Re: [PATCH v4 2/8] mm: introduce external memory hinting API

[Jann Horn <jannh@google.com>]

On Thu, Feb 13, 2020 at 12:40 AM Minchan Kim <minchan@kernel.org> wrote:
> To solve the issue, this patch introduces a new syscall process_madvise(2).
> It uses pidfd of an external process to give the hint.
[...]
> +       mm = mm_access(task, PTRACE_MODE_ATTACH_FSCREDS);
> +       if (IS_ERR_OR_NULL(mm)) {
> +               ret = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH;
> +               goto release_task;
> +       }
> +
> +       ret = do_madvise(task, start, len_in, behavior);

When you're accessing another task, you should ensure that the other
task doesn't gain new privileges by executing a setuid binary in the
middle of being accessed. mm_access() does that for you; it holds the
->cred_guard_mutex while it is looking up the task's ->mm and doing
the security check. mm_access() then returns you an mm pointer that
you're allowed to access without worrying about such things; an
mm_struct never gains privileges, since a setuid execution creates a
fresh mm_struct. However, the task may still execute setuid binaries
and such things.

This means that after you've looked up the mm with mm_access(), you
have to actually *use* that pointer. You're not allowed to simply read
task->mm yourself.

Therefore, I think you should:

 - change patch 1/8 ("mm: pass task to do_madvise") to also pass an
mm_struct* to do_madvise (but keep the task_struct* for patch 4/8)
 - in this patch, pass the mm_struct* from mm_access() into do_madvise()
 - drop patch 3/8 ("mm: validate mm in do_madvise"); it just papers
over a symptom without addressing the underlying problem