linux-kernel - Re: [PATCH v4 16/35] mm: slub: Unpoison the memchr

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Message-ID: <ff3403a257086f09db1280c5952e6f72371b10ef.camel@linux.ibm.com>
Date: Wed, 19 Jun 2024 13:46:50 +0200
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 v4 16/35] mm: slub: Unpoison the memchr_inv() return
 value

On Tue, 2024-06-18 at 16:38 +0200, Alexander Potapenko wrote:
> On Thu, Jun 13, 2024 at 5:39 PM Ilya Leoshkevich <iii@...ux.ibm.com>
> wrote:
> > 
> > Even though the KMSAN warnings generated by memchr_inv() are
> > suppressed
> > by metadata_access_enable(), its return value may still be
> > poisoned.
> > 
> > The reason is that the last iteration of memchr_inv() returns
> > `*start != value ? start : NULL`, where *start is poisoned. Because
> > of
> > this, somewhat counterintuitively, the shadow value computed by
> > visitSelectInst() is equal to `(uintptr_t)start`.
> > 
> > The intention behind guarding memchr_inv() behind
> > metadata_access_enable() is to touch poisoned metadata without
> > triggering KMSAN, so unpoison its return value.
> 
> What do you think about applying __no_kmsan_checks to these functions
> instead?

Ok, will do. The __no_kmsan_checks approach is already taken by
"mm: kfence: Disable KMSAN when checking the canary", so we might as
well be consistent in how we fix these issues.