linux-hardening - Re: [PATCH v3] x86/platform/uv: refactor deprecated strcpy and strncpy

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Date:   Wed, 13 Sep 2023 17:12:28 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Justin Stitt <justinstitt@...gle.com>,
        Steve Wahl <steve.wahl@....com>,
        Mike Travis <mike.travis@....com>,
        Dimitri Sivanich <dimitri.sivanich@....com>,
        Russ Anderson <russ.anderson@....com>,
        Darren Hart <dvhart@...radead.org>,
        Andy Shevchenko <andy@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        platform-driver-x86@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH v3] x86/platform/uv: refactor deprecated strcpy and
 strncpy

Hi,

On 9/6/23 16:09, Ingo Molnar wrote:
> 
> * Hans de Goede <hdegoede@...hat.com> wrote:
> 
>> Hi Ingo,
>>
>> On 9/6/23 14:10, Ingo Molnar wrote:
>>>
>>> * Justin Stitt <justinstitt@...gle.com> wrote:
>>>
>>>> Both `strncpy` and `strcpy` are deprecated for use on NUL-terminated
>>>> destination strings [1].
>>>>
>>>> We can see that `arg` and `uv_nmi_action` are expected to be
>>>> NUL-terminated strings due to their use within `strcmp()` and format
>>>> strings respectively.
>>>>
>>>> With this in mind, a suitable replacement is `strscpy` [2] due to the
>>>> fact that it guarantees NUL-termination on its destination buffer
>>>> argument which is _not_ the case for `strncpy` or `strcpy`!
>>>>
>>>> In this case, we can drop both the forced NUL-termination and the `... -1` from:
>>>> |       strncpy(arg, val, ACTION_LEN - 1);
>>>> as `strscpy` implicitly has this behavior.
>>>>
>>>> Link: www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings[1]
>>>> Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
>>>> Link: https://github.com/KSPP/linux/issues/90
>>>> Cc: linux-hardening@...r.kernel.org
>>>> Signed-off-by: Justin Stitt <justinstitt@...gle.com>
>>>
>>>>  arch/x86/platform/uv/uv_nmi.c | 7 +++----
>>>>  1 file changed, 3 insertions(+), 4 deletions(-)
>>>
>>> Note that this commit is already upstream:
>>>
>>>   1e6f01f72855 ("x86/platform/uv: Refactor code using deprecated strcpy()/strncpy() interfaces to use strscpy()")
>>>
>>> Below is the delta your v3 patch has compared to what is upstream - is it 
>>> really necessary to open code it, instead of using strnchrnul() as your 
>>> original patch did? Am I missing anything here?
>>
>> The new version is a result of a review from my because IMHO:
>>
>> 	strscpy(arg, val, strnchrnul(val, sizeof(arg)-1, '\n') - val + 1);
>>
>> Is really unreadable / really hard to reason about if
>> this is actually correct and does not contain any
>> of by 1 bugs.
>>
>> Note that the diff of v3 compared to the code before v2 landed is
>> actually smaller now and actually matches the subject of:
>> "refactor deprecated strcpy and strncpy"
>>
>> Where as v2 actually touches more code / refactor things
>> which fall outside of a "one change per patch" approach.
>> The:
>>
>> 	p = strchr(arg, '\n');
>> 	if (p)
>> 		*p = '\0';
>>
>> was already there before v2 landed.
>>
>> I also suggested to do a follow up patch to change things to:
>>
>> 	strscpy(arg, val, sizeof(arg));
>> 	p = strchrnul(arg, '\n');
>> 	*p = '\0';
>>
>> Which IMHO is much more readable then what has landed
>> now. But since v2 has already landed I guess the best
>> thing is just to stick with what we have upstream now...
> 
> Well, how about we do a delta patch with all the changes
> you suggested? I'm all for readability.

So I started doing this and notices that all the string
manipulation + parsing done here is really just a DYI
implementation of sysfs_match_string().

So I have prepared a patch to switch to sysfs_match_string(),
which completely removes the need to make a copy of the val
string.

I'll submit the patch right after this email.

Regards,

Hans