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Date:   Sat, 7 Jul 2018 10:01:36 +0200
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Jann Horn <jannh@google.com>
Cc:     William Hubbs <w.d.hubbs@gmail.com>,
        Chris Brannon <chris@the-brannons.com>,
        Kirk Reiser <kirk@reisers.ca>,
        Samuel Thibault <samuel.thibault@ens-lyon.org>,
        linux-kernel@vger.kernel.org, speakup@linux-speakup.org,
        devel@driverdev.osuosl.org
Subject: Re: [PATCH] staging: speakup: fix wraparound in uaccess length check
Message-ID: <20180707080136.GA27656@kroah.com>
References: <20180707015344.146672-1-jannh@google.com>
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On Sat, Jul 07, 2018 at 03:53:44AM +0200, Jann Horn wrote:
> If softsynthx_read() is called with `count < 3`, `count - 3` wraps, causing
> the loop to copy as much data as available to the provided buffer. If
> softsynthx_read() is invoked through sys_splice(), this causes an
> unbounded kernel write; but even when userspace just reads from it
> normally, a small size could cause userspace crashes.
> 
> Fixes: 425e586cf95b ("speakup: add unicode variant of /dev/softsynth")
> Cc: stable@vger.kernel.org
> Signed-off-by: Jann Horn <jannh@google.com>
> ---
> 
> Reproducer (kernel overflows userspace stack, resulting in segfault):

Nice find, thanks for the patch!

greg k-h