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

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Date:   Tue, 19 Dec 2017 10:07:58 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     "Tobin C. Harding" <me@...in.cc>
Cc:     Kees Cook <keescook@...omium.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        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 10:04 AM, Tobin C. Harding <me@...in.cc> wrote:
>> >> > <penguin-kernel@...ove.sakura.ne.jp> wrote:
>> >> >> On 2017/12/18 22:40, syzbot wrote:
>> >> >>> Hello,
>> >> >>>
>> >> >>> syzkaller hit the following crash on 6084b576dca2e898f5c101baef151f7bfdbb606d
>> >> >>> git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/master
>> >> >>> compiler: gcc (GCC) 7.1.1 20170620
>> >> >>> .config is attached
>> >> >>> Raw console output is attached.
>> >> >>>
>> >> >>> Unfortunately, I don't have any reproducer for this bug yet.
>> >> >>>
>> >> >>>
>> >> >>
>> >> >> 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.
>>
>> This is not a per-printf-site thing. It's per-machine thing.
>
> There is no way to disable the hashing currently built into the system.

Ack.
Any kind of continuous testing systems would be a use case for this.