linux-kernel - Possible race in c4.ko

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Date:   Tue, 15 Aug 2017 16:22:16 +0300
From:   Anton Volkov <avolkov@...ras.ru>
To:     isdn@...ux-pingi.de, davem@...emloft.net, calle@...le.de
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        ldv-project@...uxtesting.org,
        Alexey Khoroshilov <khoroshilov@...ras.ru>
Subject: Possible race in c4.ko

Hello.

While searching for races in the Linux kernel I've come across 
"drivers/isdn/hardware/avm/c4.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:
c4_probe
->c4_add_card
     request_irq()
                            c4_interrupt
                            ->c4_handle_interrupt
                              ->c4_handle_rx
     card->cardnr = ...         cidx = f(card->cardnr)
     (c4.c: line 1227)          (c4.c: line 526)
                                if (cidx >= card->nlogcontr) cidx = 0;
                                ctrl = &card->ctrlinfo[cidx].capi_ctrl

card->cardnr is 0 until it is initialized in c4_add_card(). If at the 
moment of read access in c4_handle_rx() it is still 0, cidx may then be 
assigned an undesirable value and wrong controller may handle messages. 
Is this case feasible from your point of view?

Thank you for your time.

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