Re: [PATCH 1/3] kasan: support backing vmalloc space with real shadow memory

[Dmitry Vyukov <dvyukov@google.com>]

On Mon, Jul 29, 2019 at 12:15 PM Daniel Axtens <dja@axtens.net> wrote:
>
> Hi Dmitry,
>
> Thanks for the feedback!
>
> >> +       addr = shadow_alloc_start;
> >> +       do {
> >> +               pgdp = pgd_offset_k(addr);
> >> +               p4dp = p4d_alloc(&init_mm, pgdp, addr);
> >
> > Page table allocations will be protected by mm->page_table_lock, right?
>
> Yes, each of those alloc functions take the lock if they end up in the
> slow-path that does the actual allocation (e.g. __p4d_alloc()).
>
> >> +               pudp = pud_alloc(&init_mm, p4dp, addr);
> >> +               pmdp = pmd_alloc(&init_mm, pudp, addr);
> >> +               ptep = pte_alloc_kernel(pmdp, addr);
> >> +
> >> +               /*
> >> +                * we can validly get here if pte is not none: it means we
> >> +                * allocated this page earlier to use part of it for another
> >> +                * allocation
> >> +                */
> >> +               if (pte_none(*ptep)) {
> >> +                       backing = __get_free_page(GFP_KERNEL);
> >> +                       backing_pte = pfn_pte(PFN_DOWN(__pa(backing)),
> >> +                                             PAGE_KERNEL);
> >> +                       set_pte_at(&init_mm, addr, ptep, backing_pte);
> >> +               }
> >> +       } while (addr += PAGE_SIZE, addr != shadow_alloc_end);
> >> +
> >> +       requested_size = round_up(requested_size, KASAN_SHADOW_SCALE_SIZE);
> >> +       kasan_unpoison_shadow(area->addr, requested_size);
> >> +       kasan_poison_shadow(area->addr + requested_size,
> >> +                           area->size - requested_size,
> >> +                           KASAN_VMALLOC_INVALID);
> >
> >
> > Do I read this correctly that if kernel code does vmalloc(64), they
> > will have exactly 64 bytes available rather than full page? To make
> > sure: vmalloc does not guarantee that the available size is rounded up
> > to page size? I suspect we will see a throw out of new bugs related to
> > OOBs on vmalloc memory. So I want to make sure that these will be
> > indeed bugs that we agree need to be fixed.
> > I am sure there will be bugs where the size is controlled by
> > user-space, so these are bad bugs under any circumstances. But there
> > will also probably be OOBs, where people will try to "prove" that
> > that's fine and will work (just based on our previous experiences :)).
>
> So the implementation of vmalloc will always round it up. The
> description of the function reads, in part:
>
>  * Allocate enough pages to cover @size from the page level
>  * allocator and map them into contiguous kernel virtual space.
>
> So in short it's not quite clear - you could argue that you have a
> guarantee that you get full pages, but you could also argue that you've
> specifically asked for @size bytes and @size bytes only.
>
> So far it seems that users are well behaved in terms of using the amount
> of memory they ask for, but you'll get a better idea than me very
> quickly as I only tested with trinity. :)

Ack.
Let's try and see then. There is always an easy fix -- round up size
explicitly before vmalloc, which will make the code more explicit and
clear. I can hardly see any potential downsides for rounding up the
size explicitly.

> I also handle vmap - for vmap there's no way to specify sub-page
> allocations so you get as many pages as you ask for.
>
> > On impl side: kasan_unpoison_shadow seems to be capable of handling
> > non-KASAN_SHADOW_SCALE_SIZE-aligned sizes exactly in the way we want.
> > So I think it's better to do:
> >
> >        kasan_unpoison_shadow(area->addr, requested_size);
> >        requested_size = round_up(requested_size, KASAN_SHADOW_SCALE_SIZE);
> >        kasan_poison_shadow(area->addr + requested_size,
> >                            area->size - requested_size,
> >                            KASAN_VMALLOC_INVALID);
>
> Will do for v2.
>
> Regards,
> Daniel
>
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