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

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Message-ID: <fc22e139-4eb1-45e0-5314-38f74ce4a643@gmail.com>
Date:   Sun, 30 Oct 2022 20:55:25 +0200
From:   Eli Billauer <eli.billauer@...il.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     gregkh@...uxfoundation.org, arnd@...db.de,
        linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
        imv4bel@...il.com
Subject: Re: [PATCH v2] char: xillybus: Prevent use-after-free due to race
 condition

On 30/10/2022 18:23, Alan Stern wrote:
> It looks like the xillybus driver already has a private mutex that would
> have been very well suited for this task: unit_mutex defined in
> xillybus_class.c.

Indeed so. The problem is that unit_mutex is global to xillybus_class.c, 
and I need the mutex to be released in xillyusb.c: The kref counter, 
which needs to be incremented with a mutex held, is inside a structure 
that only the XillyUSB driver knows about, so xillybus_class can't do 
that. Which is why xillybus_find_inode() passed a pointer to unit_mutex 
  to its caller in the v1 version of this patch. But that turned out to 
be too odd.

> Of course, there's nothing wrong with using a new
> mutex instead -- just make sure there aren't any ABBA locking order
> problems.

The kref_mutex is always taken with no other (Xillybus-related) mutex 
taken. So the locking order is assured.

But thanks for the reminder. Never hurts checking again.

Regards,
    Eli