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

[Richard Weinberger]

[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:
> >>>
> >>> 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;
> >>>>                   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