Re: KASAN: use-after-free Read in locks_delete_block

[Jeff Layton <jlayton@kernel.org>]

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@syzkaller.appspotmail.com
> 
> 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?
-- 
Jeff Layton <jlayton@kernel.org>