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Date:   Fri, 31 Aug 2018 13:12:32 -0700
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Greg Hackmann <ghackmann@android.com>
Cc:     linux-kernel@vger.kernel.org, Laura Abbott <labbott@redhat.com>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        devel@driverdev.osuosl.org, kernel-team@android.com,
        Greg Hackmann <ghackmann@google.com>, stable@vger.kernel.org
Subject: Re: [PATCH] staging: android: ion: fix ION_IOC_{MAP,SHARE}
 use-after-free
Message-ID: <20180831201232.GA23559@kroah.com>
References: <20180831200627.59712-1-ghackmann@google.com>
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On Fri, Aug 31, 2018 at 01:06:27PM -0700, Greg Hackmann wrote:
> The ION_IOC_{MAP,SHARE} ioctls drop and reacquire client->lock several
> times while operating on one of the client's ion_handles.  This creates
> windows where userspace can call ION_IOC_FREE on the same client with
> the same handle, and effectively make the kernel drop its own reference.
> For example:
> 
> - thread A: ION_IOC_ALLOC creates an ion_handle with refcount 1
> - thread A: starts ION_IOC_MAP and increments the refcount to 2
> - thread B: ION_IOC_FREE decrements the refcount to 1
> - thread B: ION_IOC_FREE decrements the refcount to 0 and frees the
>             handle
> - thread A: continues ION_IOC_MAP with a dangling ion_handle * to
>             freed memory
> 
> Fix this by holding client->lock for the duration of
> ION_IOC_{MAP,SHARE}, preventing the concurrent ION_IOC_FREE.  Also
> remove ion_handle_get_by_id(), since there's literally no way to use it
> safely.
> 
> This patch is applied on top of 4.9.y.  Kernels 4.12 and later are
> unaffected, since all the underlying ion_handle infrastructure has been
> ripped out.

Does 4.4.y or older also need this?  If so, can you send backports, as
this one does not apply there.

thanks,

greg k-h