cxl/region: Fix decoder allocation crash

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: cxl/region: Fix decoder allocation crash When an intermediate port's decoders have been exhausted by existing regions, and creating a new region with the port in question in it's hierarchical path is attempted, cxl_port_attach_region() fails to find a port decoder (as would be expected), and drops into the failure / cleanup path. However, during cleanup of the region reference, a sanity check attempts to dereference the decoder, which in the above case didn't exist. This causes a NULL pointer dereference BUG. To fix this, refactor the decoder allocation and de-allocation into helper routines, and in this 'free' routine, check that the decoder, @cxld, is valid before attempting any operations on it.

References

• https://git.kernel.org/stable/c/c6813b5610ac53af73edd87a660d23a0511faa47
• https://git.kernel.org/stable/c/71ee71d7adcba648077997a29a91158d20c40b09