linux-kernel - Re: [PATCH] x86/mm: Do not verify W^X at boot up

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Message-ID: <Y1ghRheOJeAfJvdY@hirez.programming.kicks-ass.net>
Date:   Tue, 25 Oct 2022 19:47:50 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH] x86/mm: Do not verify W^X at boot up

On Tue, Oct 25, 2022 at 09:53:27AM -0700, Linus Torvalds wrote:
> On Tue, Oct 25, 2022 at 3:16 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > This seems to boot...
> 
> This looks much better, thanks.
> 
> But this:
> 
> > diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
> > @@ -801,8 +803,9 @@ void __init poking_init(void)
> >         spinlock_t *ptl;
> >         pte_t *ptep;
> >
> > -       poking_mm = copy_init_mm();
> > -       BUG_ON(!poking_mm);
> > +       __poking_mm = init_mm;
> > +       mm_init(&__poking_mm, NULL, __poking_mm.user_ns);
> > +       poking_mm = &__poking_mm;
> 
> Should probably be just
> 
>         poking_mm = mm_alloc();
> 
> because we shouldn't be messing with 'mm_init()' in places like this,
> and we shouldn't be exporting it either:

mm_alloc() uses allocate_mm() which requires a kmem_cache to be set-up.
Using the static storage and instead calling mm_init() on it avoids
that.

So I think we can have:

static struct mm_struct __poking_mm;

	mm_init(&__poking_mm, NULL, init_mm.user_ns);

and leave out the assignment from init_mm.