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Subject: Re: [PATCH v2 13/33] kmsan: Introduce memset_no_sanitize_memory()
From:   Ilya Leoshkevich <iii@linux.ibm.com>
To:     Alexander Potapenko <glider@google.com>
Cc:     Alexander Gordeev <agordeev@linux.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Christoph Lameter <cl@linux.com>,
        David Rientjes <rientjes@google.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Marco Elver <elver@google.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Pekka Enberg <penberg@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Christian Borntraeger <borntraeger@linux.ibm.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Hyeonggon Yoo <42.hyeyoo@gmail.com>,
        kasan-dev@googlegroups.com, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-s390@vger.kernel.org,
        linux-trace-kernel@vger.kernel.org,
        Mark Rutland <mark.rutland@arm.com>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Sven Schnelle <svens@linux.ibm.com>
Date:   Fri, 08 Dec 2023 15:07:08 +0100
In-Reply-To: <CAG_fn=Vaj3hTRAMxUwofpSMPhFBOizDOWR_An-V9qLNQv-suYw@mail.gmail.com>
References: <20231121220155.1217090-1-iii@linux.ibm.com>
         <20231121220155.1217090-14-iii@linux.ibm.com>
         <CAG_fn=Vaj3hTRAMxUwofpSMPhFBOizDOWR_An-V9qLNQv-suYw@mail.gmail.com>
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On Fri, 2023-12-08 at 14:48 +0100, Alexander Potapenko wrote:
> On Tue, Nov 21, 2023 at 11:06=E2=80=AFPM Ilya Leoshkevich <iii@linux.ibm.=
com>
> wrote:
> >=20
> > Add a wrapper for memset() that prevents unpoisoning.
>=20
> 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.

> ...
>=20
> > +__no_sanitize_memory
> > +static inline void *memset_no_sanitize_memory(void *s, int c,
> > size_t n)
> > +{
> > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 return memset(s, c, n);
> > +}
>=20
> 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?