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SubjectRe: [PATCH 06/17] x86/alternative: use temporary mm for text poking
On Thu, Jan 17, 2019 at 12:27 PM Andy Lutomirski <luto@kernel.org> wrote:
>
> On Wed, Jan 16, 2019 at 4:33 PM Rick Edgecombe
> <rick.p.edgecombe@intel.com> wrote:
> >
> > From: Nadav Amit <namit@vmware.com>
> >
> > text_poke() can potentially compromise the security as it sets temporary
> > PTEs in the fixmap. These PTEs might be used to rewrite the kernel code
> > from other cores accidentally or maliciously, if an attacker gains the
> > ability to write onto kernel memory.
>
> i think this may be sufficient, but barely.
>
> > + pte_clear(poking_mm, poking_addr, ptep);
> > +
> > + /*
> > + * __flush_tlb_one_user() performs a redundant TLB flush when PTI is on,
> > + * as it also flushes the corresponding "user" address spaces, which
> > + * does not exist.
> > + *
> > + * Poking, however, is already very inefficient since it does not try to
> > + * batch updates, so we ignore this problem for the time being.
> > + *
> > + * Since the PTEs do not exist in other kernel address-spaces, we do
> > + * not use __flush_tlb_one_kernel(), which when PTI is on would cause
> > + * more unwarranted TLB flushes.
> > + *
> > + * There is a slight anomaly here: the PTE is a supervisor-only and
> > + * (potentially) global and we use __flush_tlb_one_user() but this
> > + * should be fine.
> > + */
> > + __flush_tlb_one_user(poking_addr);
> > + if (cross_page_boundary) {
> > + pte_clear(poking_mm, poking_addr + PAGE_SIZE, ptep + 1);
> > + __flush_tlb_one_user(poking_addr + PAGE_SIZE);
> > + }
>
> In principle, another CPU could still have the old translation. Your
> mutex probably makes this impossible, but it makes me nervous.
> Ideally you'd use flush_tlb_mm_range(), but I guess you can't do that
> with IRQs off. Hmm. I think you should add an inc_mm_tlb_gen() here.
> Arguably, if you did that, you could omit the flushes, but maybe
> that's silly.
>
> If we start getting new users of use_temporary_mm(), we should give
> some serious thought to the SMP semantics.
>
> Also, you're using PAGE_KERNEL. Please tell me that the global bit
> isn't set in there.
>

Much better solution: do unuse_temporary_mm() and *then*
flush_tlb_mm_range(). This is entirely non-sketchy and should be just
about optimal, too.

--Andy

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