Re: [PATCH v2] lzo: fix ip overrun during compress.

[Kees Cook]

On Fri, Dec 14, 2018 at 5:56 AM Richard Weinberger
<richard.weinberger@xxxxxxxxx> wrote:
>
> [CC'ing Kees]
>
> On Wed, Dec 12, 2018 at 1:37 PM Markus F.X.J. Oberhumer
> <markus@xxxxxxxxxxxxx> wrote:
> >
> > I still claim that (0xfffffffffffff000, 4096) is not a valid C "pointer
> > to an object" according to the C standard - please see my reply below.
> >
> > And I thought ASLR was introduced to improve security and not to create
> > new security problems - someone from the ASLR team has to comment on this.
> >
> > Cheers,
> > Markus
> >
> >
> > On 2018-12-12 06:21, Yueyi Li wrote:
> > > Hi Markus,
> > >
> > > OK, thanks. I`ll change it in v3.
> > >
> > > Thanks,
> > > Yueyi
> > >
> > > On 2018/12/6 23:03, Markus F.X.J. Oberhumer wrote:
> > >> Hi Yueyi,
> > >>
> > >> yes, my LZO patch works for all cases.
> > >>
> > >> The reason behind the issue in the first place is that if KASLR
> > >> includes the very last page 0xfffffffffffff000 then we do not have a
> > >> valid C "pointer to an object" anymore because of address wraparound.
> > >>
> > >> Unrelated to my patch I'd argue that KASLR should *NOT* include the
> > >> very last page - indeed that might cause similar wraparound problems
> > >> in lots of code.
> > >>
> > >> Eg, look at this simple clear_memory() implementation:
> > >>
> > >> void clear_memory(char *p, size_t len) {
> > >>          char *end = p + len;
> > >>          while (p < end)
> > >>                  *p++= 0;
> > >> }
> > >>
> > >> Valid code like this will fail horribly when (p, len) is the very
> > >> last virtual page (because end will be the NULL pointer in this case).
> > >>
> > >> Cheers,
> > >> Markus
> > >>
> > >>
> > >>
> > >> On 2018-12-05 04:07, Yueyi Li wrote:
> > >>> Hi Markus,
> > >>>
> > >>> Thanks for your review.
> > >>>
> > >>> On 2018/12/4 18:20, Markus F.X.J. Oberhumer wrote:
> > >>>> Hi,
> > >>>>
> > >>>> I don't think that address space wraparound is legal in C, but I
> > >>>> understand that we are in kernel land and if you really want to
> > >>>> compress the last virtual page 0xfffffffffffff000 the following
> > >>>> small patch should fix that dubious case.
> > >>> I guess the VA 0xfffffffffffff000 is available because KASLR be
> > >>> enabled. For this case we can see:

This is a weird case: I would expect the top 4k to be unmapped to
leave room of ERR_PTR, etc.

> > >>>
> > >>> crash> kmem 0xfffffffffffff000
> > >>>         PAGE               PHYSICAL      MAPPING       INDEX CNT FLAGS
> > >>> ffffffbfffffffc0        1fffff000 ffffffff1655ecb9  7181fd5  2
> > >>> 8000000000064209 locked,uptodate,owner_priv_1,writeback,reclaim,swapbacked
> > >>>
> > >>>> This also avoids slowing down the the hot path of the compression
> > >>>> core function.
> > >>>>
> > >>>> Cheers,
> > >>>> Markus
> > >>>>
> > >>>>
> > >>>>
> > >>>> diff --git a/lib/lzo/lzo1x_compress.c b/lib/lzo/lzo1x_compress.c
> > >>>> index 236eb21167b5..959dec45f6fe 100644
> > >>>> --- a/lib/lzo/lzo1x_compress.c
> > >>>> +++ b/lib/lzo/lzo1x_compress.c
> > >>>> @@ -224,8 +224,8 @@ int lzo1x_1_compress(const unsigned char *in, size_t in_len,
> > >>>>
> > >>>>           while (l > 20) {
> > >>>>                   size_t ll = l <= (M4_MAX_OFFSET + 1) ? l : (M4_MAX_OFFSET + 1);
> > >>>> -               uintptr_t ll_end = (uintptr_t) ip + ll;
> > >>>> -               if ((ll_end + ((t + ll) >> 5)) <= ll_end)
> > >>>> +               // check for address space wraparound
> > >>>> +               if (((uintptr_t) ip + ll + ((t + ll) >> 5)) <= (uintptr_t) ip)
> > >>>>                           break;

Please just use the standard add overflow checks from the kernel. See
include/linux/overflow.h

Specifically, check_add_overflow(operand1, operand2, &result). I
assume something like:

if (check_add_overflow(ip, ll, &ll_end))
   freak_out();

?

> > >>>>                   BUILD_BUG_ON(D_SIZE * sizeof(lzo_dict_t) > LZO1X_1_MEM_COMPRESS);
> > >>>>                   memset(wrkmem, 0, D_SIZE * sizeof(lzo_dict_t));
> > >>> I parsed panic ramdump and loaded CPU register values,  can see:
> > >>>
> > >>> -000|lzo1x_1_do_compress(
> > >>>       |    in = 0xFFFFFFFFFFFFF000,
> > >>>       |  ?,
> > >>>       |    out = 0xFFFFFFFF2E2EE000,
> > >>>       |    out_len = 0xFFFFFF801CAA3510,
> > >>>       |  ?,
> > >>>       |    wrkmem = 0xFFFFFFFF4EBC0000)
> > >>>       |  dict = 0xFFFFFFFF4EBC0000
> > >>>       |  op = 0x1
> > >>>       |  ip = 0x9
> > >>>       |  ii = 0x9
> > >>>       |  in_end = 0x0
> > >>>       |  ip_end = 0xFFFFFFFFFFFFFFEC
> > >>>       |  m_len = 0
> > >>>       |  m_off = 1922
> > >>> -001|lzo1x_1_compress(
> > >>>       |    in = 0xFFFFFFFFFFFFF000,
> > >>>       |    in_len = 0,
> > >>>       |    out = 0xFFFFFFFF2E2EE000,
> > >>>       |    out_len = 0x00000001616FB7D0,
> > >>>       |    wrkmem = 0xFFFFFFFF4EBC0000)
> > >>>       |  ip = 0xFFFFFFFFFFFFF000
> > >>>       |  op = 0xFFFFFFFF2E2EE000
> > >>>       |  l = 4096
> > >>>       |  t = 0
> > >>>       |  ll = 4096
> > >>>
> > >>> ll = l = in_len = 4096 in lzo1x_1_compress,  so your patch is working
> > >>> for this panic case, but, I`m
> > >>> not sure, is it possible that in = 0xFFFFFFFFFFFFF000 and  in_len < 4096?
> > >>>
> > >>>
> > >>> Thanks,
> > >>> Yueyi
> > >>>
> > >
> > >
> >
> > --
> > Markus Oberhumer, <markus@xxxxxxxxxxxxx>, http://www.oberhumer.com/
>
>
>
> --
> Thanks,
> //richard



-- 
Kees Cook