linux-kernel - Re: [PATCH RFCv4 3/4] lib/test_printf.c: split write-beyond-buffer check in two

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Date:   Fri, 17 Jun 2022 12:41:11 -0400
From:   Kent Overstreet <kent.overstreet@...il.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        Petr Mladek <pmladek@...e.com>, Jia He <justin.he@....com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFCv4 3/4] lib/test_printf.c: split write-beyond-buffer
 check in two

On 6/17/22 03:15, Rasmus Villemoes wrote:
> On 17/06/2021 16.17, Petr Mladek wrote:
>> On Tue 2021-06-15 23:49:51, 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.
>>
>> Excellent commit message.
>>
>>> 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>
> 
> Hi Petr
> 
> It seems Justin's series got stalled, but I still think this patch makes
> sense on its own (especially since another series in flight mucks about
> in this area), so can you please pick it up directly?
> 
> The lore link for the above is
> https://lore.kernel.org/lkml/20210615154952.2744-4-justin.he@arm.com/ ,
> while my original submission is at
> https://lore.kernel.org/lkml/20210615085044.1923788-1-linux@rasmusvillemoes.dk/

That patch definitely clarifies things, feel free to add my reviewed-by tag