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Subject: Re: [PATCH v2] lzo: fix ip overrun during compress.
To:     Kees Cook <keescook@chromium.org>,
        richard -rw- weinberger <richard.weinberger@gmail.com>,
        liyueyi@live.com
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From:   "Markus F.X.J. Oberhumer" <markus@oberhumer.com>
Organization: oberhumer.com
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Date:   Sun, 16 Dec 2018 17:56:01 +0100
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Yueyi,

if ASLR does indeed exclude the last page (like it should), how do
you get the invalid (0xfffffffffffff000, 4096) mapping then?

~Markus


On 2018-12-14 17:46, Kees Cook wrote:
> On Fri, Dec 14, 2018 at 5:56 AM Richard Weinberger
> <richard.weinberger@gmail.com> wrote:
>>
>> [CC'ing Kees]
>>
>> On Wed, Dec 12, 2018 at 1:37 PM Markus F.X.J. Oberhumer
>> <markus@oberhumer.com> 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@oberhumer.com>, http://www.oberhumer.com/
>>
>>
>>
>> --
>> Thanks,
>> //richard
> 
> 
> 

-- 
Markus Oberhumer, <markus@oberhumer.com>, http://www.oberhumer.com/