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

Overview

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 rb_node with an 8-byte rhash_head in struct pid, shrinking it from 160 to 144 bytes.

struct kthread (without CONFIG_BLK_CGROUP) is also 144 bytes. With CONFIG_SLAB_MERGE_DEFAULT and SLAB_HWCACHE_ALIGN both round up to 192 bytes and share the same slab cache. struct pid.rcu.func and struct kthread.affinity_node both sit at offset 0x78.

When a kthread exits via make_task_dead() it bypasses kthread_exit() and misses the affinity_node cleanup. free_kthread_struct() frees the memory while the node is still linked into the global kthread_affinity_list. 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 free_kthread_struct() to handle the missed cleanup, consolidate all kthread exit paths. Turn kthread_exit() into a macro that calls do_exit() and add kthread_do_exit() which is called from do_exit() for any task with PF_KTHREAD set. This guarantees that kthread-specific cleanup always happens regardless of the exit path - make_task_dead(), direct do_exit(), or kthread_exit().

Replace __to_kthread() with a new tsk_is_kthread() accessor in the public header. Export do_exit() since module code using the kthread_exit() macro now needs it directly.

Affected

• Linux >= 4d13f4304fa43471bfea101658a11feec7b28ac0 < 4729c7b00a347fd37d0cbc265b85f2884c3e06b6
• Linux >= 4d13f4304fa43471bfea101658a11feec7b28ac0 < 5a591d7a5e48d30100943940a30a6ab41b15c672
• Linux >= 4d13f4304fa43471bfea101658a11feec7b28ac0 < 28aaa9c39945b7925a1cc1d513c8f21ed38f5e4f
• Linux 6.14

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.