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

[Davidlohr Bueso]

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@xxxxxxxxxxx>

Reviewed-by: Davidlohr Bueso <dave@xxxxxxxxxxxx>