Re: [PATCH v7 09/12] sysfs: fix deadlock race with module removal

[Dan Williams <dan.j.williams@intel.com>]

On Fri, Sep 17, 2021 at 10:05 PM Luis Chamberlain <mcgrof@kernel.org> wrote:
>
> When sysfs attributes use a lock also used on module removal we can
> race to deadlock. This happens when for instance a sysfs file on
> a driver is used, then at the same time we have module removal call
> trigger. The module removal call code holds a lock, and then the sysfs
> file entry waits for the same lock. While holding the lock the module
> removal tries to remove the sysfs entries, but these cannot be removed
> yet as one is waiting for a lock. This won't complete as the lock is
> already held. Likewise module removal cannot complete, and so we deadlock.
>
> This can now be easily reproducible with our sysfs selftest as follows:
>
> ./tools/testing/selftests/sysfs/sysfs.sh -t 0027
>
> To fix this we extend the struct kernfs_node with a module reference and
> use the try_module_get() after kernfs_get_active() is called which
> protects integrity and the existence of the kernfs node during the
> operation.
>
> So long as the kernfs node is protected with kernfs_get_active() we know
> we can rely on its contents. And, as now just documented in the previous
> patch, we also now know that once kernfs_get_active() is called the module
> is also guarded to exist and cannot be removed.
>
> If try_module_get() fails we fail the operation on the kernfs node.
>
> We use a try method as a full lock means we'd then make our sysfs
> attributes busy us out from possible module removal, and so userspace
> could force denying module removal, a silly form of "DOS" against module
> removal. A try lock on the module removal ensures we give priority to
> module removal and interacting with sysfs attributes only comes second.
> Using a full lock could mean for instance that if you don't stop poking
> at sysfs files you cannot remove a module.
>
> Races between removal of sysfs files and the module are not possible
> given sysfs files are created by the same module, and when a sysfs file
> is being used kernfs prevents removal of the sysfs file. So if module
> removal is actually happening the removal would have to wait until
> the sysfs file operation is complete.
>
> This deadlock was first reported with the zram driver, however the live
> patching folks have acknowledged they have observed this as well with
> live patching, when a live patch is removed. I was then able to
> reproduce easily by creating a dedicated selftests.
>
> A sketch of how this can happen follows:
>
> CPU A                              CPU B
>                                    whatever_store()
> module_unload
>   mutex_lock(foo)
>                                    mutex_lock(foo)
>    del_gendisk(zram->disk);
>      device_del()
>        device_remove_groups()

This flow seems possible to trigger with:

   echo $dev > /sys/bus/$bus/drivers/$driver/unbind

I am missing why module pinning is part of the solution when it's the
device_del() path that is racing? Module removal is just a more coarse
grained way to trigger unbind => device_del(). Isn't the above a bug
in the driver, not missing synchronization in kernfs? Forgive me if
the unbind question was asked and answered elsewhere, this is my first
time taking a look at this series.