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

[Dmitry Vyukov]

On Tue, Jul 18, 2017 at 7:15 PM, Andrey Ryabinin
<aryabinin@xxxxxxxxxxxxx> 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@xxxxxxxxxx>

> Reported-by: Dave Jones <davej@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> ---
>  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
>