linux-kernel - Re: [PATCH 08/13] lib/test_printf.c: check for out-of-bound writes

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Date:	Mon, 26 Oct 2015 15:41:41 +0900
From:	Kees Cook <keescook@...omium.org>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 08/13] lib/test_printf.c: check for out-of-bound writes

On Wed, Oct 21, 2015 at 5:30 AM, Rasmus Villemoes
<linux@...musvillemoes.dk> wrote:
> Add a few padding bytes on either side of the test buffer, and check
> that these (and the part of the buffer not used) are untouched by
> vsnprintf.
>
> Cc: Kees Cook <keescook@...omium.org>
> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>

Cool, I like this. :)

Acked-by: Kees Cook <keescook@...omium.org>

-Kees

> ---
>  lib/test_printf.c | 24 +++++++++++++++++++-----
>  1 file changed, 19 insertions(+), 5 deletions(-)
>
> diff --git a/lib/test_printf.c b/lib/test_printf.c
> index 9232a2add28c..1ce1a1dd8faf 100644
> --- a/lib/test_printf.c
> +++ b/lib/test_printf.c
> @@ -16,6 +16,7 @@
>  #include <linux/in.h>
>
>  #define BUF_SIZE 256
> +#define PAD_SIZE 16
>  #define FILL_CHAR '$'
>
>  #define PTR1 ((void*)0x01234567)
> @@ -39,6 +40,7 @@
>  static unsigned total_tests __initdata;
>  static unsigned failed_tests __initdata;
>  static char *test_buffer __initdata;
> +static char *alloced_buffer __initdata;
>
>  static int __printf(4, 0) __init
>  do_test(int bufsize, const char *expect, int elen,
> @@ -49,7 +51,7 @@ do_test(int bufsize, const char *expect, int elen,
>
>         total_tests++;
>
> -       memset(test_buffer, FILL_CHAR, BUF_SIZE);
> +       memset(alloced_buffer, FILL_CHAR, BUF_SIZE + 2*PAD_SIZE);
>         va_copy(aq, ap);
>         ret = vsnprintf(test_buffer, bufsize, fmt, aq);
>         va_end(aq);
> @@ -60,8 +62,13 @@ do_test(int bufsize, const char *expect, int elen,
>                 return 1;
>         }
>
> +       if (memchr_inv(alloced_buffer, FILL_CHAR, PAD_SIZE)) {
> +               pr_warn("vsnprintf(buf, %d, \"%s\", ...) wrote before buffer\n", bufsize, fmt);
> +               return 1;
> +       }
> +
>         if (!bufsize) {
> -               if (memchr_inv(test_buffer, FILL_CHAR, BUF_SIZE)) {
> +               if (memchr_inv(test_buffer, FILL_CHAR, BUF_SIZE + PAD_SIZE)) {
>                         pr_warn("vsnprintf(buf, 0, \"%s\", ...) wrote to buffer\n",
>                                 fmt);
>                         return 1;
> @@ -76,6 +83,12 @@ do_test(int bufsize, const char *expect, int elen,
>                 return 1;
>         }
>
> +       if (memchr_inv(test_buffer + written + 1, FILL_CHAR, BUF_SIZE + PAD_SIZE - (written + 1))) {
> +               pr_warn("vsnprintf(buf, %d, \"%s\", ...) wrote beyond the nul-terminator\n",
> +                       bufsize, fmt);
> +               return 1;
> +       }
> +
>         if (memcmp(test_buffer, expect, written)) {
>                 pr_warn("vsnprintf(buf, %d, \"%s\", ...) wrote '%s', expected '%.*s'\n",
>                         bufsize, fmt, test_buffer, written, expect);
> @@ -342,16 +355,17 @@ test_pointer(void)
>  static int __init
>  test_printf_init(void)
>  {
> -       test_buffer = kmalloc(BUF_SIZE, GFP_KERNEL);
> -       if (!test_buffer)
> +       alloced_buffer = kmalloc(BUF_SIZE + 2*PAD_SIZE, GFP_KERNEL);
> +       if (!alloced_buffer)
>                 return -ENOMEM;
> +       test_buffer = alloced_buffer + PAD_SIZE;
>
>         test_basic();
>         test_number();
>         test_string();
>         test_pointer();
>
> -       kfree(test_buffer);
> +       kfree(alloced_buffer);
>
>         if (failed_tests == 0)
>                 pr_info("all %u tests passed\n", total_tests);
> --
> 2.6.1
>



-- 
Kees Cook
Chrome OS Security
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