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.
Signed-off-by: Breno Leitao <leitao@debian.org>
---
kernel/kthread.c | 1 +
1 file changed, 1 insertion(+)
diff --git a/kernel/kthread.c b/kernel/kthread.c
index 99a3808d086f..39511dd2abc9 100644
--- a/kernel/kthread.c
+++ b/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
--
2.47.3