gold: addend issues in icf.cc:get_section_contents

[Sriraman Tallam]

Ping.

On Mon, Jan 25, 2016 at 10:34 AM, Sriraman Tallam <tmsriram@google.com> wrote:
> I have attached a patch that fixes the issue described below.
>
>
> * icf.cc (get_sht_rel_merge_section_reloc_addend): New function.
> (get_section_contents):  Move merge section addend computation to a
> new function.  Ignore negative values for SHT_REL and SHT_RELA addends.
> Fix bug to not read past the length of the section.
>
>
> Thanks
> Sri
>
>
> On Mon, Sep 21, 2015 at 5:00 PM, Roland McGrath <mcgrathr@google.com> wrote:
>> I have a case where trunk gold crashes.  It's a huge case (linking Google
>> Chrome) and I don't know off hand how to go about reducing it.  But I've
>> isolated the crash to some code where things look all kinds of fishy to me.
>> I can describe in detail how things are going wrong, but I don't really
>> have any idea what would constitute things going right in this code.
>>
>> The crash happens inside get_section_contents in icf.cc.  The scenario is
>> that 'str_contents' (meant to be a pointer into section contents) has been
>> set to a wildly invalid pointer, and then we get to:
>>
>>                       // Use the entsize to determine the length.
>>                       buffer.append(reinterpret_cast<const
>>                                                      char*>(str_contents),
>>                                     entsize);
>>
>> so the crash happens inside memcpy inside std::string::append.
>>
>> Basically, all of the code inside:
>>
>>               // This reloc points to a merge section.  Hash the
>>               // contents of this section.
>>               if ((secn_flags & elfcpp::SHF_MERGE) != 0
>>                   && parameters->target().can_icf_inline_merge_sections())
>>                 {
>>
>> looks kind of fishy to me.
>>
>> It does various things to calculate 'offset', and then does:
>>
>>                   section_size_type secn_len;
>>                   const unsigned char* str_contents =
>>                   (it_v->first)->section_contents(it_v->second,
>>                                                   &secn_len,
>>                                                   false) + offset;
>>
>> So clearly both 'offset', and 'offset + N' where N is however many bytes
>> the code actually fetches from 'str_contents' must be within 'secn_len'.
>> But I see a woeful lack of any code that would give me confidence this
>> invariant is being maintained.
>>
>> As a first sanity check, I inserted:
>>                      gold_assert(offset < (long long) secn_len);
>> right after the ->section_contents call quoted above.  That alone would
>> catch my crasher case (more on that below).  But I wanted to do a little
>> more sanity checking and tried:
>>                     {
>> +                     gold_assert(secn_len >= entsize);
>> +                     gold_assert((long long) (secn_len - entsize) >= offset);
>>                       // Use the entsize to determine the length.
>>                       buffer.append(reinterpret_cast<const
>>                                                      char*>(str_contents),
>>                                     entsize);
>>                     }
>> The two gold_assert's there are my additions.  However, the second assert
>> fires in cases other than my crasher.  Unless I'm thoroughly confused,
>> those cases are wrong too even though they don't crash.  It should never be
>> OK to access past str_contents-offset+secn_len, right?
>>
>> My case is an i386 link (on an x86_64 host), so addresses and addends and
>> everything are 32-bit quantities.  In the case that crashes, the value of
>> 'reloc_addend_value' extracted from the section contents is 0xfffffffc.
>> This should be interpreted as -4, but:
>>                           reloc_addend_value =
>>                             read_from_pointer<32>(reloc_addend_ptr);
>> extracts it as 32 bits and then zero-extends it to 64 bits.  Naively, I
>> want to just sign-extend here.  But that doesn't actually make any sense in
>> the context of how this value is used.  Nearby, for the RELA case we have:
>>
>>                   unsigned long long addend = it_a->second;
>>                   // Ignoring the addend when it is a negative value.  See the
>>                   // comments in Merged_symbol_value::Value in object.h.
>>                   if (addend < 0xffffff00)
>>                     offset = offset + addend;
>>
>> I had trouble finding the comment it refers to until I realized that it's
>> actually in Merged_symbol_value::value (lower case), the method, not
>> Merged_symbol_value::Value (capitalized), which is a type.  I can almost
>> follow the logic in that comment, though not quite.  But anyway, it seems
>> pretty clear that the same logic for treating addend values should apply in
>> both RELA and REL cases.  So I change:
>> +                 if (reloc_addend_value < 0xffffff00)
>>                     offset = offset + reloc_addend_value;
>> and at least the crash goes away.  But I have no idea if things are
>> actually coming out correct now.
>>
>> If that is the "correct" fix, then I think it would certainly be cleaner to
>> separate this logic from the collection of the addend value.  Collecting
>> the addend value is done differently for RELA vs REL, but the logic applied
>> to that value is the same either way.  But I don't know enough of the
>> context here to be confident about how to refactor that correctly.  (Is
>> the '*it_addend_size == 0' case for RELA, or something else?  Is there ever
>> a case when 'addend' (aka 'it_a->second') is nonzero and '*it_addend_size'
>> is also nonzero?)
>>
>> To what little extent I understand the context here, I think things being
>> wrong in this code is most likely to just cause ICF to miss a folding
>> opportunity.  So it might not really be noticed otherwise.  This makes me
>> think this whole function needs a careful audit by someone who thoroughly
>> understands its context (which I do not).
>>
>> What is most troubling to me is that there appears to be no input
>> validation being done here at all.  Even if an addend value is not huge
>> (e.g. because it's really negative, as in my crasher) then you can't just
>> blithely assume that it's a valid offset within the section from which to
>> read 'entsize' bytes of data (or worse, a null-terminated string for the
>> SHF_STRINGS case).
>>
>> Is there someone who groks this code and can tackle this mess?
>>
>>
>> Thanks,
>> Roland