Re: [PATCH v4 00/10] Function Granular KASLR

[Joe Lawrence <joe.lawrence@redhat.com>]

On 8/21/20 7:02 PM, Kristen Carlson Accardi wrote:
> On Wed, 2020-07-22 at 16:33 -0500, Josh Poimboeuf wrote:
>> On Wed, Jul 22, 2020 at 12:56:10PM -0700, Kristen Carlson Accardi
>> wrote:
>>> On Wed, 2020-07-22 at 12:42 -0700, Kees Cook wrote:
>>>> On Wed, Jul 22, 2020 at 11:07:30AM -0500, Josh Poimboeuf wrote:
>>>>> On Wed, Jul 22, 2020 at 07:39:55AM -0700, Kees Cook wrote:
>>>>>> On Wed, Jul 22, 2020 at 11:27:30AM +0200, Miroslav Benes
>>>>>> wrote:
>>>>>>> Let me CC live-patching ML, because from a quick glance
>>>>>>> this is
>>>>>>> something
>>>>>>> which could impact live patching code. At least it
>>>>>>> invalidates
>>>>>>> assumptions
>>>>>>> which "sympos" is based on.
>>>>>>
>>>>>> In a quick skim, it looks like the symbol resolution is using
>>>>>> kallsyms_on_each_symbol(), so I think this is safe? What's a
>>>>>> good
>>>>>> selftest for live-patching?
>>>>>
>>>>> The problem is duplicate symbols.  If there are two static
>>>>> functions
>>>>> named 'foo' then livepatch needs a way to distinguish them.
>>>>>
>>>>> Our current approach to that problem is "sympos".  We rely on
>>>>> the
>>>>> fact
>>>>> that the second foo() always comes after the first one in the
>>>>> symbol
>>>>> list and kallsyms.  So they're referred to as foo,1 and foo,2.
>>>>
>>>> Ah. Fun. In that case, perhaps the LTO series has some solutions.
>>>> I
>>>> think builds with LTO end up renaming duplicate symbols like
>>>> that, so
>>>> it'll be back to being unique.
>>>>
>>>
>>> Well, glad to hear there might be some precendence for how to solve
>>> this, as I wasn't able to think of something reasonable off the top
>>> of
>>> my head. Are you speaking of the Clang LTO series?
>>> https://lore.kernel.org/lkml/20200624203200.78870-1-samitolvanen@google.com/
>>
>> I'm not sure how LTO does it, but a few more (half-brained) ideas
>> that
>> could work:
>>
>> 1) Add a field in kallsyms to keep track of a symbol's original
>> offset
>>     before randomization/re-sorting.  Livepatch could use that field
>> to
>>     determine the original sympos.
>>
>> 2) In fgkaslr code, go through all the sections and mark the ones
>> which
>>     have duplicates (i.e. same name).  Then when shuffling the
>> sections,
>>     skip a shuffle if it involves a duplicate section.  That way all
>> the
>>     duplicates would retain their original sympos.
>>
>> 3) Livepatch could uniquely identify symbols by some feature other
>> than
>>     sympos.  For example:
>>
>>     Symbol/function size - obviously this would only work if
>> duplicately
>>     named symbols have different sizes.
>>
>>     Checksum - as part of a separate feature we're also looking at
>> giving
>>     each function its own checksum, calculated based on its
>> instruction
>>     opcodes.  Though calculating checksums at runtime could be
>>     complicated by IP-relative addressing.
>>
>> I'm thinking #1 or #2 wouldn't be too bad.  #3 might be harder.
>>
> 
> Hi there! I was trying to find a super easy way to address this, so I
> thought the best thing would be if there were a compiler or linker
> switch to just eliminate any duplicate symbols at compile time for
> vmlinux. I filed this question on the binutils bugzilla looking to see
> if there were existing flags that might do this, but H.J. Lu went ahead
> and created a new one "-z unique", that seems to do what we would need
> it to do.
> 
> https://sourceware.org/bugzilla/show_bug.cgi?id=26391
> 
> When I use this option, it renames any duplicate symbols with an
> extension - for example duplicatefunc.1 or duplicatefunc.2.

I tried out H.J. Lu's branch and built some of the livepatch selftests 
with -z unique-symbol and indeed observe the following pattern:

  foo, foo.1, foo.2, etc.

for homonym symbol names.

 > You could
> either match on the full unique name of the specific binary you are
> trying to patch, or you match the base name and use the extension to
> determine original position. Do you think this solution would work? 

I think it could work for klp-relocations.

As a quick test, I was able to hack the WIP klp-convert branch [1] to 
generate klp-relocations with the following hack:

   const char *foo(void) __asm__("foo.1");

when building foo's target with -z unique-symbol.  (The target contained 
two static foo() functions.)  The asm rename trick exercised the 
klp-convert implicit conversion feature, as the symbol was now uniquely 
named and included a non-valid C symbol character.  User-defined 
klp-convert annotation support will require some refactoring, but 
shouldn't be too difficult to support as well.

> If
> so, I can modify livepatch to refuse to patch on duplicated symbols if
> CONFIG_FG_KASLR and when this option is merged into the tool chain I
> can add it to KBUILD_LDFLAGS when CONFIG_FG_KASLR and livepatching
> should work in all cases.
> 

I don't have a grasp on how complicated the alternatives might be, so 
I'll let others comment on best paths forward.  I just wanted to note 
that -z unique-symbol looks like it could reasonable work well for this 
niche case.

[1] 
https://github.com/joe-lawrence/linux/tree/klp-convert-v5-expanded-v5.8 
(not modified for -z unique-symbol, but noted for reference)

-- Joe