CVE-2026-43402

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

kthread: consolidate kthread exit paths to prevent use-after-free

Guillaume reported crashes via corrupted RCU callback function pointers during KUnit testing. The crash was traced back to the pidfs rhashtable conversion which replaced the 24-byte rbnode with an 8-byte rhashhead in struct pid, shrinking it from 160 to 144 bytes.

struct kthread (without CONFIGBLKCGROUP) is also 144 bytes. With CONFIGSLABMERGEDEFAULT and SLABHWCACHEALIGN both round up to 192 bytes and share the same slab cache. struct pid.rcu.func and struct kthread.affinitynode both sit at offset 0x78.

When a kthread exits via maketaskdead() it bypasses kthreadexit() and misses the affinitynode cleanup. freekthreadstruct() frees the memory while the node is still linked into the global kthreadaffinitylist. A subsequent list_del() by another kthread writes through dangling list pointers into the freed and reused memory, corrupting the pid's rcu.func pointer.

Instead of patching freekthreadstruct() to handle the missed cleanup, consolidate all kthread exit paths. Turn kthreadexit() into a macro that calls doexit() and add kthreaddoexit() which is called from doexit() for any task with PFKTHREAD set. This guarantees that kthread-specific cleanup always happens regardless of the exit path - maketaskdead(), direct doexit(), or kthreadexit().

Replace _tokthread() with a new tskiskthread() accessor in the public header. Export doexit() since module code using the kthreadexit() macro now needs it directly.