linux-kernel - Re: [PATCH v2 3/4] lib/test_printf.c: split write-beyond-buffer check in two

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Message-ID: <YNL72KwP8oyNzons@smile.fi.intel.com>
Date:   Wed, 23 Jun 2021 12:16:08 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Jia He <justin.he@....com>
Cc:     Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Jonathan Corbet <corbet@....net>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Eric Biggers <ebiggers@...gle.com>,
        "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        Matthew Wilcox <willy@...radead.org>,
        Christoph Hellwig <hch@...radead.org>, nd@....com
Subject: Re: [PATCH v2 3/4] lib/test_printf.c: split write-beyond-buffer
 check in two

On Wed, Jun 23, 2021 at 01:50:10PM +0800, Jia He wrote:
> From: Rasmus Villemoes <linux@...musvillemoes.dk>
> 
> Before each invocation of vsnprintf(), do_test() memsets the entire
> allocated buffer to a sentinel value. That buffer includes leading and
> trailing padding which is never included in the buffer area handed to
> vsnprintf (spaces merely for clarity):
> 
>   pad  test_buffer      pad
>   **** **************** ****
> 
> Then vsnprintf() is invoked with a bufsize argument <=
> BUF_SIZE. Suppose bufsize=10, then we'd have e.g.
> 
>  |pad |   test_buffer    |pad |
>   **** pizza0 **** ****** ****
>  A    B      C    D           E
> 
> where vsnprintf() was given the area from B to D.
> 
> It is obviously a bug for vsnprintf to touch anything between A and B
> or between D and E. The former is checked for as one would expect. But
> for the latter, we are actually a little stricter in that we check the
> area between C and E.
> 
> Split that check in two, providing a clearer error message in case it
> was a genuine buffer overrun and not merely a write within the
> provided buffer, but after the end of the generated string.
> 
> So far, no part of the vsnprintf() implementation has had any use for
> using the whole buffer as scratch space, but it's not unreasonable to
> allow that, as long as the result is properly nul-terminated and the
> return value is the right one. However, it is somewhat unusual, and
> most %<something> won't need this, so keep the [C,D] check, but make
> it easy for a later patch to make that part opt-out for certain tests.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>

> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
> Tested-by: Jia He <justin.he@....com>
> Signed-off-by: Jia He <justin.he@....com>
> Reviewed-by: Petr Mladek <pmladek@...e.com>
> ---
>  lib/test_printf.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/test_printf.c b/lib/test_printf.c
> index ec0d5976bb69..d1d2f898ebae 100644
> --- a/lib/test_printf.c
> +++ b/lib/test_printf.c
> @@ -78,12 +78,17 @@ 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))) {
> +	if (memchr_inv(test_buffer + written + 1, FILL_CHAR, bufsize - (written + 1))) {
>  		pr_warn("vsnprintf(buf, %d, \"%s\", ...) wrote beyond the nul-terminator\n",
>  			bufsize, fmt);
>  		return 1;
>  	}
>  
> +	if (memchr_inv(test_buffer + bufsize, FILL_CHAR, BUF_SIZE + PAD_SIZE - bufsize)) {
> +		pr_warn("vsnprintf(buf, %d, \"%s\", ...) wrote beyond buffer\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);
> -- 
> 2.17.1
> 

-- 
With Best Regards,
Andy Shevchenko