Date:   Tue, 21 Dec 2021 11:08:11 +0100
From:   Greg KH <gregkh@linuxfoundation.org>
To:     Todd Kjos <tkjos@google.com>
Cc:     christian@brauner.io, arve@android.com, devel@driverdev.osuosl.org,
        linux-kernel@vger.kernel.org, maco@google.com,
        joel@joelfernandes.org, kernel-team@android.com,
        stable <stable@vger.kernel.org>
Subject: Re: [PATCH] binder: fix async_free_space accounting for empty parcels
Message-ID: <YcGnizJKAQ9pxGBY@kroah.com>
References: <20211220190150.2107077-1-tkjos@google.com>
 <CAHRSSExTHHOdqEnRF0g435BrO5L-X6M3pxPg3OmLz8xUWDuNKA@mail.gmail.com>
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On Mon, Dec 20, 2021 at 11:06:09AM -0800, Todd Kjos wrote:
> On Mon, Dec 20, 2021 at 11:02 AM Todd Kjos <tkjos@google.com> wrote:
> >
> > In 4.13, commit 74310e06be4d ("android: binder: Move buffer out of area shared with user space")
> > fixed a kernel structure visibility issue. As part of that patch,
> > sizeof(void *) was used as the buffer size for 0-length data payloads so
> > the driver could detect abusive clients sending 0-length asynchronous
> > transactions to a server by enforcing limits on async_free_size.
> >
> > Unfortunately, on the "free" side, the accounting of async_free_space
> > did not add the sizeof(void *) back. The result was that up to 8-bytes of
> > async_free_space were leaked on every async transaction of 8-bytes or
> > less.  These small transactions are uncommon, so this accounting issue
> > has gone undetected for several years.
> >
> > The fix is to use "buffer_size" (the allocated buffer size) instead of
> > "size" (the logical buffer size) when updating the async_free_space
> > during the free operation. These are the same except for this
> > corner case of asynchronous transactions with payloads < 8 bytes.
> >
> > Fixes: 74310e06be4d ("android: binder: Move buffer out of area shared with user space")
> > Signed-off-by: Todd Kjos <tkjos@google.com>
> 
> I forgot to CC stable. This applies to all stable branches back to 4.14.
> Cc: stable@vger.kernel.org # 4.14+

Thanks, I've added that to the patch when committing it.

greg k-h