Re: [PATCH v2] kcov: update pos before writing pc in trace function

[Liu, Congyu]

It was actually first found in the kernel trace module I wrote for my research
project. For each call instruction I instrumented one trace function before it
and one trace function after it, then expected traces generated from
them would match since I only instrumented calls that return. But it turns
out that it didn't match from time to time in a non-deterministic manner.
Eventually I figured out it was actually caused by the overwritten issue
from interrupt. I then referred to kcov for a solution but it also suffered from
the same issue...so here's this patch :).

________________________________________
From: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Sent: Monday, May 23, 2022 4:38
To: Liu, Congyu
Cc: andreyknvl@xxxxxxxxx; kasan-dev@xxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
Subject: Re: [PATCH v2] kcov: update pos before writing pc in trace function

On Mon, 23 May 2022 at 07:35, Congyu Liu <liu3101@xxxxxxxxxx> wrote:
>
> In __sanitizer_cov_trace_pc(), previously we write pc before updating pos.
> However, some early interrupt code could bypass check_kcov_mode()
> check and invoke __sanitizer_cov_trace_pc(). If such interrupt is raised
> between writing pc and updating pos, the pc could be overitten by the
> recursive __sanitizer_cov_trace_pc().
>
> As suggested by Dmitry, we cold update pos before writing pc to avoid
> such interleaving.
>
> Apply the same change to write_comp_data().
>
> Signed-off-by: Congyu Liu <liu3101@xxxxxxxxxx>

This version looks good to me.
I wonder how you encountered this? Do you mind sharing a bit about
what you are doing with kcov?

Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>

Thanks

> ---
> PATCH v2:
> * Update pos before writing pc as suggested by Dmitry.
>
> PATCH v1:
> https://lore.kernel.org/lkml/20220517210532.1506591-1-liu3101@xxxxxxxxxx/
> ---
>  kernel/kcov.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/kcov.c b/kernel/kcov.c
> index b3732b210593..e19c84b02452 100644
> --- a/kernel/kcov.c
> +++ b/kernel/kcov.c
> @@ -204,8 +204,16 @@ void notrace __sanitizer_cov_trace_pc(void)
>         /* The first 64-bit word is the number of subsequent PCs. */
>         pos = READ_ONCE(area[0]) + 1;
>         if (likely(pos < t->kcov_size)) {
> -               area[pos] = ip;
> +               /* Previously we write pc before updating pos. However, some
> +                * early interrupt code could bypass check_kcov_mode() check
> +                * and invoke __sanitizer_cov_trace_pc(). If such interrupt is
> +                * raised between writing pc and updating pos, the pc could be
> +                * overitten by the recursive __sanitizer_cov_trace_pc().
> +                * Update pos before writing pc to avoid such interleaving.
> +                */
>                 WRITE_ONCE(area[0], pos);
> +               barrier();
> +               area[pos] = ip;
>         }
>  }
>  EXPORT_SYMBOL(__sanitizer_cov_trace_pc);
> @@ -236,11 +244,13 @@ static void notrace write_comp_data(u64 type, u64 arg1, u64 arg2, u64 ip)
>         start_index = 1 + count * KCOV_WORDS_PER_CMP;
>         end_pos = (start_index + KCOV_WORDS_PER_CMP) * sizeof(u64);
>         if (likely(end_pos <= max_pos)) {
> +               /* See comment in __sanitizer_cov_trace_pc(). */
> +               WRITE_ONCE(area[0], count + 1);
> +               barrier();
>                 area[start_index] = type;
>                 area[start_index + 1] = arg1;
>                 area[start_index + 2] = arg2;
>                 area[start_index + 3] = ip;
> -               WRITE_ONCE(area[0], count + 1);
>         }
>  }
>
> --
> 2.34.1
>