linux-kernel - Re: [PATCH v6 9/9] vsprintf: Avoid confusion between invalid address and value

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Message-ID: <CAHp75Vckf_aJqpK75-7xmN-u3aVs8nUrAOWiRzSLP5Gh3uHUEQ@mail.gmail.com>
Date:   Tue, 19 Feb 2019 13:06:17 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Petr Mladek <pmladek@...e.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Tobin C . Harding" <me@...in.cc>, Joe Perches <joe@...ches.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.cz>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 9/9] vsprintf: Avoid confusion between invalid address
 and value

On Tue, Feb 19, 2019 at 5:07 AM Sergey Senozhatsky
<sergey.senozhatsky.work@...il.com> wrote:

> Suppose, in my driver I want to sprintf() IPv4 address. The longest
> possible address is 3 * 4 (%d%d%d) + 3 bytes (dots) + terminating NULL.
> E.g. 111.111.111.111
>
> So I can allocate a 16-bytes buffer (stack or slab) and accidentally
> do an %piS sprintf() on a corrupted in_addr struct:
>
>                 char buf[16];
>                 sprintf(buf, "%piS", in_addr);
>
> forcing sprintf() to write "(invalid address)" to a 16-bytes buffer,
> but the thing is - strlen("(invalid address)") > 16.

It would print as many characters as buffer allows. In your case above
the use of sprintf() is a bit fragile.

But yes, the error messages should not be longer than 8 / 16 bytes
depending on 32- or 64-bit build we have.

-- 
With Best Regards,
Andy Shevchenko