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

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Message-ID: <20230201155729.wnil24dijtd2rfhl@offworld>
Date:   Wed, 1 Feb 2023 07:57:29 -0800
From:   Davidlohr Bueso <dave@...olabs.net>
To:     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>,
        "Jonathan.Cameron@...wei.com" <Jonathan.Cameron@...wei.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>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cxl/region: Fix null pointer dereference for resetting
 decoder

On Thu, 15 Dec 2022, Fan Ni 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>

Reviewed-by: Davidlohr Bueso <dave@...olabs.net>