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SubjectRe: [PATCH v2] lzo: fix ip overrun during compress.
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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@oberhumer.com>, http://www.oberhumer.com/

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