++++++++++++++
> >  lib/Kconfig.debug |  8 ++++++++
> >  2 files changed, 39 insertions(+)
> >
> > diff --git a/kernel/kcov.c b/kernel/kcov.c
> > index c3124f6d5536..72a5bf55107f 100644
> > --- a/kernel/kcov.c
> > +++ b/kernel/kcov.c
> > @@ -11,6 +11,7 @@
> >  #include <linux/fs.h>
> >  #include <linux/hashtable.h>
> >  #include <linux/init.h>
> > +#include <linux/jiffies.h>
> >  #include <linux/kmsan-checks.h>
> >  #include <linux/mm.h>
> >  #include <linux/preempt.h>
> > @@ -1057,6 +1058,32 @@ u64 kcov_common_handle(void)
> >  }
> >  EXPORT_SYMBOL(kcov_common_handle);
> >
> > +#ifdef CONFIG_KCOV_SELFTEST
> > +static void __init selftest(void)
> > +{
> > +     unsigned long start;
> > +
> > +     pr_err("running self test\n");
> > +     /*
> > +      * Test that interrupts don't produce spurious coverage.
> > +      * The coverage callback filters out interrupt code, but only
> > +      * after the handler updates preempt count. Some code periodically
> > +      * leaks out of that section and leads to spurious coverage.
> > +      * It's hard to call the actual interrupt handler directly,
> > +      * so we just loop here for a bit waiting for a timer interrupt.
> > +      * We set kcov_mode to enable tracing, but don't setup the area,
> > +      * so any attempt to trace will crash. Note: we must not call any
> > +      * potentially traced functions in this region.
> > +      */
> > +     start = jiffies;
> > +     current->kcov_mode = KCOV_MODE_TRACE_PC;
>
>         barrier();
>
> > +     while ((jiffies - start) * MSEC_PER_SEC / HZ < 300)
> > +             ;
>
>         barrier();
>
> > +     current->kcov_mode = 0;
> > +     pr_err("done running self test\n");
> > +}
> > +#endif
>
> --
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