CVE-2022-49895

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, cxlportattach_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.