linux-kernel - Possible race in pc87413

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Message-ID: <da42be1b-f442-6294-a163-84e1b971a651@ispras.ru>
Date:   Mon, 7 Aug 2017 16:22:17 +0300
From:   Anton Volkov <avolkov@...ras.ru>
To:     wim@...ana.be
Cc:     linux@...ck-us.net, linux-watchdog@...r.kernel.org,
        linux-kernel@...r.kernel.org, ldv-project@...uxtesting.org,
        Alexey Khoroshilov <khoroshilov@...ras.ru>
Subject: Possible race in pc87413_wdt.ko

Hello.

While searching for races in the Linux kernel I've come across 
"drivers/watchdog/pc87413_wdt.ko" module. Here is a question that I came 
up with while analyzing results. Lines are given using the info from 
Linux v4.12.

Consider the following case:

Thread 1:                          Thread 2:
pc87413_init
    misc_register(&pc87413_miscdev)
-> pc87413_get_swc_base_addr       pc87413_open
                                    -> pc87413_refresh
                                       -> pc87413_swc_bank3
      swc_base_addr = ...                  <read access to swc_base_addr>
      (pc87413_wdt.c: line 133)            (pc87413_wdt.c: line 146)

So in this case preemptive registration of the device leads to a 
possibility of race between the initialization process and a callback to 
the registered device.

Is this race feasible from your point of view? And if it is, is it 
possible to move the device registration a bit further down in the 
pc87413_init function?

Thank you for your time.

-- Anton Volkov
Linux Verification Center, ISPRAS
web: http://linuxtesting.org
e-mail: avolkov@...ras.ru