linux-kernel - Re: [PATCH v2 13/33] kmsan: Introduce memset_no_sanitize

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Message-ID: <69e7bc8e8c8a38c429a793e991e0509cb97a53e1.camel@linux.ibm.com>
Date:   Fri, 08 Dec 2023 15:07:08 +0100
From:   Ilya Leoshkevich <iii@...ux.ibm.com>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     Alexander Gordeev <agordeev@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Marco Elver <elver@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Pekka Enberg <penberg@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        kasan-dev@...glegroups.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-s390@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org,
        Mark Rutland <mark.rutland@....com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Sven Schnelle <svens@...ux.ibm.com>
Subject: Re: [PATCH v2 13/33] kmsan: Introduce memset_no_sanitize_memory()

On Fri, 2023-12-08 at 14:48 +0100, Alexander Potapenko wrote:
> On Tue, Nov 21, 2023 at 11:06 PM Ilya Leoshkevich <iii@...ux.ibm.com>
> wrote:
> > 
> > Add a wrapper for memset() that prevents unpoisoning.
> 
> We have __memset() already, won't it work for this case?

A problem with __memset() is that, at least for me, it always ends
up being a call. There is a use case where we need to write only 1
byte, so I thought that introducing a call there (when compiling
without KMSAN) would be unacceptable.

> On the other hand, I am not sure you want to preserve the redzone in
> its previous state (unless it's known to be poisoned).

That's exactly the problem with unpoisoning: it removes the distinction
between a new allocation and a UAF.

> You might consider explicitly unpoisoning the redzone instead.

That was my first attempt, but it resulted in test failures due to the
above.

> ...
> 
> > +__no_sanitize_memory
> > +static inline void *memset_no_sanitize_memory(void *s, int c,
> > size_t n)
> > +{
> > +       return memset(s, c, n);
> > +}
> 
> I think depending on the compiler optimizations this might end up
> being a call to normal memset, that would still change the shadow
> bytes.

Interesting, do you have some specific scenario in mind? I vaguely
remember that in the past there were cases when sanitizer annotations
were lost after inlining, but I thought they were sorted out?

And, in any case, if this were to happen, would not it be considered a
compiler bug that needs fixing there, and not in the kernel?