kthread: Warn if mm_struct lacks user_ns in kthread_use_mm()

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 a5baf582f4
("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.

Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

kernel/kthread.c

@@ -1599,6 +1599,7 @@ void kthread_use_mm(struct mm_struct *mm)
 
 	WARN_ON_ONCE(!(tsk->flags & PF_KTHREAD));
 	WARN_ON_ONCE(tsk->mm);
+	WARN_ON_ONCE(!mm->user_ns);
 
 	/*
 	 * It is possible for mm to be the same as tsk->active_mm, but