Commit cfe54f45 authored by Breno Leitao's avatar Breno Leitao Committed by Ard Biesheuvel
Browse files

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 a5baf582
("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: default avatarBreno Leitao <leitao@debian.org>
Acked-by: default avatarRik van Riel <riel@surriel.com>
Signed-off-by: default avatarArd Biesheuvel <ardb@kernel.org>
parent 61ed08c2

kernel/kthread.c

+1 −0
Original line number Diff line number Diff line
@@ -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