Crash in dm_done()

[Hannes Reinecke <hare@suse.de>]

Hi all,

I'm trying to hunt down a mysterious crash:

Unable to handle kernel pointer dereference at virtual kernel
address 000003c001762000
Oops: 0011 [#1] SMP
Modules linked in: dm_round_robin sg sd_mod crc_t10dif ipv6 loop
dm_multipath scsi_dh dm_mod qeth_l3 ipv6_lib zfcp scsi_transport_fc
scsi_tgt scsi_mod dasd_eckd_mod dasd_mod qeth qdio ccwgroup ext3
mbcache jbd
Supported: Yes
CPU: 0 Not tainted 3.0.13-0.5-default #1
Process kworker/0:1 (pid: 51750, task: 0000000008450138, ksp:
0000000007323620)
Krnl PSW : 0704200180000000 000003c001953750 (dm_done+0x44/0x188
[dm_mod])
           R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:0 CC:2 PM:0 EA:3
Krnl GPRS: 000000002adb5e80 000003c001762100 0000000037de9e80
0000000000000000
           0000000000000400 000003c00195e178 0000000000000380
0000000000000000
           0000000000000100 0000000000000001 0000000037de9e80
0000000037de9e68
           000003c00194f000 000003c00195e168 000000007ef97d30
000000007ef97cd8
Krnl Code: 000003c001953740: e3b021580004       lg     %r11,344(%r2)
           000003c001953746: e310b0080004       lg     %r1,8(%r11)
           000003c00195374c: bf4fb180           icm %r4,15,384(%r11)
          >000003c001953750: e32010080004       lg      %r2,8(%r1)
           000003c001953756: e38020500004       lg      %r8,80(%r2)
           000003c00195375c: a7740062           brc    7,3c001953820
           000003c001953760: b9020099           ltgr    %r9,%r9
           000003c001953764: a7840049           brc    8,3c0019537f6

r11 is struct dm_rq_target_io *tio = clone->end_io_data;
r1 is tio->ti (ie struct dm_target), which is invalid.
r2 is tio->ti->type, likewise.

Apparently the table got replaced between map_io() and dm_done(),
causing this invalid pointer.
While we do hold a reference on the mapped_device in map_request(),
we only take a _single_ reference to the table in dm_request_fn(),
which is dropped again at the end.
And as the table holds the pointer to the targets, they'll be
invalidated upon table swapping, causing dm_done() accessing an
invalid pointer.

I can't really believe that is the case, so please do correct me if
the above analysis is wrong.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)