linux-kernel - Re: [PATCH v4 0/4] SROP Mitigation: Sigreturn Cookies

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Message-ID: <87d1qafldd.fsf@x220.int.ebiederm.org>
Date:	Thu, 31 Mar 2016 15:22:38 -0500
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Scotty Bauer <sbauer@....utah.edu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"kernel-hardening\@lists.openwall.com" 
	<kernel-hardening@...ts.openwall.com>, X86 ML <x86@...nel.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, wmealing@...hat.com,
	criu@...nvz.org
Subject: Re: [PATCH v4 0/4] SROP Mitigation: Sigreturn Cookies


Cc' the Criu list to attempt to give them a heads up.

Scotty Bauer <sbauer@....utah.edu> writes:

> On 03/29/2016 04:34 PM, Linus Torvalds wrote:
>> On Tue, Mar 29, 2016 at 4:38 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>>
>>> Then there's an unanswered question: is this patch acceptable given
>>> that it's an ABI break?  Security fixes are sometimes an exception to
>>> the "no ABI breaks" rule, but it's by no means an automatic exception.
>> 
>> So there isn't any "no ABI break" rule - there is only a "does it
>> break real applications" rule.
>> 
>> (This can also be re-stated as: "Talk is cheap", aka "reality trumps
>> documentation".
>> 
>> Documentation is meaningless if it doesn't match reality, and what we
>> actually *do* is what matters.
>> 
>> So the ABI isn't about some theoretical interface documentation, the
>> ABI is about what people use and have tested.
>> 
>> On the one hand, that means that that our ABI is _stricter_ than any
>> documentatiuon, and that "but we can make this change that breaks app
>> XYZ, because XYZ is depending on undocumented behavior" is not an
>> acceptable excuse.
>> 
>> But on the other hand it *also* means that since the ABI is about real
>> programs, not theoretical issues, we can also change things as long as
>> we don't actually break anything that people can notice and depend
>> on).
>> 
>> And while *acute* security holes will be fixed regardless of ABI
>> issues, something like this that is only hardening rather than fixing
>> a particular security hole, really needs to not break any
>> applications.
>> 
>> Because if it does break anything, it needs to be turned off by
>> default. That's a hard rule. And since that would be largely defeating
>> the whole point o fthe series, I think we really need to have made
>> sure nothing breaks before a patch series like this can be accepted.
>> 
>> That said, if this is done right, I don't think it will break
>> anything. CRIU may indeed be a special case, but CRIU isn't really a
>> normal application, and the CRIU people may need to turn this off
>> explicitly, if it does break.
>> 
>> But yes, dosemu needs to be tested, and needs to just continue
>> working. But does dosemu actually create a signal stack, as opposed to
>> just playing with one that has been created for it? I thought it was
>> just the latter case, which should be ok even with a magic cookie in
>> there.
>> 
>>                    Linus
>> 
>
>
> For what it's worth this series is breaking CRIU, I just tested:
>
> root@...e0:/mnt/criu# criu restore -vvvv -o restore.log --shell-job
> root@...e0:/mnt/criu# tail -3 /var/log/syslog
> Mar 29 17:12:08 localhost kernel: [ 3554.625535] Possible exploit attempt or buggy program!
> Mar 29 17:12:08 localhost kernel: [ 3554.625535] If you believe this is an error you can disable SROP  Protection by #echo 1 > /proc/sys/kernel/disable-srop-protection
> Mar 29 17:12:08 localhost kernel: [ 3554.625545] test_[25305] bad frame in rt_sigreturn frame:000000000001e540 ip:7f561542cf20 sp:7ffe004ecfd8 orax:ffffffffffffffff in libc-2.19.so[7f561536c000+1bb0]
> root@...e0:/mnt/criu# echo 1 > /proc/sys/kernel/disable-srop-protection 
> root@...e0:/mnt/criu# criu restore -vvvv -o restore.log --shell-job
> slept for one second
> slept for one second
> slept for one second
> slept for one second
> root@...e0:/mnt/criu#

Which means that if checkpoint/restart is going to continue to be
something that is possible in Linux it should be possible to
save/restore the per process sig_cookie.  Perhaps with a prctl?

This should be addressed as part of this patchset as making that
information too easily accessible/changable will defeat the security
guarantees.  Making it too difficult to do destroys the ability to
migrate a process from one kernel to another.  As the existence of CRIU
attests it is desirable to have a checkpoint/restart capability in the
kernel.

> I'm working on getting dosemu up and running-- are there any other applications
> off the top of your head that I should be testing with?

There are a set of POSIX functions setcontext, getcontext, makecontext
and swapcontext that to the best of my knowledge deal in signal stacks.
Although I don't know that they use sigreturn.  Anything that makes use
of those is potentially affected.

Perhaps you can find binaries that care by looking for libraries and
executables that import those elf symbols.  glibc certainly provides
them.


Eric