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

[Kees Cook <keescook@chromium.org>]

On Tue, Aug 26, 2014 at 7:43 AM, Will Deacon <will.deacon@arm.com> wrote:
> 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)

I guess it's not enforced (how do I make sure preempt is off?), but
it's only used during the ftrace patching. I'm open to suggestions! (I
had avoided going the fixmap route since I would have expected that to
be much slower.)

-Kees

-- 
Kees Cook
Chrome OS Security