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

[NeilBrown <neilb@suse.com>]

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On Wed, Jan 02 2019, Jeff Layton wrote:

> On Wed, 2019-01-02 at 02:31 -0800, syzbot wrote:
>> Hello,
>> 
>> syzbot found the following crash on:
>> 
>> HEAD commit:    e1ef035d272e Merge tag 'armsoc-defconfig' of git://git.ker..
>> git tree:       upstream
>> console output: https://syzkaller.appspot.com/x/log.txt?x=16bb4c4b400000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=9c6a26e22579190b
>> dashboard link: https://syzkaller.appspot.com/bug?extid=239d99847eb49ecb3899
>> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=128aa377400000
>> 
>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>> Reported-by: syzbot+239d99847eb49ecb3899@syzkaller.appspotmail.com
>> 
>> IPv6: ADDRCONF(NETDEV_UP): vxcan1: link is not ready
>> IPv6: ADDRCONF(NETDEV_UP): vxcan1: link is not ready
>> 8021q: adding VLAN 0 to HW filter on device batadv0
>> 8021q: adding VLAN 0 to HW filter on device batadv0
>> ==================================================================
>> BUG: KASAN: use-after-free in what_owner_is_waiting_for fs/locks.c:1000  
>> [inline]
>> BUG: KASAN: use-after-free in posix_locks_deadlock fs/locks.c:1023 [inline]
>> BUG: KASAN: use-after-free in posix_lock_inode+0x1f9e/0x2750 fs/locks.c:1163
>> Read of size 8 at addr ffff88808791b000 by task syz-executor2/10100
>> 
>> CPU: 1 PID: 10100 Comm: syz-executor2 Not tainted 4.20.0+ #3
>> 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+0x1db/0x2d0 lib/dump_stack.c:113
>>   print_address_description.cold+0x7c/0x20d mm/kasan/report.c:187
>>   kasan_report.cold+0x1b/0x40 mm/kasan/report.c:317
>>   __asan_report_load8_noabort+0x14/0x20 mm/kasan/generic_report.c:135
>>   what_owner_is_waiting_for fs/locks.c:1000 [inline]
>>   posix_locks_deadlock fs/locks.c:1023 [inline]
>>   posix_lock_inode+0x1f9e/0x2750 fs/locks.c:1163
>>   posix_lock_file fs/locks.c:1346 [inline]
>>   vfs_lock_file fs/locks.c:2314 [inline]
>>   vfs_lock_file+0xc7/0xf0 fs/locks.c:2309
>>   do_lock_file_wait.part.0+0xe5/0x260 fs/locks.c:2328
>>   do_lock_file_wait fs/locks.c:2324 [inline]
>>   fcntl_setlk+0x2f1/0xfe0 fs/locks.c:2413
>>   do_fcntl+0x843/0x12b0 fs/fcntl.c:370
>>   __do_sys_fcntl fs/fcntl.c:463 [inline]
>>   __se_sys_fcntl fs/fcntl.c:448 [inline]
>>   __x64_sys_fcntl+0x16d/0x1e0 fs/fcntl.c:448
>>   do_syscall_64+0x1a3/0x800 arch/x86/entry/common.c:290
>>   entry_SYSCALL_64_after_hwframe+0x49/0xbe
>> RIP: 0033:0x457ec9
>> Code: 6d b7 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 3b b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00
>> RSP: 002b:00007f58bbb50c78 EFLAGS: 00000246 ORIG_RAX: 0000000000000048
>> RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000000457ec9
>> RDX: 0000000020000140 RSI: 0000000000000007 RDI: 0000000000000003
>> RBP: 000000000073bf00 R08: 0000000000000000 R09: 0000000000000000
>> R10: 0000000000000000 R11: 0000000000000246 R12: 00007f58bbb516d4
>> R13: 00000000004be5f0 R14: 00000000004ceab0 R15: 00000000ffffffff
>> 
>> Allocated by task 10100:
>>   save_stack+0x45/0xd0 mm/kasan/common.c:73
>>   set_track mm/kasan/common.c:85 [inline]
>>   kasan_kmalloc mm/kasan/common.c:482 [inline]
>>   kasan_kmalloc+0xcf/0xe0 mm/kasan/common.c:455
>>   kasan_slab_alloc+0xf/0x20 mm/kasan/common.c:397
>>   kmem_cache_alloc+0x12d/0x710 mm/slab.c:3541
>>   kmem_cache_zalloc include/linux/slab.h:730 [inline]
>>   locks_alloc_lock+0x8e/0x2f0 fs/locks.c:344
>>   fcntl_setlk+0xa9/0xfe0 fs/locks.c:2362
>>   do_fcntl+0x843/0x12b0 fs/fcntl.c:370
>>   __do_sys_fcntl fs/fcntl.c:463 [inline]
>>   __se_sys_fcntl fs/fcntl.c:448 [inline]
>>   __x64_sys_fcntl+0x16d/0x1e0 fs/fcntl.c:448
>>   do_syscall_64+0x1a3/0x800 arch/x86/entry/common.c:290
>>   entry_SYSCALL_64_after_hwframe+0x49/0xbe
>> 
>> Freed by task 10100:
>>   save_stack+0x45/0xd0 mm/kasan/common.c:73
>>   set_track mm/kasan/common.c:85 [inline]
>>   __kasan_slab_free+0x102/0x150 mm/kasan/common.c:444
>>   kasan_slab_free+0xe/0x10 mm/kasan/common.c:452
>>   __cache_free mm/slab.c:3485 [inline]
>>   kmem_cache_free+0x86/0x260 mm/slab.c:3747
>>   locks_free_lock+0x27a/0x3f0 fs/locks.c:381
>>   fcntl_setlk+0x7b5/0xfe0 fs/locks.c:2439
>>   do_fcntl+0x843/0x12b0 fs/fcntl.c:370
>>   __do_sys_fcntl fs/fcntl.c:463 [inline]
>>   __se_sys_fcntl fs/fcntl.c:448 [inline]
>>   __x64_sys_fcntl+0x16d/0x1e0 fs/fcntl.c:448
>>   do_syscall_64+0x1a3/0x800 arch/x86/entry/common.c:290
>>   entry_SYSCALL_64_after_hwframe+0x49/0xbe
>> 
>> The buggy address belongs to the object at ffff88808791b000
>>   which belongs to the cache file_lock_cache of size 264
>> The buggy address is located 0 bytes inside of
>>   264-byte region [ffff88808791b000, ffff88808791b108)
>> The buggy address belongs to the page:
>> page:ffffea00021e46c0 count:1 mapcount:0 mapping:ffff8880aa16a1c0 index:0x0
>> flags: 0x1fffc0000000200(slab)
>> raw: 01fffc0000000200 ffffea0002333508 ffffea00021d76c8 ffff8880aa16a1c0
>> raw: 0000000000000000 ffff88808791b000 000000010000000c 0000000000000000
>> page dumped because: kasan: bad access detected
>> 
>> Memory state around the buggy address:
>>   ffff88808791af00: fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc fc
>>   ffff88808791af80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>> > ffff88808791b000: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>>                     ^
>>   ffff88808791b080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>>   ffff88808791b100: fb fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb
>> ==================================================================
>> 
>> 
>> 
>
> The interesting bit is that the crash, alloc and free all seem to have
> occurred in the same kernel task (PID 10100).
>
> Here's the loop in what_owner_is_waiting_for():
>
> ----------------8<------------------
>         hash_for_each_possible(blocked_hash, fl, fl_link, posix_owner_key(block_fl)) {
>                 if (posix_same_owner(fl, block_fl)) {                           
>                         while (fl->fl_blocker)            <<<<<< CRASH HERE                           
>                                 fl = fl->fl_blocker;                            
>                         return fl;                                              
>                 }                                                               
>         }        
> ----------------8<------------------
>
> So fl got freed while we were walking down the chain of blocked locks.
> At a quick glance, I'm now wondering whether the lockless optimization
> to avoid the blocked_lock_lock in locks_delete_block is actually ok.
>
> Neil, any thoughts?

The repro didn't trigger for me, but code inspection suggested I should
invest in brown paper bags.

Note that while this patch clearly fixes a bug, and I suspect it is
the cause of this report, I cannot confirm with testing as I cannot
trigger the bug.  I enabled KASAN and ran "qemu -smp 4" to no avail.

Thanks,
NeilBrown

From: NeilBrown <neilb@suse.com>
Date: Thu, 3 Jan 2019 10:59:50 +1100
Subject: [PATCH] locks: fix error in locks_move_blocks()

After moving all requests from
   fl->fl_blocked_requests
to
   new->fl_blocked_requests

it is nonsensical to do anything to all the remaining elements, there
aren't any.
This should do something to all the requests that have been
moved - for simplicity, it does it to all requests in the target
list.
Setting "f->fl_blocker = new" to all members of
new->fl_blocked_requests is "obviously correct" as it preserves
the invariant of the linkage among requests.

Reported-by: syzbot+239d99847eb49ecb3899@syzkaller.appspotmail.com
Fixes: 5946c4319ebb ("fs/locks: allow a lock request to block other requests.")
Signed-off-by: NeilBrown <neilb@suse.com>
---
 fs/locks.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/locks.c b/fs/locks.c
index f0b24d98f36b..ff6af2c32601 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -453,7 +453,7 @@ static void locks_move_blocks(struct file_lock *new, struct file_lock *fl)
 		return;
 	spin_lock(&blocked_lock_lock);
 	list_splice_init(&fl->fl_blocked_requests, &new->fl_blocked_requests);
-	list_for_each_entry(f, &fl->fl_blocked_requests, fl_blocked_member)
+	list_for_each_entry(f, &new->fl_blocked_requests, fl_blocked_member)
 		f->fl_blocker = new;
 	spin_unlock(&blocked_lock_lock);
 }
-- 
2.14.0.rc0.dirty


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