UBUNTU-CVE-2026-98038: Medium severity ubuntu/linux-hwe-edge vulnerability

In the Linux kernel, the following vulnerability has been resolved: bpf: Keep refcountacquire nullable for borrowed RCU kptrs bpfrefcountacquire() is fallible for a borrowed reference because the object may have reached a zero refcount. The verifier therefore keeps KFRETNULL on the return value unless the argument is an owning reference. An RCU-protected load of a local kptr is marked MEMALLOC, but it only receives NONOWNREF when the pointee contains a graph node. A refcounted object without a graph node consequently looks like an owning reference even though the loaded register has no acquired reference state. If the program drops the last real reference while remaining in the RCU critical section, refcountincnotzero() returns NULL while the verifier treats the result as non-NULL. Only classify the argument as owning when it is backed by a verifier-tracked reference. This retains the non-NULL return for pointers from bpfobjnew(), bpfkptrxchg(), or an earlier successful acquisition, while requiring a NULL check for borrowed RCU kptrs. [ kkd: Rewrote commit log ]