linux-kernel - Re: [PATCH] lib/strscpy: avoid KASAN false positive

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Message-ID: <CACT4Y+YLW50EtJHjdv5VuFsE+vYo13523oF4Z5GFn4naMqt0Nw@mail.gmail.com>
Date:   Tue, 18 Jul 2017 19:14:27 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Dave Jones <davej@...emonkey.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Potapenko <glider@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lib/strscpy: avoid KASAN false positive

On Tue, Jul 18, 2017 at 7:15 PM, Andrey Ryabinin
<aryabinin@...tuozzo.com> wrote:
> strscpy() performs the word-at-a-time optimistic reads. So it may
> may access the memory past the end of the object, which is perfectly fine
> since strscpy() doesn't use that (past-the-end) data and makes sure the
> optimistic read won't cross a page boundary.
>
> But KASAN doesn't know anything about that so it will complain.
> Let's just fallback to the byte-at-a-time reads under CONFIG_KASAN=y
> to avoid false-positives.

Acked-by: Dmitry Vyukov <dvyukov@...gle.com>

> Reported-by: Dave Jones <davej@...emonkey.org.uk>
> Signed-off-by: Andrey Ryabinin <aryabinin@...tuozzo.com>
> ---
>  lib/string.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/lib/string.c b/lib/string.c
> index ebbb99c775bd..8b93d2519d5a 100644
> --- a/lib/string.c
> +++ b/lib/string.c
> @@ -199,6 +199,13 @@ ssize_t strscpy(char *dest, const char *src, size_t count)
>                 max = 0;
>  #endif
>
> +       /*
> +        * KASAN won't be happy about word-at-a-time
> +        * optimistic reads, so let's avoid them.
> +        */
> +       if (IS_ENABLED(CONFIG_KASAN))
> +               max = 0;
> +
>         while (max >= sizeof(unsigned long)) {
>                 unsigned long c, data;
>
> --
> 2.13.0
>