Re: [PATCH v2] char: xillybus: Prevent use-after-free due to race condition

[Hyunwoo Kim <imv4bel@gmail.com>]

On Sun, Nov 13, 2022 at 11:03:20AM +0200, Eli Billauer wrote:
> On 13/11/2022 10:47, Hyunwoo Kim wrote:
> > And, even if the mutex_unlock(&unit_mutex); of xillybus_find_inode()
> > is finally moved, xdev may be released before kref_get() is executed
> > if xillyusb_disconnect() ends just before the function returns.
> > (Of course, this is an extremely rare case.)
> > 
> > So, in xillyusb_open() we need to move kref_get() above xillybus_find_inode().
> 
> First of all, that's impossible. kref_get() is called on a member of a
> specific @xdev's struct, and it's xillybus_find_inode()'s job to find it. So
> before the call to xillybus_find_inode(), we don't know which @xdev it is.
> That's the tricky part of all this.
> 
> The solution of this submitted patch was a lock that briefly prevents the
> kref_put() of all @xdevs. The way it works is that if an @xdev is found by
> xillybus_find_inode(), it necessarily means that xillyusb_disconnect()'s
> call to xillybus_cleanup_chrdev() hasn't returned (yet). Therefore, holding
> @kref_mutex guarantees that the kref_put() call, which is later on, isn't
> reached for the @xdev that has been found.
> 
> If you've found a flaw in this mechanism, please be more specific about it.

you're right.

It seems that you only need to move the location of unit_mutex 
in xillybus_find_inode().


Regards,
Hyunwoo Kim