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

[Gregory Price]

On Thu, Dec 15, 2022 at 05:09:14PM +0000, 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>
> ---
>  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);
> -- 
> 2.25.1


Should we try to get this upstreamed in 6.2-final?  Seems like a good
stable addition. Probably doesn't affect real hardware, but it certainly
affects QEMU.


Tested-by: Gregory Price <gregory.price@xxxxxxxxxxxx>
Reviewed-by: Gregory Price <gregory.price@xxxxxxxxxxxx>