Re: [Qemu-devel] [PATCH v1 2/2] s390x/tcg: Store only the necessary amount of doublewords for STFLE

[David Hildenbrand <david@redhat.com>]

On 03.06.19 12:38, Stefan Liebler wrote:
> On 5/31/19 4:56 PM, David Hildenbrand wrote:
>> The PoP (z14, 7-382) says:
>>      Doublewords to the right of the doubleword in which the
>>      highest-numbered facility bit is assigned for a model
>>      may or may not be stored.
>>
>> However, stack protection in certain binaries can't deal with that.
>> "gzip" example code:
>>
>> f1b4:       a7 08 00 03             lhi     %r0,3
>> f1b8:       b2 b0 f0 a0             stfle   160(%r15)
>> f1bc:       e3 20 f0 b2 00 90       llgc    %r2,178(%r15)
>> f1c2:       c0 2b 00 00 00 01       nilf    %r2,1
>> f1c8:       b2 4f 00 10             ear     %r1,%a0
>> f1cc:       b9 14 00 22             lgfr    %r2,%r2
>> f1d0:       eb 11 00 20 00 0d       sllg    %r1,%r1,32
>> f1d6:       b2 4f 00 11             ear     %r1,%a1
>> f1da:       d5 07 f0 b8 10 28       clc     184(8,%r15),40(%r1)
>> f1e0:       a7 74 00 06             jne     f1ec <file_read@@Base+0x1bc>
>> f1e4:       eb ef f1 30 00 04       lmg     %r14,%r15,304(%r15)
>> f1ea:       07 fe                   br      %r14
>> f1ec:       c0 e5 ff ff 9d 6e       brasl   %r14,2cc8 <__stack_chk_fail@plt>
>>
>> In QEMU, we currently have:
>>      max_bytes = 24
>> the code asks for (3 + 1) doublewords == 32 bytes.
>>
>> If we write 32 bytes instead of only 24, and return "2 + 1" doublewords
>> ("one less than the number of doulewords needed to contain all of the
>>   facility bits"), the example code detects a stack corruption.
>>
>> In my opinion, the code is wrong. However, it seems to work fine on
>> real machines. So let's limit storing to the minimum of the requested
>> and the maximum doublewords.
> Hi David,
> 
> Thanks for catching this. I've reported the "gzip" example to Ilya and 
> indeed, r0 is setup too large. He will fix it in gzip.
> 
> You've mentioned, that this is detected in certain binaries.
> Can you please share those occurrences.

Hi Stafan,

thanks for your reply.

I didn't track all occurrences, it *could* be that it was only gzip in
the background making other processes fail.

For example, the systemd "vitual console setup" unit failed, too, which
was fixed by this change.

I also remember, seeing segfaults in rpmbuild, for example. They only
"changed" with this fix - I m still chasing different errors. :)

You mentioned "He will fix it in gzip", so I assume this is a gzip issue
and not a gcc/glibc/whatever toolchain issue?

-- 

Thanks,

David / dhildenb