On Tue, Aug 26, 2014 at 7:43 AM, Will Deacon <will.deacon@xxxxxxx> 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@xxxxxxx> 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
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