UBUNTU-CVE-2026-89965: High severity ubuntu/linux-aws vulnerability

In the Linux kernel, the following vulnerability has been resolved: nvdimm/btt: reject an arena whose nfree is below the lane count The BTT info block's nfree field, the number of reserve free blocks, is read from the medium without validation. bttfreelistinit() and bttrttinit() size the per-lane freelist[] and rtt[] arrays by nfree, but the I/O path indexes them by the lane from ndregionacquirelane(), which is bounded by ndregion->numlanes (NDMAXLANES), not by nfree. A crafted or foreign arena whose nfree is below the lane count makes freelist[lane]/rtt[lane] run past the allocation: an out-of-bounds write. btt.rst documents the nlanes = min(nfree, numcpus) invariant, which the code does not currently honor: numlanes is NDMAXLANES regardless of nfree. Reject an arena whose nfree is below numlanes at discovery, before the per-lane arrays are allocated, enforcing that invariant.