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Message-ID: <cd4c22c3-2901-4dee-b6b4-e6981848cb70@suse.cz>
Date: Tue, 2 Jan 2024 17:10:53 +0100
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Subject: Re: [PATCH v3 15/34] mm: slub: Unpoison the memchr_inv() return value
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To: Ilya Leoshkevich <iii@linux.ibm.com>,
 Alexander Gordeev <agordeev@linux.ibm.com>,
 Alexander Potapenko <glider@google.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>
Cc: 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>
References: <20231213233605.661251-1-iii@linux.ibm.com>
 <20231213233605.661251-16-iii@linux.ibm.com>
From: Vlastimil Babka <vbabka@suse.cz>
In-Reply-To: <20231213233605.661251-16-iii@linux.ibm.com>
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On 12/14/23 00:24, Ilya Leoshkevich 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.
> 
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  mm/slub.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 2d29d368894c..802702748925 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1076,6 +1076,7 @@ static int check_bytes_and_report(struct kmem_cache *s, struct slab *slab,
>  	metadata_access_enable();
>  	fault = memchr_inv(kasan_reset_tag(start), value, bytes);
>  	metadata_access_disable();
> +	kmsan_unpoison_memory(&fault, sizeof(fault));
>  	if (!fault)
>  		return 1;
>  
> @@ -1182,6 +1183,7 @@ static void slab_pad_check(struct kmem_cache *s, struct slab *slab)
>  	metadata_access_enable();
>  	fault = memchr_inv(kasan_reset_tag(pad), POISON_INUSE, remainder);
>  	metadata_access_disable();
> +	kmsan_unpoison_memory(&fault, sizeof(fault));
>  	if (!fault)
>  		return;
>  	while (end > fault && end[-1] == POISON_INUSE)