linux-kernel - Re: + lib-stringc-strlcpy-might-read-too-far.patch added to -mm tree

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Message-ID: <CACVxJT-f+K_z-XxqjDVU_QK-VJPqnCEbWuMJF1fyTL0V0xiTgA@mail.gmail.com>
Date:	Tue, 15 Apr 2014 13:49:38 +0300
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	Linux Kernel <linux-kernel@...r.kernel.org>
Cc:	Vegard Nossum <vegard.nossum@...il.com>,
	Dan Carpenter <dan.carpenter@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: + lib-stringc-strlcpy-might-read-too-far.patch added to -mm tree

On Tue, Apr 15, 2014 at 12:47 AM,  <akpm@...ux-foundation.org> wrote:
> Subject: lib/string.c: strlcpy() might read too far
>
> Imagine you have a user controlled variable at the end of a struct which
> is allocated at the end of a page.  The strlen() could read beyond the
> mapped memory and cause an oops.
>
> Probably there are two reasons why we have never hit this condition in
> real life.  First you would have to be really unlucky for all the
> variables to line up so the oops can happen.  Second we don't do a lot of
> fuzzing with invalid strings.
>
> The strnlen() call is obviously a little bit slower than strlen() but I
> have tested it and I think it's probably ok.

> --- a/lib/string.c~lib-stringc-strlcpy-might-read-too-far
> +++ a/lib/string.c
> @@ -148,10 +148,10 @@ EXPORT_SYMBOL(strncpy);
>   */
>  size_t strlcpy(char *dest, const char *src, size_t size)
>  {
> -       size_t ret = strlen(src);
> +       size_t ret = strnlen(src, size);
>
>         if (size) {
> -               size_t len = (ret >= size) ? size - 1 : ret;
> +               size_t len = (ret < size) ? ret : ret - 1;
>                 memcpy(dest, src, len);
>                 dest[len] = '\0';
>         }

Return value matters. It may not matter for kernel, because kernel is
not heavy string user.
But it is better to not diverge from master code:
http://www.openbsd.org/cgi-bin/cvsweb/src/lib/libc/string/strlcpy.c?rev=1.11

Counter-rationale:
* strlcpy() accepts strings, so if you're giving raw buffer you're
doing it wrong.
* last byte of last page argument is bogus because kernel copies data
from userspace first.
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