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

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Date:   Wed, 12 Dec 2018 05:21:29 +0000
From:   Yueyi Li <liyueyi@...e.com>
To:     "Markus F.X.J. Oberhumer" <markus@...rhumer.com>,
        "dsterba@...e.cz" <dsterba@...e.cz>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "w@....eu" <w@....eu>,
        "donb@...uritymouse.com" <donb@...uritymouse.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] lzo: fix ip overrun during compress.

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
>>