s390/qeth: fix NULL deref in qeth_clear_working_pool_list()

In the Linux kernel, the following vulnerability has been resolved:

s390/qeth: fix NULL deref in qeth_clear_working_pool_list()

When qeth_set_online() calls qeth_clear_working_pool_list() to roll

back after an error exit from qeth_hardsetup_card(), we are at risk of

accessing card->qdio.in_q before it was allocated by

qeth_alloc_qdio_queues() via qeth_mpc_initialize().

qeth_clear_working_pool_list() then dereferences NULL, and by writing to

queue->bufs[i].pool_entry scribbles all over the CPU's lowcore.

Resulting in a crash when those lowcore areas are used next (eg. on

the next machine-check interrupt).

Such a scenario would typically happen when the device is first set

online and its queues aren't allocated yet. An early IO error or certain

misconfigs (eg. mismatched transport mode, bad portno) then cause us to

error out from qeth_hardsetup_card() with card->qdio.in_q still being

NULL.

Fix it by checking the pointer for NULL before accessing it.

Note that we also have (rare) paths inside qeth_mpc_initialize() where

a configuration change can cause us to free the existing queues,

expecting that subsequent code will allocate them again. If we then

error out before that re-allocation happens, the same bug occurs.

Root-caused-by: Heiko Carstens <[email protected]>