Re: [PATCH] staging: speakup: fix wraparound in uaccess length check

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Thu, Jul 12, 2018 at 04:12:39PM -0700, Jann Horn wrote:
> On Thu, Jul 12, 2018 at 3:47 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > On Fri, Jul 13, 2018 at 12:29:36AM +0200, Jann Horn wrote:
> > > From: Samuel Thibault <samuel.thibault@ens-lyon.org>
> > >
> > > From: Samuel Thibault <samuel.thibault@ens-lyon.org>
> > >
> > > 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.
> >
> > Or you could try this (completely untested, though):
> 
> I think this has the same problem as my original buggy patch: At the
> point where you notice that you'd overflow the buffer, you've already
> consumed a character from the synth buffer. You'd have to put it back,
> and since the spinlock protecting it has been dropped, that's a bit
> weird.
> 
> Also, I'm not sure whether Greg prefers fixes for stable kernels that
> don't also contain cleanup?

For staging code, I really don't care, as long as it's fixing an issue :)

thanks,

greg k-h