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SubjectRe: spi: Add call to spi_slave_abort() function when spidev driver is released
Hi Geert,

> Hi Lukasz,
>
> On Thu, Sep 26, 2019 at 12:14 PM Lukasz Majewski <lukma@denx.de>
> wrote:
> > > 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.
>
> Worse, this makes me realize spidev->spi may be a NULL pointer, which
> will be dereferenced by spi_slave_abort(), so caching it before the
> call to kfree() won't work.
>

The patch as it is now can be fixed as follows:

static int spidev_release(struct inode *inode, struct file *filp)
{
struct spidev_data *spidev;

mutex_lock(&device_list_lock);
spidev = filp->private_data;
filp->private_data = NULL;

#ifdef CONFIG_SPI_SLAVE
if (spidev->spi)
spi_slave_abort(spidev->spi);
#endif

/* last close? */
spidev->users--;
if (!spidev->users) {
int dofree;

/* free buffers */

spin_lock_irq(&spidev->spi_lock);
if (spidev->spi)
spidev->speed_hz = spidev->spi->max_speed_hz;

/* ... after we unbound from the underlying device? */
//
// [*]
//
dofree = (spidev->spi == NULL);
spin_unlock_irq(&spidev->spi_lock);

if (dofree)
kfree(spidev);
}

mutex_unlock(&device_list_lock);

return 0;
}

The question is if we shall call the spi_slave_abort() when cleaning up
spi after releasing last reference, or each time release callback is
called ?

> > However, Geert (CC'ed) had some questions about placement of this
> > function call, so I will wait with providing fix until he replies.
>
> Seems like this needs more thought...

Could you be more specific?

Do you mean to move the spi_slave_abort() call just before dofree
evaluation ? ([*]).

>
> Gr{oetje,eeting}s,
>
> Geert
>




Best regards,

Lukasz Majewski

--

DENX Software Engineering GmbH, Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
Phone: (+49)-8142-66989-59 Fax: (+49)-8142-66989-80 Email: lukma@denx.de
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