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

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Date:   Wed, 22 Dec 2021 15:30:56 +0100
From:   Martijn Coenen <maco@...roid.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

LGTM,

On Mon, Dec 20, 2021 at 8:02 PM 'Todd Kjos' via kernel-team
<kernel-team@...roid.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@...gle.com>
Reviewed-by: Martijn Coenen <maco@...roid.com>

> ---
>  drivers/android/binder_alloc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
> index 340515f54498..47bc74a8c7b6 100644
> --- a/drivers/android/binder_alloc.c
> +++ b/drivers/android/binder_alloc.c
> @@ -671,7 +671,7 @@ static void binder_free_buf_locked(struct binder_alloc *alloc,
>         BUG_ON(buffer->user_data > alloc->buffer + alloc->buffer_size);
>
>         if (buffer->async_transaction) {
> -               alloc->free_async_space += size + sizeof(struct binder_buffer);
> +               alloc->free_async_space += buffer_size + sizeof(struct binder_buffer);
>
>                 binder_alloc_debug(BINDER_DEBUG_BUFFER_ALLOC_ASYNC,
>                              "%d: binder_free_buf size %zd async free %zd\n",
> --
> 2.34.1.307.g9b7440fafd-goog
>
>