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Date:   Wed, 29 May 2019 12:48:52 -0700
From:   Matthew Wilcox <willy@infradead.org>
To:     Dianzhang Chen <dianzhangchen0@gmail.com>
Cc:     cl@linux.com, penberg@kernel.org, rientjes@google.com,
        iamjoonsoo.kim@lge.com, akpm@linux-foundation.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/slab_common.c: fix possible spectre-v1 in
 kmalloc_slab()
Message-ID: <20190529194852.GA23461@bombadil.infradead.org>
References: <1559133448-31779-1-git-send-email-dianzhangchen0@gmail.com>
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On Wed, May 29, 2019 at 08:37:28PM +0800, Dianzhang Chen wrote:
> The `size` in kmalloc_slab() is indirectly controlled by userspace via syscall: poll(defined in fs/select.c), hence leading to a potential exploitation of the Spectre variant 1 vulnerability.
> The `size` can be controlled from: poll -> do_sys_poll -> kmalloc -> __kmalloc -> kmalloc_slab.
> 
> Fix this by sanitizing `size` before using it to index size_index.

I think it makes more sense to sanitize size in size_index_elem(),
don't you?

 static inline unsigned int size_index_elem(unsigned int bytes)
 {
-	return (bytes - 1) / 8;
+	return array_index_nospec((bytes - 1) / 8, ARRAY_SIZE(size_index));
 }

(untested)