fs: GPF in locked_inode_to_wb_and_lock_list

[Dmitry Vyukov]

Hello,

I've hit the following GPF while running syzkaller fuzzer on commit
806fdcce017dc98c4dbf8ed001750a0d7d2bb0af (Apr 14):

general protection fault: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN
Modules linked in:
CPU: 3 PID: 32 Comm: kworker/u10:1 Not tainted 4.6.0-rc3+ #349
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
Workqueue: writeback wb_workfn (flush-11:0)
task: ffff88006ccf1840 ti: ffff88006cda8000 task.ti: ffff88006cda8000
RIP: 0010:[<ffffffff818884d2>]  [<ffffffff818884d2>]
locked_inode_to_wb_and_lock_list+0xa2/0x750
RSP: 0018:ffff88006cdaf7d0  EFLAGS: 00010246
RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff88006ccf2050
RDX: 0000000000000000 RSI: 000000114c8a8484 RDI: 0000000000000286
RBP: ffff88006cdaf820 R08: ffff88006ccf1840 R09: 0000000000000000
R10: 000229915090805f R11: 0000000000000001 R12: ffff88006a72f5e0
R13: dffffc0000000000 R14: ffffed000d4e5eed R15: ffffffff8830cf40
FS:  0000000000000000(0000) GS:ffff88006d500000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000003301bf8 CR3: 000000006368f000 CR4: 00000000000006e0
DR0: 0000000000001ec9 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000600
Stack:
 ffff88006a72f680 ffff88006a72f768 ffff8800671230d8 03ff88006cdaf948
 ffff88006a72f668 ffff88006a72f5e0 ffff8800671230d8 ffff88006cdaf948
 ffff880065b90cc8 ffff880067123100 ffff88006cdaf970 ffffffff8188e12e
Call Trace:
 [<     inline     >] inode_to_wb_and_lock_list fs/fs-writeback.c:309
 [<ffffffff8188e12e>] writeback_sb_inodes+0x4de/0x1250 fs/fs-writeback.c:1554
 [<ffffffff8188efa4>] __writeback_inodes_wb+0x104/0x1e0 fs/fs-writeback.c:1600
 [<ffffffff8188f9ae>] wb_writeback+0x7ce/0xc90 fs/fs-writeback.c:1709
 [<     inline     >] wb_do_writeback fs/fs-writeback.c:1844
 [<ffffffff81891079>] wb_workfn+0x2f9/0x1000 fs/fs-writeback.c:1884
 [<ffffffff813bcd1e>] process_one_work+0x78e/0x15c0 kernel/workqueue.c:2094
 [<ffffffff813bdc2b>] worker_thread+0xdb/0xfc0 kernel/workqueue.c:2228
 [<ffffffff813cdeef>] kthread+0x23f/0x2d0 drivers/block/aoe/aoecmd.c:1303
 [<ffffffff867bc5d2>] ret_from_fork+0x22/0x50 arch/x86/entry/entry_64.S:392
Code: 05 94 4a a8 06 85 c0 0f 85 03 03 00 00 e8 07 15 d0 ff 41 80 3e
00 0f 85 64 06 00 00 49 8b 9c 24 88 01 00 00 48 89 d8 48 c1 e8 03 <42>
80 3c 28 00 0f 85 17 06 00 00 48 8b 03 48 83 c0 50 48 39 c3
RIP  [<     inline     >] wb_get include/linux/backing-dev-defs.h:212
RIP  [<ffffffff818884d2>] locked_inode_to_wb_and_lock_list+0xa2/0x750
fs/fs-writeback.c:281
 RSP <ffff88006cdaf7d0>
---[ end trace 986a4d314dcb2694 ]---


The crash happened here:

        if (wb != &wb->bdi->wb)
ffffffff818884cb:       48 89 d8                mov    %rbx,%rax
ffffffff818884ce:       48 c1 e8 03             shr    $0x3,%rax
ffffffff818884d2:       42 80 3c 28 00          cmpb   $0x0,(%rax,%r13,1)
ffffffff818884d7:       0f 85 17 06 00 00       jne
ffffffff81888af4 <locked_inode_to_wb_and_lock_list+0x6c4>
ffffffff818884dd:       48 8b 03                mov    (%rbx),%rax
ffffffff818884e0:       48 83 c0 50             add    $0x50,%rax
ffffffff818884e4:       48 39 c3                cmp    %rax,%rbx
ffffffff818884e7:       0f 84 c3 00 00 00       je
ffffffff818885b0 <locked_inode_to_wb_and_lock_list+0x180>

Which means that bdi is NULL (if I get indirections right).

The block device is flush-11:0 (/dev/sr0).

I was able to reproduce it once by re-running programs from the crash log:
https://gist.githubusercontent.com/dvyukov/7c9e6358b661e2d50a774a70b5ee348a/raw/5cd94e32bbcf2a788b5efd0768cfd883c5921070/gistfile1.txt
But it took half an hour, and then another hour long run did not
trigger it. So it is probably triggered by some rare race.

Thank you