[kernel-hardening] Re: [RFC][PATCH v2 6/4] mm: disallow user copy to/from separately allocated pages

From: Rik van Riel <riel@redhat.com>
To: Kees Cook <keescook@chromium.org>
Cc: "kernel-hardening@lists.openwall.com"
	<kernel-hardening@lists.openwall.com>,
	Brad Spengler <spender@grsecurity.net>,
	PaX Team <pageexec@freemail.hu>,
	Casey Schaufler <casey.schaufler@intel.com>,
	Christoph Lameter <cl@linux.com>,
	Pekka Enberg <penberg@kernel.org>,
	David Rientjes <rientjes@google.com>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: [kernel-hardening] Re: [RFC][PATCH v2 6/4] mm: disallow user copy to/from separately allocated pages
Date: Fri, 24 Jun 2016 16:57:27 -0400	[thread overview]
Message-ID: <1466801847.22723.5.camel@redhat.com> (raw)
In-Reply-To: <CAGXu5j+BvZTNW=CqrEHNMiTvwpRvmLsSBKpKHd++woFpO0JBWg@mail.gmail.com>

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On Fri, 2016-06-24 at 13:53 -0700, Kees Cook wrote:
> On Fri, Jun 10, 2016 at 12:44 PM, Rik van Riel <riel@redhat.com>
> wrote:
> > 
> > v2 of yesterday's patch, this one seems to completely work on my
> > system, after taking _bdata & _edata into account.
> > 
> > I am still looking into CMA, but am leaning towards doing that as
> > a follow-up patch.
> > 
> > ---8<---
> > 
> > Subject: mm: disallow user copy to/from separately allocated pages
> > 
> > A single copy_from_user or copy_to_user should go to or from a
> > single
> > kernel object. Inside the slab, or on the stack, we can track the
> > individual objects.
> > 
> > For the general kernel heap, we do not know exactly where each
> > object
> > is, but we can tell whether the whole range from ptr to ptr + n is
> > inside the same page, or inside the same compound page.
> > 
> > If the start and end of the "object" are in pages that were not
> > allocated
> > together, we are likely dealing with an overflow from one object
> > into
> > the next page, and should disallow this copy.
> > 
> > Signed-off-by: Rik van Riel <riel@redhat.com>
> > ---
> > v2:
> >  - also test against _bdata & _edata, this appears to be necessary
> > for
> >    some kernfs/sysfs stuff
> >  - clean up the code a little bit
> > 
> >  mm/usercopy.c | 27 +++++++++++++++++++++++----
> >  1 file changed, 23 insertions(+), 4 deletions(-)
> > 
> > diff --git a/mm/usercopy.c b/mm/usercopy.c
> > index e09c33070759..78869ea73194 100644
> > --- a/mm/usercopy.c
> > +++ b/mm/usercopy.c
> > @@ -109,7 +109,8 @@ static inline bool
> > check_kernel_text_object(const void *ptr, unsigned long n)
> > 
> >  static inline const char *check_heap_object(const void *ptr,
> > unsigned long n)
> >  {
> > -       struct page *page;
> > +       struct page *page, *endpage;
> > +       const void *end = ptr + n - 1;
> > 
> >         if (ZERO_OR_NULL_PTR(ptr))
> >                 return "<null>";
> > @@ -118,11 +119,29 @@ static inline const char
> > *check_heap_object(const void *ptr, unsigned long n)
> >                 return NULL;
> > 
> >         page = virt_to_head_page(ptr);
> > -       if (!PageSlab(page))
> > +       if (PageSlab(page))
> > +               /* Check allocator for flags and size. */
> > +               return __check_heap_object(ptr, n, page);
> > +
> > +       /* Is the object wholly within one base page? */
> > +       if (likely(((unsigned long)ptr & (unsigned long)PAGE_MASK)
> > ==
> > +                  ((unsigned long)end & (unsigned
> > long)PAGE_MASK)))
> > +               return NULL;
> > +
> > +       /* Are the start and end inside the same compound page? */
> > +       endpage = virt_to_head_page(end);
> > +       if (likely(endpage == page))
> > +               return NULL;
> > +
> > +       /* Is this a special area, eg. .rodata, .bss, or device
> > memory? */
> > +       if (ptr >= (const void *)_sdata && end <= (const void
> > *)_edata)
> > +               return NULL;
> > +
> > +       if (PageReserved(page) && PageReserved(endpage))
> >                 return NULL;
> Shouldn't PageReserved cover the .data, .rodata, and .bss areas
> already? Is the concern for the check being added here that a copy
> might span multiple pages and that they're not allocated together
> when
> laying out the kernel data regions?

Having just the PageReserved check was not enough, I had
to specifically add the _sdata & _edata test to get things
to work.

It looks like PageReserved is simply not set in some cases,
perhaps we are mapping those kernel sections with huge pages,
but not setting up the compound page stuff for the backing
pages, since they never pass through the buddy allocator?

-- 
All Rights Reversed.

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