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

[NeilBrown]

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@xxxxxxxxxxxxxxxxxxxxxxxxx
>> 
>> 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@xxxxxxxx>
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@xxxxxxxxxxxxxxxxxxxxxxxxx

Signed-off-by: NeilBrown <neilb@xxxxxxxx>
---
 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;
 }
-- 
2.14.0.rc0.dirty

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