Message-ID: <87bm6pewnm.fsf@notabene.neil.brown.name>
Date:   Fri, 16 Nov 2018 10:41:33 +1100
From:   NeilBrown <neilb@...e.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>,
        Jeff Layton <jlayton@...nel.org>
Cc:     syzbot <syzbot+a4a3d526b4157113ec6a@...kaller.appspotmail.com>,
        Bruce Fields <bfields@...ldses.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs@...glegroups.com, Al Viro <viro@...iv.linux.org.uk>
Subject: Re: KASAN: use-after-free Read in locks_delete_block

On Thu, Nov 15 2018, Dmitry Vyukov wrote:

> On Wed, Nov 14, 2018 at 2:36 AM, Jeff Layton <jlayton@...nel.org> wrote:
>> On Wed, 2018-11-14 at 07:40 +1100, NeilBrown wrote:
>>> On Tue, Nov 13 2018, Jeff Layton wrote:
>>>
>>> > On Mon, 2018-11-12 at 12:34 -0800, syzbot wrote:
>>> > > Hello,
>>> > >
>>> > > syzbot found the following crash on:
>>> > >
>>> > > HEAD commit:    442b8cea2477 Add linux-next specific files for 20181109
>>> > > git tree:       linux-next
>>> > > console output: https://syzkaller.appspot.com/x/log.txt?x=115dbad5400000
>>> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=2f72bdb11df9fbe8
>>> > > dashboard link: https://syzkaller.appspot.com/bug?extid=a4a3d526b4157113ec6a
>>> > > compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
>>> > >
>>> > > Unfortunately, I don't have any reproducer for this crash yet.
>>> > >
>>> > > IMPORTANT: if you fix the bug, please add the following tag to the commit:
>>> > > Reported-by: syzbot+a4a3d526b4157113ec6a@...kaller.appspotmail.com
>
> /\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\
>
> Hi Neil,
>
> Please include the Reported-by tag next time.

I did, as you can see below.

When the fix is merged into the patch that introduced the bug, do you
still want the Reported-by there, even though the bug and the fix are no
longer visible?  What if I were to completely rewrite the patch - do I
still need the Reported-by??

I'm certainly happy to give credit where due, but keeping a complete
history of past bugs in a single commit seems excessive.
Please help me to understand your needs.

>
> I see the linux-next patch is already update,
> so let's tell syzbot that this is fixed here:
>
> #syz fix: fs/locks: always delete_block after waiting.
>
> If the bug is still open on syzbot dashboard:
> https://syzkaller.appspot.com#upstream
> syzbot will not report new bugs in these functions in future.
>
> Thanks
>
>>> > > device loop0 blocksize: 4096
>>> > > __find_get_block_slow() failed. block=1, b_blocknr=8
>>> > > b_state=0x00000029, b_size=512
>>> > > device loop0 blocksize: 4096
>>> > > ==================================================================
>>> > > BUG: KASAN: use-after-free in __list_del_entry_valid+0xf1/0x100
>>> > > lib/list_debug.c:51
>>> > > Read of size 8 at addr ffff88017eb47b70 by task syz-executor3/13461
>>> > >
>>> > > CPU: 0 PID: 13461 Comm: syz-executor3 Not tainted 4.20.0-rc1-next-20181109+
>>> > > #110
>>> > > 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+0x244/0x39d lib/dump_stack.c:113
>>> > >   print_address_description.cold.7+0x9/0x1ff mm/kasan/report.c:256
>>> > >   kasan_report_error mm/kasan/report.c:354 [inline]
>>> > >   kasan_report.cold.8+0x242/0x309 mm/kasan/report.c:412
>>> > >   __asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:433
>>> > >   __list_del_entry_valid+0xf1/0x100 lib/list_debug.c:51
>>> > >   __list_del_entry include/linux/list.h:117 [inline]
>>> > >   list_del_init include/linux/list.h:159 [inline]
>>> > >   __locks_delete_block fs/locks.c:683 [inline]
>>> > >   locks_delete_block+0xce/0x3d0 fs/locks.c:716
>>> > >   locks_mandatory_area+0x48b/0x6a0 fs/locks.c:1398
>>> > >   rw_verify_area+0x2f2/0x360 fs/read_write.c:386
>>> > >   vfs_write+0x149/0x560 fs/read_write.c:544
>>> > >   ksys_write+0x101/0x260 fs/read_write.c:598
>>> > >   __do_sys_write fs/read_write.c:610 [inline]
>>> > >   __se_sys_write fs/read_write.c:607 [inline]
>>> > >   __x64_sys_write+0x73/0xb0 fs/read_write.c:607
>>> > >   do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
>>> > >   entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>> > > RIP: 0033:0x457569
>>> > > Code: fd b3 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7
>>> > > 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff
>>> > > ff 0f 83 cb b3 fb ff c3 66 2e 0f 1f 84 00 00 00 00
>>> > > RSP: 002b:00007ff2e8194c78 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
>>> > > RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000000457569
>>> > > RDX: 0000000000000010 RSI: 0000000020000180 RDI: 0000000000000006
>>> > > RBP: 000000000072c0e0 R08: 0000000000000000 R09: 0000000000000000
>>> > > R10: 0000000000000000 R11: 0000000000000246 R12: 00007ff2e81956d4
>>> > > R13: 00000000004c571f R14: 00000000004d9360 R15: 00000000ffffffff
>>> > >
>>> > > The buggy address belongs to the page:
>>> > > page:ffffea0005fad1c0 count:0 mapcount:0 mapping:0000000000000000 index:0x0
>>> > > flags: 0x2fffc0000000000()
>>> > > raw: 02fffc0000000000 0000000000000000 ffffea0005fad1c8 0000000000000000
>>> > > raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
>>> > > page dumped because: kasan: bad access detected
>>> > >
>>> > > Memory state around the buggy address:
>>> > >   ffff88017eb47a00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>>> > >   ffff88017eb47a80: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>>> > > > ffff88017eb47b00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>>> > >
>>> > >                                                               ^
>>> > >   ffff88017eb47b80: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>>> > >   ffff88017eb47c00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>>> > > ==================================================================
>>> > >
>>> >
>>> > Ouch, crash down in the mandatory locking code. This is with Neil's set
>>> > from last week. I haven't merged the series he sent the other day yet,
>>> > but they don't seem to be different in this regard.
>>> >
>>> > Looks like the fl_blocked list might have had an entry on it that was
>>> > freed without being removed? locks_mandatory_area declares a file_lock
>>> > on the stack, but it seems to be initialized properly.
>>> >
>>> > The one weird thing is that locks_mandatory_area sets FL_ACCESS and
>>> > FL_SLEEP, but I don't see anything wrong with that right offhand.
>>> >
>>> > Neil, any thoughts?
>>>
>>> I'm not certain, but probably this:
>>>
>>> From: NeilBrown <neilb@...e.com>
>>> Date: Wed, 14 Nov 2018 07:38:05 +1100
>>> Subject: [PATCH] fs/locks: always delete_block after waiting - mandatory locks
>>>
>>> The patch
>>>       fs/locks: always delete_block after waiting.
>>> should have moved the locks_delete_block() call in
>>> locks_mandatory_area() too.
>>>
>>> This might fix the bug:
>>> Reported-by: syzbot+a4a3d526b4157113ec6a@...kaller.appspotmail.com

Here you see the Reported-by line that I included.

NeilBrown

>>>
>>> Signed-off-by: NeilBrown <neilb@...e.com>
>>> ---
>>>  fs/locks.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/fs/locks.c b/fs/locks.c
>>> index f456cd3d9d50..eb0c0b33fb7b 100644
>>> --- a/fs/locks.c
>>> +++ b/fs/locks.c
>>> @@ -1436,9 +1436,9 @@ int locks_mandatory_area(struct inode *inode, struct file *filp, loff_t start,
>>>                               continue;
>>>               }
>>>
>>> -             locks_delete_block(&fl);
>>>               break;
>>>       }
>>> +     locks_delete_block(&fl);
>>>
>>>       return error;
>>>  }
>>
>> That makes sense. I went ahead and squashed this patch into the earlier
>> one and pushed the result to my locks-next branch. linux-next should
>> pick it up soon.
>>
>> Thanks!
>> --
>> Jeff Layton <jlayton@...nel.org>
>>
>> --
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