[PATCH 2/2] kthread: Warn if mm_struct lacks user_ns in kthread_use_mm()

[Rik van Riel]

On Tue, 2025-12-23 at 02:55 -0800, Breno Leitao wrote:
> 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 at debian.org>

Acked-by: Rik van Riel <riel at surriel.com>

-- 
All Rights Reversed.