linux-kernel - Re: [PATCH 0/5] x86/ftrace: Cure boot time W+X mapping

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Date:   Wed, 26 Oct 2022 10:59:29 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     rostedt@...dmis.org, dave.hansen@...el.com,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        keescook@...omium.org, seanjc@...gle.com
Subject: Re: [PATCH 0/5] x86/ftrace: Cure boot time W+X mapping

On Wed, Oct 26, 2022 at 12:15 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> Right, and we have it all over the place. Something like the below
> perhaps? I'll feed it to the robots, see if something breaks.

I was nodding along with the patches like this:

> -       set_memory_ro(base, pages);
> -       set_memory_x(base, pages);
> +       set_memory_rox(base, pages);

but then I got to this part:

> +static inline int set_memory_rox(unsigned long addr, int numpages)
> +{
> +       int ret = set_memory_ro(addr, numpages);
> +       if (ret)
> +               return ret;
> +       return set_memory_x(addr, numpages);
> +}

and that's when I went "no, I really meant make it one single call".

set_memory_ro() and set_memory_x() basically end up doing the exact
same thing, just with different bits.  So it's not only silly to have
the callers do two different calls, it's silly to have the
*implementation* do two different scans of the page tables and page
merging/splitting.

I think in the case of x86, the set_memory_rox() function would
basically just be

    int set_memory_rox(unsigned long addr, int numpages)
    {
        pgprot_t clr = __pgprot(_PAGE_RW);
        pgprot_t set = { 0 };

        if (__supported_pte_mask & _PAGE_NX)
                set.pgprot |= _PAGE_NX;

        return change_page_attr_set_clr(&addr, numpages, set, clr, 0, NULL);
    }

or something close to that. (NOTE! The above was cobbled together in
the MUA, hasn't seen a compiler much less been booted, and might be
completely broken for some reason - it's meant to be the concept, not
some kind of final word).

IOW, the whole "set_rox" is really just a _single_
change_page_attr_set_clr() call.

Maybe you meant to do that, and this patch was just prep-work for the
arch code being the second stage?

                  Linus