CVE-2026-74274: cxl/region: Fill first free targets[] slot during auto-discovery

In the Linux kernel, the following vulnerability has been resolved:

cxl/region: Fill first free targets[] slot during auto-discovery

Any invalid endpoint decoder pointer in the target array of an active region is not allowed by cxl driver. This means cxl driver always assumes the first p->nrtargets entries of the target array in an auto-assembly region are valid. However, there are scenarios that could leave NULL endpoint decoder pointer holes in the target array.

1. When cxlcancelautoattach() removes an endpoint decoder from a target array, the target slot is set to NULL. If the removed endpoint decoder is not the last element in the target array, the target array will contain a NULL hole.

2. When a auto-assembly region removes an assigned endpoint decoder, if the removed endpoint decoder is not the last element in the target array, always remains a NULL hole in the target array.

When a NULL pointer hole exists in a region's target array, it introduces two potential problems: 1. Access an endpoint decoder via a NULL pointer. it always trigger calltrace like that. Oops: general protection fault, probably for non-canonical address 0xdffffc0000000008: 0000 [#1] SMP KASAN PTI RIP: 0010:cxlcalcinterleavepos+0x26/0x810 [cxlcore] Call Trace: <TASK> cxlregionattach+0xc50/0x2140 [cxlcore] cxladdtoregion+0x321/0x2330 [cxlcore] discoverregion+0x92/0x150 [cxlport] deviceforeachchild+0xf3/0x170 cxlportprobe+0x150/0x200 [cxlport] cxlbusprobe+0x4f/0xa0 [cxlcore] reallyprobe+0x1c8/0x960 driverprobedevice+0x323/0x450 driverprobedevice+0x45/0x120 deviceattachdriver+0x15d/0x280 busforeachdrv+0x10f/0x190

2. Not having enough valid endpoint decoders attached to an auto-assembly region. if an auto-assembly region is created with lock flag or assigned endpoint decoder with lock flag, which means assigned endpoint decoder will not be reset during detaching, they could re-attach to the auto-assembly region again. But cxl region driver relies on p->nrtargets to verify whether the required number of endpoint decoders has been attached, and NULL endpoint decoder pointers are still counted in that case.

To fix above issues, adjust cxlregionattachauto() logic to find the first free target slot for endpoint decoder attachment, this ensures NULL holes in the target array are filled, rather than adding new endpoint decoders at the tail of the target array.