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

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Date:   Fri, 13 Jan 2023 11:01:08 +0000
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
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>,
        "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

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.

either way
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>

> ---
>  drivers/cxl/core/region.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index f9ae5ad284ff..3931793a13ac 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -131,7 +131,7 @@ static int cxl_region_decode_reset(struct cxl_region *cxlr, int count)
>  		struct cxl_memdev *cxlmd = cxled_to_memdev(cxled);
>  		struct cxl_port *iter = cxled_to_port(cxled);
>  		struct cxl_ep *ep;
> -		int rc;
> +		int rc = 0;
>  
>  		while (!is_cxl_root(to_cxl_port(iter->dev.parent)))
>  			iter = to_cxl_port(iter->dev.parent);
> @@ -143,7 +143,8 @@ static int cxl_region_decode_reset(struct cxl_region *cxlr, int count)
>  
>  			cxl_rr = cxl_rr_load(iter, cxlr);
>  			cxld = cxl_rr->decoder;
> -			rc = cxld->reset(cxld);
> +			if (cxld->reset)
> +				rc = cxld->reset(cxld);
>  			if (rc)
>  				return rc;
>  		}
> @@ -186,7 +187,8 @@ static int cxl_region_decode_commit(struct cxl_region *cxlr)
>  			     iter = ep->next, ep = cxl_ep_load(iter, cxlmd)) {
>  				cxl_rr = cxl_rr_load(iter, cxlr);
>  				cxld = cxl_rr->decoder;
> -				cxld->reset(cxld);
> +				if (cxld->reset)
> +					cxld->reset(cxld);
>  			}
>  
>  			cxled->cxld.reset(&cxled->cxld);