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

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Date:   Sun, 13 Nov 2022 11:03:20 +0200
From:   Eli Billauer <eli.billauer@...il.com>
To:     Hyunwoo Kim <imv4bel@...il.com>
Cc:     gregkh@...uxfoundation.org, arnd@...db.de,
        linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
        stern@...land.harvard.edu
Subject: Re: [PATCH v2] char: xillybus: Prevent use-after-free due to race
 condition

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.

Regards,
   Eli