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

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Date:   Wed, 6 Sep 2023 14:16:30 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     Ingo Molnar <mingo@...nel.org>,
        Justin Stitt <justinstitt@...gle.com>
Cc:     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 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...

Regards,

Hans





> 
> Thanks,
> 
> 	Ingo
> 
> --- a/arch/x86/platform/uv/uv_nmi.c
> +++ b/arch/x86/platform/uv/uv_nmi.c
> @@ -202,10 +202,13 @@ static int param_set_action(const char *val, const struct kernel_param *kp)
>  {
>  	int i;
>  	int n = ARRAY_SIZE(valid_acts);
> -	char arg[ACTION_LEN];
> +	char arg[ACTION_LEN], *p;
>  
>  	/* (remove possible '\n') */
> -	strscpy(arg, val, strnchrnul(val, sizeof(arg)-1, '\n') - val + 1);
> +	strscpy(arg, val, sizeof(arg));
> +	p = strchr(arg, '\n');
> +	if (p)
> +		*p = '\0';
>  
>  	for (i = 0; i < n; i++)
>  		if (!strcmp(arg, valid_acts[i].action))
>