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Date:   Mon, 16 Jan 2017 15:56:06 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     Neil Horman <nhorman@...driver.com>,
        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

On Mon, Jan 16, 2017 at 3:50 PM, David Laight <David.Laight@...lab.com> wrote:
> 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??


Just in case, I don't have CONFIG_VMAP_STACK selected.
If it is a generic issue, then CONFIG_HARDENED_USERCOPY_PAGESPAN looks
considerably broken as there are tons of copies onto stack. I don't
see what's special in this particular case.

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