linux-kernel - Re: [PATCH] cxl/region: Fix null pointer dereference for resetting decoder

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Date:   Wed, 1 Feb 2023 09:58:36 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Fan Ni <fan.ni@...sung.com>
CC:     "alison.schofield@...el.com" <alison.schofield@...el.com>,
        "vishal.l.verma@...el.com" <vishal.l.verma@...el.com>,
        "ira.weiny@...el.com" <ira.weiny@...el.com>,
        "bwidawsk@...nel.org" <bwidawsk@...nel.org>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        "dan.carpenter@...cle.com" <dan.carpenter@...cle.com>,
        "linux-cxl@...r.kernel.org" <linux-cxl@...r.kernel.org>,
        Adam Manzanares <a.manzanares@...sung.com>,
        "dave@...olabs.net" <dave@...olabs.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cxl/region: Fix null pointer dereference for resetting
 decoder

Jonathan Cameron wrote:
> On Thu, 15 Dec 2022 17:09:14 +0000
> Fan Ni <fan.ni@...sung.com> wrote:
> 
> > Not all decoders have a reset callback.
> > 
> > The CXL specification allows a host bridge with a single root port to
> > have no explicit HDM decoders. Currently the region driver assumes there
> > are none.  As such the CXL core creates a special pass through decoder
> > instance without a commit/reset callback.
> > 
> > Prior to this patch, the ->reset() callback was called unconditionally when
> > calling cxl_region_decode_reset. Thus a configuration with 1 Host Bridge,
> > 1 Root Port, and one directly attached CXL type 3 device or multiple CXL
> > type 3 devices attached to downstream ports of a switch can cause a null
> > pointer dereference.
> > 
> > Before the fix, a kernel crash was observed when we destroy the region, and
> > a pass through decoder is reset.
> > 
> > The issue can be reproduced as below,
> >     1) create a region with a CXL setup which includes a HB with a
> >     single root port under which a memdev is attached directly.
> >     2) destroy the region with cxl destroy-region regionX -f.
> > 
> > Fixes: 176baefb2eb5 ("cxl/hdm: Commit decoder state to hardware")
> > Signed-off-by: Fan Ni <fan.ni@...sung.com>
> 
> Explanation seems correct to me.  Only question (and it's one for the
> Maintainers) is whether they prefer optionality here or a stub reset()
> implementation for the pass through decoder.

Yeah, I think this fix as is works for the purposes of the -stable
backport and then a follow-on can add the optionality.