[PATCH 5.7 020/124] scsi: zfcp: Fix use-after-free in request timeout handlers

Greg Kroah-Hartman
2020-08-24T08:39:31Z

From: Steffen Maier <maier@linux.ibm.com>

commit 2d9a2c5f581be3991ba67fa9e7497c711220ea8e upstream.

Before v4.15 commit 75492a51568b ("s390/scsi: Convert timers to use
timer_setup()"), we intentionally only passed zfcp_adapter as context
argument to zfcp_fsf_request_timeout_handler(). Since we only trigger
adapter recovery, it was unnecessary to sync against races between timeout
and (late) completion. Likewise, we only passed zfcp_erp_action as context
argument to zfcp_erp_timeout_handler(). Since we only wakeup an ERP action,
it was unnecessary to sync against races between timeout and (late)
completion.

Meanwhile the timeout handlers get timer_list as context argument and do a
timer-specific container-of to zfcp_fsf_req which can have been freed.

Fix it by making sure that any request timeout handlers, that might just
have started before del_timer(), are completed by using del_timer_sync()
instead. This ensures the request free happens afterwards.

Space time diagram of potential use-after-free:

Basic idea is to have 2 or more pending requests whose timeouts run out at
almost the same time.

req 1 timeout ERP thread req 2 timeout
---------------- ---------------- ---------------------------------------
zfcp_fsf_request_timeout_handler
fsf_req = from_timer(fsf_req, t, timer)
adapter = fsf_req->adapter
zfcp_qdio_siosl(adapter)
zfcp_erp_adapter_reopen(adapter,...)
zfcp_erp_strategy
...
zfcp_fsf_req_dismiss_all
list_for_each_entry_safe
zfcp_fsf_req_complete 1
del_timer 1
zfcp_fsf_req_free 1
zfcp_fsf_req_complete 2
zfcp_fsf_request_timeout_handler
del_timer 2
fsf_req = from_timer(fsf_req, t, timer)
zfcp_fsf_req_free 2
adapter = fsf_req->adapter
^^^^^^^ already freed

Link: https://lore.kernel.org/r/20200813152856.50088-1-maier@linux.ibm.com
Fixes: 75492a51568b ("s390/scsi: Convert timers to use timer_setup()")
Cc: <stable@vger.kernel.org> #4.15+
Suggested-by: Julian Wiedmann <jwi@linux.ibm.com>
Reviewed-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Steffen Maier <maier@linux.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/s390/scsi/zfcp_fsf.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

--- a/drivers/s390/scsi/zfcp_fsf.c
+++ b/drivers/s390/scsi/zfcp_fsf.c
@@ -430,7 +430,7 @@ static void zfcp_fsf_req_complete(struct
return;
}

- del_timer(&req->timer);
+ del_timer_sync(&req->timer);
zfcp_fsf_protstatus_eval(req);
zfcp_fsf_fsfstatus_eval(req);
req->handler(req);
@@ -905,7 +905,7 @@ static int zfcp_fsf_req_send(struct zfcp
req->qdio_req.qdio_outb_usage = atomic_read(&qdio->req_q_free);
req->issued = get_tod_clock();
if (zfcp_qdio_send(qdio, &req->qdio_req)) {
- del_timer(&req->timer);
+ del_timer_sync(&req->timer);
/* lookup request again, list might have changed */
zfcp_reqlist_find_rm(adapter->req_list, req_id);
zfcp_erp_adapter_reopen(adapter, 0, "fsrs__1");