Messages in this thread | | | Date | Fri, 29 Aug 2014 09:04:41 -0700 | Subject | Re: [PATCH v4 7/8] ARM: mm: allow non-text sections to be non-executable | From | Kees Cook <> |
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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
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