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Date:   Mon, 16 Jan 2017 14:50:48 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Dmitry Vyukov' <dvyukov@...gle.com>,
        Neil Horman <nhorman@...driver.com>
CC:     Vladislav Yasevich <vyasevich@...il.com>,
        David Miller <davem@...emloft.net>,
        "linux-sctp@...r.kernel.org" <linux-sctp@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...gle.com>,
        syzkaller <syzkaller@...glegroups.com>
Subject: RE: sctp: kernel memory overwrite attempt detected in
 sctp_getsockopt_assoc_stats

From: Dmitry Vyukov
> Sent: 16 January 2017 14:04
> >> >> I've enabled CONFIG_HARDENED_USERCOPY_PAGESPAN on syzkaller fuzzer and
...
> >> The code also takes into account compound pages. As far as I
> >> understand the intention of the check is to effectively find
> >> out-of-bounds copies (e.g. goes beyond the current heap allocation). I
> >> would expect that stacks are allocated as compound pages and don't
> >> trigger this check. I don't see it is firing in other similar places.
> >>
> > Honestly, I'm not overly familiar with stack page allocation, at least not so
> > far as compound vs. single page allocation is concerned.  I suppose the question
> > your really asking here is: Have you found a case in which the syscall fuzzer
> > has forced the allocation of an insecure non-compound page on the stack, or is
> > this a false positive warning.  I can't provide the answer to that.
> 
> Yes. I added Kees, author of CONFIG_HARDENED_USERCOPY_PAGESPAN, to To line.
> Kees, is this a false positive?

I'd guess that the kernel stack is (somehow) allocated page by page
rather than by a single multi-page allocate.
Or maybe vmalloc() isn't setting the required flag??

	David

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