Re: spi: Add call to spi_slave_abort() function when spidev driver is released

From: Colin Ian King <colin.king@canonical.com>
To: Lukasz Majewski <lukma@denx.de>
Cc: Mark Brown <broonie@kernel.org>,
	linux-spi@vger.kernel.org,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Geert Uytterhoeven <geert@linux-m68k.org>
Subject: Re: spi: Add call to spi_slave_abort() function when spidev driver is released
Date: Thu, 26 Sep 2019 11:15:33 +0100	[thread overview]
Message-ID: <abeb7029-0b24-64f0-0d1c-63f12ba8fffe@canonical.com> (raw)
In-Reply-To: <20190926121438.655f1f10@jawa>

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On 26/09/2019 11:14, Lukasz Majewski wrote:
> Hi Colin,
> 
>> Hi,
>>
>> Static analysis with Coverity has detected an potential dereference
>> of a free'd object with commit:
>>
>> commit 9f918a728cf86b2757b6a7025e1f46824bfe3155
>> Author: Lukasz Majewski <lukma@denx.de>
>> Date:   Wed Sep 25 11:11:42 2019 +0200
>>
>>     spi: Add call to spi_slave_abort() function when spidev driver is
>> released
>>
>> In spidev_release() in drivers/spi/spidev.c the analysis is as
>> follows:
>>
>> 600static int spidev_release(struct inode *inode, struct file *filp)
>> 601{
>> 602        struct spidev_data      *spidev;
>> 603
>> 604        mutex_lock(&device_list_lock);
>>
>>    1. alias: Assigning: spidev = filp->private_data. Now both point to
>> the same storage.
>>
>> 605        spidev = filp->private_data;
>> 606        filp->private_data = NULL;
>> 607
>> 608        /* last close? */
>> 609        spidev->users--;
>>
>>    2. Condition !spidev->users, taking true branch.
>>
>> 610        if (!spidev->users) {
>> 611                int             dofree;
>> 612
>> 613                kfree(spidev->tx_buffer);
>> 614                spidev->tx_buffer = NULL;
>> 615
>> 616                kfree(spidev->rx_buffer);
>> 617                spidev->rx_buffer = NULL;
>> 618
>> 619                spin_lock_irq(&spidev->spi_lock);
>>
>>    3. Condition spidev->spi, taking false branch.
>>
>> 620                if (spidev->spi)
>> 621                        spidev->speed_hz =
>> spidev->spi->max_speed_hz; 622
>> 623                /* ... after we unbound from the underlying
>> device? */
>>
>>    4. Condition spidev->spi == NULL, taking true branch.
>>
>> 624                dofree = (spidev->spi == NULL);
>> 625                spin_unlock_irq(&spidev->spi_lock);
>> 626
>>
>>    5. Condition dofree, taking true branch.
>>
>> 627                if (dofree)
>>
>>    6. freed_arg: kfree frees spidev.
>>
>> 628                        kfree(spidev);
>> 629        }
>> 630#ifdef CONFIG_SPI_SLAVE
>>
>>    CID 89726 (#1 of 1): Read from pointer after free (USE_AFTER_FREE)
>> 7. deref_after_free: Dereferencing freed pointer spidev.
>>
>> 631        spi_slave_abort(spidev->spi);
>> 632#endif
>> 633        mutex_unlock(&device_list_lock);
>> 634
>> 635        return 0;
>> 636}
>>
>> The call to spi_slave_abort() on spidev is reading an earlier kfree'd
>> spidev.
> 
> Thanks for spotting this issue - indeed there is a possibility to use
> spidev after being kfree'd. 
> 
> However, Geert (CC'ed) had some questions about placement of this
> function call, so I will wait with providing fix until he replies.

Cool, thanks for the update.

Colin
> 
>>
>> Colin
>>
> 
> 
> Best regards,
> 
> Lukasz Majewski
> 
> --
> 
> DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
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> 

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