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

[Dave Jiang]

On 12/15/22 10:09 AM, 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@xxxxxxxxxxx>

Makes sense, especially with the emulated decoders coming w/o ->reset().

Reviewed-by: Dave Jiang <dave.jiang@xxxxxxxxx>

> ---
>   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);