linux-kernel - Re: BUG: bad usercopy in memdup

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Date:   Tue, 19 Dec 2017 15:12:05 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Matthew Wilcox <willy@...radead.org>,
        "Tobin C. Harding" <me@...in.cc>,
        Kees Cook <keescook@...omium.org>,
        Linux-MM <linux-mm@...ck.org>,
        syzbot 
        <bot+719398b443fd30155f92f2a888e749026c62b427@...kaller.appspotmail.com>,
        David Windsor <dave@...lcore.net>, keun-o.park@...kmatter.ae,
        Laura Abbott <labbott@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        Ingo Molnar <mingo@...nel.org>,
        syzkaller-bugs@...glegroups.com, Will Deacon <will.deacon@....com>
Subject: Re: BUG: bad usercopy in memdup_user

On Tue, Dec 19, 2017 at 3:08 PM, Tetsuo Handa
<penguin-kernel@...ove.sakura.ne.jp> wrote:
> Dmitry Vyukov wrote:
>> On Tue, Dec 19, 2017 at 2:22 PM, Matthew Wilcox <willy@...radead.org> wrote:
>> >> > >> This BUG is reporting
>> >> > >>
>> >> > >> [   26.089789] usercopy: kernel memory overwrite attempt detected to 0000000022a5b430 (kmalloc-1024) (1024 bytes)
>> >> > >>
>> >> > >> line. But isn't 0000000022a5b430 strange for kmalloc(1024, GFP_KERNEL)ed kernel address?
>> >> > >
>> >> > > The address is hashed (see the %p threads for 4.15).
>> >> >
>> >> >
>> >> > +Tobin, is there a way to disable hashing entirely? The only
>> >> > designation of syzbot is providing crash reports to kernel developers
>> >> > with as much info as possible. It's fine for it to leak whatever.
>> >>
>> >> We have new specifier %px to print addresses in hex if leaking info is
>> >> not a worry.
>> >
>> > Could we have a way to know that the printed address is hashed and not just
>> > a pointer getting completely scrogged?  Perhaps prefix it with ... a hash!
>> > So this line would look like:
>> >
>> > [   26.089789] usercopy: kernel memory overwrite attempt detected to #0000000022a5b430 (kmalloc-1024) (1024 bytes)
>> >
>> > Or does that miss the point of hashing the address, so the attacker
>> > thinks its a real address?
>>
>> If we do something with this, I would suggest that we just disable
>> hashing. Any of the concerns that lead to hashed pointers are not
>> applicable in this context, moreover they are harmful, cause confusion
>> and make it harder to debug these bugs. That perfectly can be an
>> opt-in CONFIG_DEBUG_INSECURE_BLA_BLA_BLA.
>>
> Why not a kernel command line option? Hashing by default.


Would work for continuous testing systems too.
I just thought that since it has security implications, a config would
be more reliable. Say if a particular distribution builds kernel
without this config, then there is no way to enable it on the fly,
intentionally or not.