Re: [PATCH 2/2] time/namespace: Forbid timens page faults under kthread_use_mm()

[Thomas Gleixner <tglx@linutronix.de>]

On Tue, Nov 29 2022 at 20:18, Jann Horn wrote:

> find_timens_vvar_page() doesn't work when current's timens does not match
> the timens associated with current->mm.
> v6 of the series adding this code [1] had some complicated code to deal
> with this case, but v7 [2] removed that.
>
> Since the vvar region is designed to only be accessed by vDSO code, and
> vDSO code can't run in kthread context, it should be fine to error out in
> this case.

Should? Either it is correct or not.

But the way more interesting question is:
  
>  struct page *find_timens_vvar_page(struct vm_area_struct *vma)
>  {
> +	/*
> +	 * We can't handle faults where current's timens does not match the
> +	 * timens associated with the mm_struct. This can happen if a page fault
> +	 * occurs in a kthread that is using kthread_use_mm().
> +	 */

How does a kthread, which obvioulsy did kthread_use_mm(), end up trying to
fault in the time namespace vvar page?

It's probably something nasty, but the changelog has a big information
void.

It neither answers the obvious question why this is a problem of the
time namespace vvar page and not a general issue versus a kthread, which
borrowed a user mm, ending up in vdso_fault() in the first place?

None of those VDSO (user space) addresses are subject to be faulted in
by anything else than the associated user space task(s).

Thanks,

        tglx