linux-hardening - Re: [PATCH v6 4/4] powerpc/64s: Initialize and use a temporary mm for patching on Radix

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Date:   Wed, 15 Sep 2021 19:45:24 -0500
From:   "Christopher M. Riedl" <cmr@...escreens.de>
To:     "Jordan Niethe" <jniethe5@...il.com>
Cc:     "linuxppc-dev" <linuxppc-dev@...ts.ozlabs.org>,
        <linux-hardening@...r.kernel.org>
Subject: Re: [PATCH v6 4/4] powerpc/64s: Initialize and use a temporary mm
 for patching on Radix

On Tue Sep 14, 2021 at 11:24 PM CDT, Jordan Niethe wrote:
> On Sat, Sep 11, 2021 at 12:39 PM Christopher M. Riedl
> <cmr@...escreens.de> wrote:
> > ... 
> > +/*
> > + * This can be called for kernel text or a module.
> > + */
> > +static int map_patch_mm(const void *addr, struct patch_mapping *patch_mapping)
> > +{
> > +       struct page *page;
> > +       struct mm_struct *patching_mm = __this_cpu_read(cpu_patching_mm);
> > +       unsigned long patching_addr = __this_cpu_read(cpu_patching_addr);
> > +
> > +       if (is_vmalloc_or_module_addr(addr))
> > +               page = vmalloc_to_page(addr);
> > +       else
> > +               page = virt_to_page(addr);
> > +
> > +       patch_mapping->ptep = get_locked_pte(patching_mm, patching_addr,
> > +                                            &patch_mapping->ptl);
> > +       if (unlikely(!patch_mapping->ptep)) {
> > +               pr_warn("map patch: failed to allocate pte for patching\n");
> > +               return -1;
> > +       }
> > +
> > +       set_pte_at(patching_mm, patching_addr, patch_mapping->ptep,
> > +                  pte_mkdirty(mk_pte(page, PAGE_KERNEL)));
>
> I think because switch_mm_irqs_off() will not necessarily have a
> barrier so a ptesync would be needed.
> A spurious fault here from __patch_instruction() would not be handled
> correctly.

Sorry I don't quite follow - can you explain this to me in a bit more
detail?