Date:   Mon, 14 Sep 2020 07:47:30 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     syzbot <syzbot+d9ae84069cff753e94bf@...kaller.appspotmail.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christian Brauner <christian@...uner.io>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Eric Sandeen <sandeen@...deen.net>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>
Subject: Re: KASAN: unknown-crash Read in do_exit

Dmitry Vyukov <dvyukov@...gle.com> writes:

> On Mon, Sep 14, 2020 at 2:15 PM Eric W. Biederman <ebiederm@...ssion.com> wrote:
>>
>> syzbot <syzbot+d9ae84069cff753e94bf@...kaller.appspotmail.com> writes:
>>
>> > Hello,
>> >
>> > syzbot found the following issue on:
>>
>> Skimming the code it appears this is a feature not a bug.
>>
>> The stack_not_used code deliberately reads the unused/unitiailized
>> portion of the stack, to see if that part of the stack was used.
>>
>> Perhaps someone wants to make this play nice with KASAN?
>>
>> KASAN should be able to provide better information than reading the
>> stack to see if it is still zeroed out.
>>
>> Eric
>
> Hi Eric,
>
> Thanks for looking into this.
>
> There may be something else in play here. Unused parts of the stack
> are supposed to have zero shadow. The stack instrumentation code
> assumes that. If there is some garbage left in the shadow (like these
> "70 07 00 00 77" in this case), then it will lead to very obscure
> false positives later (e.g. some out-of-bounds on stack which can't be
> explained easily).
> If some code does something like "jongjmp", then we should clear the
> stack at the point of longjmp. I think we did something similar for
> something called jprobles, but jprobes were removed at some point.
>
> Oh, wait, the reproducer uses /dev/fb. And as far as I understand
> /dev/fd smashes kernel memory left and right. So most likely it's some
> wild out of bounds write in /dev/fb.

So I am confused.  The output in the console does not match the log
below.  Further the memory addresses in the report don't make a bit
of sense.  Incrementing by 0x80 and only printing 16 bytes which is 0x10.

I am simply responding to the fact that KASAN is complaining about an
out of bounds/uniitialized access in stack_not_used.

Which seems a legitimate thing to do, but that seems to indicate
two debugging primitives are fighting each other.

So why we have several very different traces I don't understand.
Unless you are right and something is causing corruption.

At which point this needs to be delivered to whomever can dig into this.


Eric

>> > HEAD commit:    729e3d09 Merge tag 'ceph-for-5.9-rc5' of git://github.com/..
>> > git tree:       upstream
>> > console output: https://syzkaller.appspot.com/x/log.txt?x=170a7cf1900000
>> > kernel config:  https://syzkaller.appspot.com/x/.config?x=c61610091f4ca8c4
>> > dashboard link: https://syzkaller.appspot.com/bug?extid=d9ae84069cff753e94bf
>> > compiler:       gcc (GCC) 10.1.0-syz 20200507
>> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=10642545900000
>> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=141f2bed900000
>> >
>> > Bisection is inconclusive: the issue happens on the oldest tested release.
>> >
>> > bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=17b9ffcd900000
>> > final oops:     https://syzkaller.appspot.com/x/report.txt?x=1479ffcd900000
>> > console output: https://syzkaller.appspot.com/x/log.txt?x=1079ffcd900000
>> >
>> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
>> > Reported-by: syzbot+d9ae84069cff753e94bf@...kaller.appspotmail.com
>> >
>> > ==================================================================
>> > BUG: KASAN: unknown-crash in stack_not_used include/linux/sched/task_stack.h:101 [inline]
>> > BUG: KASAN: unknown-crash in check_stack_usage kernel/exit.c:692 [inline]
>> > BUG: KASAN: unknown-crash in do_exit+0x24a6/0x29f0 kernel/exit.c:849
>> > Read of size 8 at addr ffffc9000cf30130 by task syz-executor624/10359
>> >
>> > CPU: 1 PID: 10359 Comm: syz-executor624 Not tainted 5.9.0-rc4-syzkaller #0
>> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
>> > Call Trace:
>> >  __dump_stack lib/dump_stack.c:77 [inline]
>> >  dump_stack+0x198/0x1fd lib/dump_stack.c:118
>> >  print_address_description.constprop.0.cold+0x5/0x497 mm/kasan/report.c:383
>> >  __kasan_report mm/kasan/report.c:513 [inline]
>> >  kasan_report.cold+0x1f/0x37 mm/kasan/report.c:530
>> >  stack_not_used include/linux/sched/task_stack.h:101 [inline]
>> >  check_stack_usage kernel/exit.c:692 [inline]
>> >  do_exit+0x24a6/0x29f0 kernel/exit.c:849
>> >  do_group_exit+0x125/0x310 kernel/exit.c:903
>> >  get_signal+0x428/0x1f00 kernel/signal.c:2757
>> >  arch_do_signal+0x82/0x2520 arch/x86/kernel/signal.c:811
>> >  exit_to_user_mode_loop kernel/entry/common.c:159 [inline]
>> >  exit_to_user_mode_prepare+0x1ae/0x200 kernel/entry/common.c:190
>> >  syscall_exit_to_user_mode+0x7e/0x2e0 kernel/entry/common.c:265
>> >  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>> > RIP: 0033:0x446b99
>> > Code: Bad RIP value.
>> > RSP: 002b:00007f70f5ed9d18 EFLAGS: 00000246 ORIG_RAX: 0000000000000038
>> > RAX: 0000000000002878 RBX: 00000000006dbc58 RCX: 0000000000446b99
>> > RDX: 9999999999999999 RSI: 0000000000000000 RDI: 0000020002004ffc
>> > RBP: 00000000006dbc50 R08: ffffffffffffffff R09: 0000000000000000
>> > R10: 0000000000000000 R11: 0000000000000246 R12: 00000000006dbc5c
>> > R13: 00007f70f5ed9d20 R14: 00007f70f5ed9d20 R15: 000000000000002d
>> >
>> >
>> > Memory state around the buggy address:
>> >  ffffc9000cf30000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>> >  ffffc9000cf30080: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>> >>ffffc9000cf30100: 00 00 00 00 00 00 70 07 00 00 77 00 00 00 00 00
>> >                                      ^
>> >  ffffc9000cf30180: 00 00 70 07 00 00 70 07 00 00 00 00 77 00 70 07
>> >  ffffc9000cf30200: 00 70 07 00 77 00 00 00 00 00 70 07 00 00 00 00
>> > ==================================================================
>> >
>> >
>> > ---
>> > This report is generated by a bot. It may contain errors.
>> > See https://goo.gl/tpsmEJ for more information about syzbot.
>> > syzbot engineers can be reached at syzkaller@...glegroups.com.
>> >
>> > syzbot will keep track of this issue. See:
>> > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
>> > For information about bisection process see: https://goo.gl/tpsmEJ#bisection
>> > syzbot can test patches for this issue, for details see:
>> > https://goo.gl/tpsmEJ#testing-patches

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