Add a WARN_ON_ONCE() check to detect mm_struct instances that are missing user_ns initialization when passed to kthread_use_mm(). When a kthread adopts an mm via kthread_use_mm(), LSM hooks and capability checks may access current->mm->user_ns for credential validation. If user_ns is NULL, this leads to a NULL pointer dereference crash. This was observed with efi_mm on arm64, where commit a5baf582f4c0 ("arm64/efi: Call EFI runtime services without disabling preemption") introduced kthread_use_mm(&efi_mm), but efi_mm lacked user_ns initialization, causing crashes during /proc access. Adding this warning helps catch similar bugs early during development rather than waiting for hard-to-debug NULL pointer crashes in production.
Unified · Split
| @@ -1599,6 +1599,7 @@ void kthread_use_mm(struct mm_struct *mm) · 1598 unmodified lines | ||
| 1599 | 1599 | |
| 1600 | 1600 | WARN_ON_ONCE(!(tsk->flags & PF_KTHREAD)); |
| 1601 | 1601 | WARN_ON_ONCE(tsk->mm); |
| 1602 | + WARN_ON_ONCE(!mm->user_ns); | |
| 1602 | 1603 | |
| 1603 | 1604 | /* |
| 1604 | 1605 | * It is possible for mm to be the same as tsk->active_mm, but |