linux-kernel - Re: [PATCH] crypto: xor - avoid division by zero crash

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Message-ID: <CAMj1kXEfw2a7QAfFUGemV-uiXF1oSeWnBzWouXFbp2u6g7rojQ@mail.gmail.com>
Date:   Sun, 24 Jan 2021 15:30:36 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Helge Deller <deller@....de>
Cc:     Douglas Anderson <dianders@...omium.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:PARISC ARCHITECTURE" <linux-parisc@...r.kernel.org>
Subject: Re: [PATCH] crypto: xor - avoid division by zero crash

On Sun, 24 Jan 2021 at 15:28, Helge Deller <deller@....de> wrote:
>
> On some of my parisc machines, this patch c055e3eae0f1 ("crypto: xor -
> use ktime for template benchmarking") triggers a dividy-by-zero
> exception because "min" becomes zero, which then leads to a kernel
> crash.
>
> It's not clear yet, why I see the issue only on some machines. At least
> on those the measured time to run the xor speed tests becomes zero
> nanoseconds. Maybe it's because gcc-10 optimizes the speed test out, or
> because of some other changes in the time keeping routines.
>
> In either case, the kernel should not crash.
>
> This patch adds a workaround by reporting such cases with a kernel
> warning and continues as if the xor tests would have run in 1 ns.
>
> Signed-off-by: Helge Deller <deller@....de>
> Cc: stable@...r.kernel.org # 5.10+
>
> ---
>
> diff --git a/crypto/xor.c b/crypto/xor.c
> index eacbf4f93990..3639341bac7e 100644
> --- a/crypto/xor.c
> +++ b/crypto/xor.c
> @@ -100,6 +100,8 @@ do_xor_speed(struct xor_block_template *tmpl, void *b1, void *b2)
>                         mb();
>                 }
>                 diff = ktime_sub(ktime_get(), start);
> +               if (WARN_ON(diff == 0))
> +                       diff = 1;
>                 if (diff < min)
>                         min = diff;
>         }

This should already be fixed in mainline - please check whether that
fix works for you.