Re: [PATCH v2 00/29] Media device lifetime management

[Sakari Ailus <sakari.ailus@linux.intel.com>]

Hi Laurent,

On Wed, Dec 20, 2023 at 12:52:32PM +0200, Laurent Pinchart wrote:
> On Wed, Dec 20, 2023 at 12:36:44PM +0200, Sakari Ailus wrote:
> > Hi folks,
> > 
> > This is a refresh of my 2016 RFC patchset to start addressing object
> > lifetime issues in Media controller. It further allows continuing work to
> > address lifetime management of media entities.
> 
> I think you win the prize of the refresh for the oldest patch series.
> Thanks for not dropping the ball. One day we'll make the media subsystem
> healthy :-)

Maybe so, indeed. The problem has always been around and I think it needs
to be addressed, although this only solves a part of it. It's been a low
priority for me and figuring out how to prevent old drivers getting worse
in this respect wasn't obvious at first, as you see from changes since v1.

> 
> > The underlying problem is described in detail in v4 of the previous RFC:
> > <URL:https://lore.kernel.org/linux-media/20161108135438.GO3217@valkosipuli.retiisi.org.uk/>.
> > In brief, there is currently no connection between releasing media device
> > (and related) memory and IOCTL calls, meaning that there is a time window
> > during which released kernel memory can be accessed, and that access can be
> > triggered from the user space. The only reason why this is not a grave
> > security issue is that it is not triggerable by the user alone but requires
> > unbinding a device. That is still not an excuse for not fixing it.
> > 
> > This set differs from the earlier RFC to address the issue in the
> > following respects:
> > 
> > - Make changes for ipu3-cio2 driver, too.
> > 
> > - Continue to provide best effort attempt to keep the window between device
> >   removal and user space being able to access released memory as small as
> >   possible. This means the problem won't become worse for drivers for which
> >   Media device lifetime management has not been implemented.
> > 
> > The latter is achieved by adding a new object, Media devnode compat
> > reference, which is allocated, refcounted and eventually released by the
> > Media controller framework itself, and where the information on registration
> > and open filehandles is maintained. This is only done if the driver does not
> > manage the lifetime of the media device itself, i.e. its release operation
> > is NULL.
> 
> Interesting. I'll check that when reviewing the patches.

Please. :-)

> 
> > Due to this, Media device file handles will also be introduced by this
> > patchset. I thought the first user of this would be Media device events but
> > it seems we already need them here.
> 
> Nice, it will become a useful feature.
> 
> > Both ipu3-cio2 and omap3isp drivers are relieved of devm_request_irq() use,
> > as device_release() releases the resources before calling the driver's
> > remove function.
> 
> Are you sure about that ? device_release() is the .release() function
> for device_ktype, which means it's called when the last reference to a
> struct device disappears. That should be way after .remove().

It's a bit grey area. If an interrupt arrives from the device in the
meantime, bad things will still happen. Of course it's not supposed to.
Either way, I've dropped these changes as requested in review comments in
v1 and as outlined below in changes since v1, but forgot to change this
text (unchanged since v1).

> 
> > While further work will be required also on these drivers
> > to safely stop he hardware at unbind time, I don't see a reason not to merge
> > these patches now.
> > 
> > Some patches are temporarily reverted in order to make reworks easier, then
> > applied later on.
> > 
> > I've tested this on ipu3-cio2 with and without the refcounting patch (media:
> > ipu3-cio2: Release the cio2 device context by media device callback),
> > including failures in a few parts of the driver initialisation process in
> > the MC framework.
> > 
> > Questions and comments are welcome.
> > 
> > since v1:
> > 
> > - Align subject prefixes with current media tree practices.
> > 
> > - Make release changes to the vimc driver (last patch of the set). This
> >   was actually easy as vimc already centralised resource release to struct
> >   v4l2_device, so it was just moved to the media device.
> > 
> > - Move cdev field to struct media_devnode_compat_ref and add dev field to
> >   the struct, these are needed during device release. This now includes
> >   also the character device which is accessed by __fput(). I've now tested
> >   ipu3-cio2 and vimc with KASAN. As a by-product the kref in struct
> >   media_devnode_compat_ref becomes redundant and is removed. Both devices
> >   are registered in case of best effort memory safety support and used for
> >   refcounting.
> > 
> > - Drop omap3isp driver patch moving away from devm_request_irq().
> > 
> > - Add a patch to warn of drivers not releasing media device safely (i.e.
> >   relying on the best effort memory safety mechanism without refcounting).
> > 
> > - Add a patch to document how the best effort memory release safety helper
> >   works.
> > 
> > - Add a note on releasing driver's context with the media device, not the
> >   V4L2 device, in MC documentation.
> > 
> > - Check media device is registered before accessing its fops in
> >   media_read(), media_write(), media_ioctl and media_compat_ioctl().
> > 
> > - Document best effort media device lifetime management (new patch).
> > 
> > - Use media_devnode_free_minor() in unallocating device node minor number
> >   in media_devnode_register().
> > 
> > - Continue to rely on devm_register_irq() in ipu3-cio2 driver but register
> >   the IRQ later on (compared to v1).
> > 
> > - Drop the patch to move away from devm_request_irq() in omap3isp.
> > 
> > - Fix putting references to media device and V4L2 device in 
> >   v4l2_device_release().
> > 
> > - Add missing media_device_get() (in v1) for M2M devices in
> >   video_register_media_controller().
> > 
> > - Unconditionally set the media devnode release function in
> >   media_device_init(). There's no harm doing so and the caller of
> >   media_device_init() may set the ops after calling the function.
> > 
> > Daniel Axtens (1):
> >   media: uvcvideo: Refactor teardown of uvc on USB disconnect
> > 
> > Laurent Pinchart (1):
> >   media: mc: Add per-file-handle data support
> > 
> > Logan Gunthorpe (1):
> >   media: mc: utilize new cdev_device_add helper function
> > 
> > Sakari Ailus (26):
> >   Revert "[media] media: fix media devnode ioctl/syscall and unregister
> >     race"
> >   Revert "media: utilize new cdev_device_add helper function"
> >   Revert "[media] media: fix use-after-free in cdev_put() when app exits
> >     after driver unbind"
> >   Revert "media: uvcvideo: Refactor teardown of uvc on USB disconnect"
> >   Revert "[media] media-device: dynamically allocate struct
> >     media_devnode"
> >   media: mc: Drop nop release callback
> >   media: mc: Do not call cdev_device_del() if cdev_device_add() fails
> >   media: mc: Delete character device early
> >   media: mc: Split initialising and adding media devnode
> >   media: mc: Shuffle functions around
> >   media: mc: Initialise media devnode in media_device_init()
> >   media: mc: Refactor media devnode minor clearing
> >   media: mc: Unassign minor only if it has been assigned
> >   media: mc: Refcount the media device
> >   media: v4l: Acquire a reference to the media device for every video
> >     device
> >   media: mc: Postpone graph object removal until free
> >   media: omap3isp: Release the isp device struct by media device
> >     callback
> >   media: ipu3-cio2: Call v4l2_device_unregister() earlier
> >   media: ipu3-cio2: Request IRQ earlier
> >   media: ipu3-cio2: Release the cio2 device context by media device
> >     callback
> >   media: vimc: Release resources on media device release
> >   media: Documentation: Document how Media device resources are released
> >   media: mc: Maintain a list of open file handles in a media device
> >   media: mc: Implement best effort media device removal safety sans
> >     refcount
> >   media: mc: Warn about drivers not releasing media device safely
> >   media: Documentation: Document media device memory safety helper
> > 
> >  Documentation/driver-api/media/mc-core.rst  |  18 +-
> >  drivers/media/cec/core/cec-core.c           |   2 +-
> >  drivers/media/mc/mc-device.c                | 260 ++++++++++++--------
> >  drivers/media/mc/mc-devnode.c               | 230 +++++++++++------
> >  drivers/media/pci/intel/ipu3/ipu3-cio2.c    |  70 ++++--
> >  drivers/media/platform/ti/omap3isp/isp.c    |  24 +-
> >  drivers/media/test-drivers/vimc/vimc-core.c |  15 +-
> >  drivers/media/usb/au0828/au0828-core.c      |   4 +-
> >  drivers/media/usb/uvc/uvc_driver.c          |   2 +-
> >  drivers/media/v4l2-core/v4l2-dev.c          |  65 +++--
> >  drivers/staging/media/sunxi/cedrus/cedrus.c |   2 +-
> >  include/media/media-device.h                |  46 +++-
> >  include/media/media-devnode.h               | 136 +++++++---
> >  include/media/media-fh.h                    |  32 +++
> >  14 files changed, 632 insertions(+), 274 deletions(-)
> >  create mode 100644 include/media/media-fh.h
> 

-- 
Kind regards,

Sakari Ailus