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

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Date:   Fri, 7 Sep 2018 12:57:41 -0700
From:   Kees Cook <keescook@...gle.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     syzbot <syzbot+a3c9d2673837ccc0f22b@...kaller.appspotmail.com>,
        Chris von Recklinghausen <crecklin@...hat.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>, Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>
Subject: Re: BUG: bad usercopy in __check_object_size (2)

On Fri, Sep 7, 2018 at 9:17 AM, Tetsuo Handa
<penguin-kernel@...ove.sakura.ne.jp> wrote:
> On 2018/09/08 0:29, syzbot wrote:
>> syzbot has found a reproducer for the following crash on:
>>
>> HEAD commit:    28619527b8a7 Merge git://git.kernel.org/pub/scm/linux/kern..
>> git tree:       bpf
>> console output: https://syzkaller.appspot.com/x/log.txt?x=124e64d1400000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=62e9b447c16085cf
>> dashboard link: https://syzkaller.appspot.com/bug?extid=a3c9d2673837ccc0f22b
>> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=179f9cd1400000
>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=11b3e8be400000
>>
>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>> Reported-by: syzbot+a3c9d2673837ccc0f22b@...kaller.appspotmail.com
>>
>>  entry_SYSCALL_64_after_hwframe+0x49/0xbe
>> RIP: 0033:0x440479
>> usercopy: Kernel memory overwrite attempt detected to spans multiple pages (offset 0, size 64)!
>
> Kees, is this because check_page_span() is failing to allow on-stack variable
>
>    u8 opcodes[OPCODE_BUFSIZE];
>
> which by chance crossed PAGE_SIZE boundary?

There are a lot of failure conditions for the PAGESPAN check. This
might be one (and one that I'm hoping to solve separately).

-Kees

-- 
Kees Cook
Pixel Security