Re: [PATCH RFC v2 2/2] USB: ldusb: fix ring-buffer locking

[Johan Hovold <johan@kernel.org>]

On Fri, Oct 18, 2019 at 11:54:58AM -0700, Greg Kroah-Hartman wrote:
> On Fri, Oct 18, 2019 at 05:19:55PM +0200, Johan Hovold wrote:
> > The custom ring-buffer implementation was merged without any locking
> > whatsoever, but a spinlock was later added by commit 9d33efd9a791
> > ("USB: ldusb bugfix").
> > 
> > The lock did not cover the loads from the ring-buffer entry after
> > determining the buffer was non-empty, nor the update of the tail index
> > once the entry had been processed. The former could lead to stale data
> > being returned, while the latter could lead to memory corruption on
> > sufficiently weakly ordered architectures.
> 
> Ugh.
> 
> This almost looks sane, but what's the odds there is some other issue in
> here as well?  Would it make sense to just convert the code to use the
> "standard" ring buffer code instead?

Yeah, long term that may be the right thing to do, but I wanted a
minimal fix addressing the issue at hand without having to reimplement
the driver and fix all other (less-critical) issues in there...

For the ring-buffer corruption / info-leak issue, these two patches
should be sufficient though.

Copying the ring-buffer entry to a temporary buffer while holding the
lock might still be preferred to avoid having to deal with barrier
subtleties. But unless someone speaks out against 2/2, I'd just go ahead
and apply it.

Johan