linux-kernel - Possible race in xilinx-xadc.ko

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Date:   Fri, 18 Aug 2017 18:10:32 +0300
From:   Anton Volkov <avolkov@...ras.ru>
To:     lars@...afoo.de
Cc:     jic23@...nel.org, knaack.h@....de, pmeerw@...erw.net,
        linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
        ldv-project@...uxtesting.org,
        Alexey Khoroshilov <khoroshilov@...ras.ru>
Subject: Possible race in xilinx-xadc.ko

Hello.

While searching for races in the Linux kernel I've come across
"drivers/iio/adc/xilinx-xadc.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:
xadc_probe
   iio_triggered_buffer_setup
                              xadc_trigger_handler
                              ->xadc_read_adc_reg
                                ->_xadc_read_adc_reg
                                  ->xadc_zynq_read_adc_reg
->xadc_zynq_setup                  ->xadc_zynq_update_intmsk
    xadc->zynq_intmask = ~0            xadc->zynq_intmask &= ~mask
    (xilinx-xadc-core.c: line 340)     (xilinx-xadc-core.c: line 152)
    xadc_write_reg(xadc->zynq_intmask) xadc_write_reg(xadc->zynq_intmask)

Judging by the comments to iio_triggered_buffer_setup() the best 
practice is to call it after the indio_dev structure had been fully 
initialized. In the suggested case xadc_trigger_handler() interferes in 
the initialization process. Both xadc_zynq_setup() and 
xadc_zynq_update_intmsk() will write the same xadc->zynq_intmask though 
it should probably be different. Is this race 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