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

> 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...

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds

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