Re: [PATCH v4 7/8] ARM: mm: allow non-text sections to be non-executable

[Will Deacon <will.deacon@arm.com>]

On Wed, Aug 20, 2014 at 01:37:14PM +0100, Kees Cook wrote:
> On Tue, Aug 19, 2014 at 7:33 AM, Will Deacon <will.deacon@arm.com> wrote:
> > On Wed, Aug 13, 2014 at 06:06:32PM +0100, Kees Cook wrote:
> >> +/*
> >> + * Updates section permissions only for the current mm (sections are
> >> + * copied into each mm). During startup, this is the init_mm.
> >> + */
> >> +static inline void section_update(unsigned long addr, pmdval_t mask,
> >> +                               pmdval_t prot)
> >> +{
> >> +     struct mm_struct *mm;
> >> +     pmd_t *pmd;
> >> +
> >> +     mm = current->active_mm;
> >> +     pmd = pmd_offset(pud_offset(pgd_offset(mm, addr), addr), addr);
> >> +
> >> +#ifdef CONFIG_ARM_LPAE
> >> +     pmd[0] = __pmd((pmd_val(pmd[0]) & mask) | prot);
> >> +#else
> >> +     if (addr & SECTION_SIZE)
> >> +             pmd[1] = __pmd((pmd_val(pmd[1]) & mask) | prot);
> >> +     else
> >> +             pmd[0] = __pmd((pmd_val(pmd[0]) & mask) | prot);
> >> +#endif
> >> +     flush_pmd_entry(pmd);
> >> +     local_flush_tlb_kernel_range(addr, addr + SECTION_SIZE);
> >
> > Why only a local flush? You're changing global mappings here, right?
> 
> Yes, but with the a15 errata, it cannot use a global flush. As a
> result, section_update can only be used by a single CPU which is how
> the usage is managed. Perhaps I should add some comments to that
> effect? (There was a thread a few months ago on this problem and this
> shook out as a solution.)

Hmm, so do you mandate that preemption is disabled during sections of code
where the permissions must be changed after boot? (e.g. ftrace patching)

Will