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

From: NeilBrown
Date: Wed Jan 02 2019 - 20:59:51 EST

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On Wed, Jan 02 2019, Jeff Layton wrote:

> On Thu, 2019-01-03 at 11:04 +1100, NeilBrown wrote:
>> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
>> > >
>> > > 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@xxxxxxxx>
>> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
>> Fixes: 5946c4319ebb ("fs/locks: allow a lock request to block other requests.")
>> 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 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);
>> }
>
> Thanks Neil,
>
> The VM I was using seemed to reproduce this pretty readily, and the
> patch does seem to fix it. I've gone ahead and added it to -next for
> now, and will plan to pass it on to Linus in the next few days after
> I've tested it a bit more.

Thanks, that's encouraging.

NeilBrown

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In reply to: Jeff Layton: "Re: KASAN: use-after-free Read in posix_lock_inode"
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