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Message-ID: <a29e8960-916b-8a5b-f8ed-ec040eddbbde@redhat.com>
Date: Tue, 25 Aug 2020 12:16:39 -0400
From: Joe Lawrence <joe.lawrence@...hat.com>
To: Kristen Carlson Accardi <kristen@...ux.intel.com>,
Josh Poimboeuf <jpoimboe@...hat.com>
Cc: Kees Cook <keescook@...omium.org>, Miroslav Benes <mbenes@...e.cz>,
tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, arjan@...ux.intel.com,
x86@...nel.org, linux-kernel@...r.kernel.org,
kernel-hardening@...ts.openwall.com, rick.p.edgecombe@...el.com,
live-patching@...r.kernel.org, Hongjiu Lu <hongjiu.lu@...el.com>
Subject: Re: [PATCH v4 00/10] Function Granular KASLR
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
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