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

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Date:   Wed, 6 Sep 2023 10:07:03 -0500
From:   Steve Wahl <steve.wahl@....com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Hans de Goede <hdegoede@...hat.com>,
        Justin Stitt <justinstitt@...gle.com>,
        Steve Wahl <steve.wahl@....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

On Wed, Sep 06, 2023 at 04:09:01PM +0200, 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.

For whatever it's worth, I vote in favor of adopting an increased
readability version.

I was on vacation when the patch came through, and by the time I
reviewed it it was already accepted.  I still puzzled through the
-1/+1 stuff to be sure it functioned correctly; since it worked and
was already accepted, I let it go.

When Hans' comments on readability later came through, I was thinking
"Yes, he's exactly right! Why, when I worked so hard on verifying that
the code worked properly, did it not occur to me to suggest re-writing
this in a simpler fashion to make the intent clear?"

Thanks,

--> Steve Wahl

-- 
Steve Wahl, Hewlett Packard Enterprise