linux-kernel - Re: [PATCH] binder: fix async_free_space accounting for empty parcels

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Date:   Thu, 23 Dec 2021 10:47:41 +0100
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Todd Kjos <tkjos@...gle.com>
Cc:     gregkh@...uxfoundation.org, christian@...uner.io, arve@...roid.com,
        devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org,
        maco@...gle.com, joel@...lfernandes.org, kernel-team@...roid.com
Subject: Re: [PATCH] binder: fix async_free_space accounting for empty parcels

On Mon, Dec 20, 2021 at 11:01:50AM -0800, Todd Kjos 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@...gle.com>
> ---

Looks good.
Acked-by: Christian Brauner <christian.brauner@...ntu.com>