From: Andy Lutomirski <luto@amacapital.net>
To: Rich Felker <dalias@libc.org>
Cc: Kees Cook <keescook@chromium.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
libc-alpha <libc-alpha@sourceware.org>,
"musl@lists.openwall.com" <musl@lists.openwall.com>,
gcc@gcc.gnu.org, Binutils <binutils@sourceware.org>
Subject: Re: [musl] RFC: adding Linux vsyscall-disable and similar backwards-incompatibility flags to ELF headers?
Date: Tue, 1 Sep 2015 22:03:27 -0700 [thread overview]
Message-ID: <CALCETrWUk=A2fPE6a9njKFqNT9yYwmV45v1GQhcDkgw5M2QrGg@mail.gmail.com> (raw)
In-Reply-To: <20150902045523.GI17773@brightrain.aerifal.cx>
On Tue, Sep 1, 2015 at 9:55 PM, Rich Felker <dalias@libc.org> wrote:
> On Tue, Sep 01, 2015 at 09:32:22PM -0700, Andy Lutomirski wrote:
>> On Tue, Sep 1, 2015 at 9:18 PM, Rich Felker <dalias@libc.org> wrote:
>> > On Tue, Sep 01, 2015 at 08:39:27PM -0700, Andy Lutomirski wrote:
>> >> On Tue, Sep 1, 2015 at 7:54 PM, Rich Felker <dalias@libc.org> wrote:
>> >> > On Tue, Sep 01, 2015 at 05:51:44PM -0700, Andy Lutomirski wrote:
>> >> >> Hi all-
>> >> >>
>> >> >> Linux has a handful of weird features that are only supported for
>> >> >> backwards compatibility. The big one is the x86_64 vsyscall page, but
>> >> >> uselib probably belongs on the list, too, and we might end up with
>> >> >> more at some point.
>> >> >>
>> >> >> I'd like to add a way that new programs can turn these features off.
>> >> >> In particular, I want the vsyscall page to be completely gone from the
>> >> >> perspective of any new enough program. This is straightforward if we
>> >> >> add a system call to ask for the vsyscall page to be disabled, but I'm
>> >> >> wondering if we can come up with a non-syscall way to do it.
>> >> >>
>> >> >> I think that the ideal behavior would be that anything linked against
>> >> >> a sufficiently new libc would be detected, but I don't see a good way
>> >> >> to do that using existing toolchain features.
>> >> >>
>> >> >> Ideas? We could add a new phdr for this, but then we'd need to play
>> >> >> linker script games, and I'm not sure that could be done in a clean,
>> >> >> extensible way.
>> >> >
>> >> > Is there a practical problem you're trying to solve? My understanding
>> >> > is that the vsyscall nonsense is fully emulated now and that the ways
>> >> > it could be used as an attack vector have been mitigated.
>> >>
>> >> They've been mostly mitigated, but not fully. See:
>> >>
>> >> http://googleprojectzero.blogspot.com/2015/08/three-bypasses-and-fix-for-one-of.html
>> >
>> > That looks like it would be mitigated by not having any mapping there
>> > at all and having the kernel just catch the page fault and emulate
>> > rather than filling it with trapping opcodes for the kernel to catch.
>> >
>>
>> Oddly, that causes a compatibility problem. There's a program called
>> pin that does dynamic instrumentation and actually expects to be able
>> to read the targets of calls. The way that Linux handles this now is
>
> Um, do people seriously need to do this dynamic instrumentation on
> ancient obsolete binaries? This sounds to me like confused
> requirements.
Unclear. They certainly did, and I got a bug report, the first time
around. That was a couple years ago.
I suppose we could have a sysctl that you need to set to enable that
use case. OTOH, I think that, as long as we have a way to distinguish
new and old binaries, it's not that much harder to twiddle vsyscall
readability per process than it is to twiddle vsyscall executability
per process.
--Andy
next prev parent reply other threads:[~2015-09-02 5:03 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-09-02 0:51 Andy Lutomirski
2015-09-02 1:12 ` Ian Lance Taylor
2015-09-02 2:23 ` Andy Lutomirski
2015-09-02 1:53 ` Brian Gerst
2015-09-02 2:21 ` Andy Lutomirski
2015-09-02 13:57 ` Brian Gerst
2015-09-02 14:08 ` Andy Lutomirski
2015-09-02 2:54 ` [musl] " Rich Felker
2015-09-02 3:39 ` Andy Lutomirski
2015-09-02 4:18 ` Rich Felker
2015-09-02 4:32 ` Andy Lutomirski
2015-09-02 4:55 ` Rich Felker
2015-09-02 5:03 ` Andy Lutomirski [this message]
2015-09-02 5:22 ` Rich Felker
2015-09-02 12:48 ` Austin S Hemmelgarn
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