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Message-ID: <201907082140.51E0B9E2@keescook>
Date: Mon, 8 Jul 2019 21:51:11 -0700
From: Kees Cook <keescook@...omium.org>
To: Salvatore Mesoraca <s.mesoraca16@...il.com>
Cc: Al Viro <viro@...iv.linux.org.uk>, linux-kernel@...r.kernel.org,
Kernel Hardening <kernel-hardening@...ts.openwall.com>,
linux-mm@...ck.org, linux-security-module@...r.kernel.org,
Brad Spengler <spender@...ecurity.net>,
Casey Schaufler <casey@...aufler-ca.com>,
Christoph Hellwig <hch@...radead.org>, Jann Horn <jannh@...gle.com>,
PaX Team <pageexec@...email.hu>,
"Serge E. Hallyn" <serge@...lyn.com>,
Thomas Gleixner <tglx@...utronix.de>,
James Morris <jmorris@...ei.org>
Subject: Re: [PATCH v5 06/12] S.A.R.A.: WX protection
On Sun, Jul 07, 2019 at 05:49:35PM +0200, Salvatore Mesoraca wrote:
> Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> > On Sat, Jul 06, 2019 at 12:54:47PM +0200, Salvatore Mesoraca wrote:
> >
> > > +#define sara_warn_or_return(err, msg) do { \
> > > + if ((sara_wxp_flags & SARA_WXP_VERBOSE)) \
> > > + pr_wxp(msg); \
> > > + if (!(sara_wxp_flags & SARA_WXP_COMPLAIN)) \
> > > + return -err; \
> > > +} while (0)
> > > +
> > > +#define sara_warn_or_goto(label, msg) do { \
> > > + if ((sara_wxp_flags & SARA_WXP_VERBOSE)) \
> > > + pr_wxp(msg); \
> > > + if (!(sara_wxp_flags & SARA_WXP_COMPLAIN)) \
> > > + goto label; \
> > > +} while (0)
> >
> > No. This kind of "style" has no place in the kernel.
> >
> > Don't hide control flow. It's nasty enough to reviewers,
> > but it's pure hell on anyone who strays into your code while
> > chasing a bug or doing general code audit. In effect, you
> > are creating your oh-so-private C dialect and assuming that
> > everyone who ever looks at your code will start with learning
> > that *AND* incorporating it into their mental C parser.
> > I'm sorry, but you are not that important.
> >
> > If it looks like a function call, a casual reader will assume
> > that this is exactly what it is. And when one is scanning
> > through a function (e.g. to tell if handling of some kind
> > of refcounts is correct, with twentieth grep through the
> > tree having brought something in your code into the view),
> > the last thing one wants is to switch between the area-specific
> > C dialects. Simply because looking at yours is sandwiched
> > between digging through some crap in drivers/target/ and that
> > weird thing in kernel/tracing/, hopefully staying limited
> > to 20 seconds of glancing through several functions in your
> > code.
> >
> > Don't Do That. Really.
>
> I understand your concerns.
> The first version of SARA didn't use these macros,
> they were added because I was asked[1] to do so.
>
> I have absolutely no problems in reverting this change.
> I just want to make sure that there is agreement on this matter.
> Maybe Kees can clarify his stance.
>
> Thank you for your suggestions.
>
> [1] https://lkml.kernel.org/r/CAGXu5jJuQx2qOt_aDqDQDcqGOZ5kmr5rQ9Zjv=MRRCJ65ERfGw@mail.gmail.com
I just didn't like how difficult it was to review the repeated checking.
I thought then (and still think now) it's worth the unusual style to
improve the immediate readability. Obviously Al disagrees. I'm not
against dropping my suggestion; it's just a pain to review it and it
seems like an area that would be highly prone to subtle typos. Perhaps
some middle ground:
#define sara_warn(msg) ({ \
if ((sara_wxp_flags & SARA_WXP_VERBOSE)) \
pr_wxp(msg); \
!(sara_wxp_flags & SARA_WXP_COMPLAIN); \
})
...
if (unlikely(sara_wxp_flags & SARA_WXP_WXORX &&
vm_flags & VM_WRITE &&
vm_flags & VM_EXEC &&
sara_warn("W^X")))
return -EPERM;
that way the copy/pasting isn't present but the control flow is visible?
--
Kees Cook
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