linux-kernel - Re: [PATCH 2/2] vsprintf: Stop using obsolete simple

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Message-ID: <CAHk-=wi2kvoDh3z_X3ZCBcyAX1MU4OedPkQjA-4=bR=6q7AT3w@mail.gmail.com>
Date:   Tue, 11 Dec 2018 09:22:22 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     thomas.preston@...ethink.co.uk
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Petr Mladek <pmladek@...e.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Steven Rostedt <rostedt@...dmis.org>, geert+renesas@...der.be,
        Jonathan Corbet <corbet@....net>, tcharding <me@...in.cc>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        ben.dooks@...ethink.co.uk
Subject: Re: [PATCH 2/2] vsprintf: Stop using obsolete simple_strtoul()

On Tue, Dec 11, 2018 at 7:21 AM Thomas Preston
<thomas.preston@...ethink.co.uk> wrote:
>
> Stop using the obsolete functions simple_strtoul() and
> simple_strtoull(). Instead, we should use the improved kstrtol() and
> kstrtoll() functions. To do this, we must copy the current field into a
> null-terminated tmpstr and advance the variable `next` manually.

I see what you're trying to do, but this fix is much much worse than
the bug was.

> +               if (field_width > 0) {
> +                       char tmpstr[INT_BUF_LEN];
> +                       int ret;
> +
> +                       strscpy(tmpstr, str, field_width+1);

If field_width is larger than INT_BUF_LEN, you are now corrupting kernel stack.

And no, you can't fix it by limiting field_width, since a large
field_width is quite possible and might even be valid - and still fit
in an int. Maybe the number is

    000000000000000000000001

or something?

A fix might be to skip leading zeroes.

Honestly, just do it by hand. Don't use kstrol and friends at all.
Just do something like

    unsigned long long val = 0;
    p = str;
    for (;;) {
        int c;
        if (field_width > 0 && p - str >= field_width)
            break;
        c = hexval(*p++);
        if (c < 0 || c > base)
            break;
        val = val * base + c;
        // check for overflow
    }
    /* Now do "sign" and range checking on val */
    /* Ta-daa, all done */

or similar.  Treat the above as pseudo-code, I didn't fill in all the details.

            Linus