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Re: [musl] RFC: adding Linux vsyscall-disable and similar backwards-incompatibility flags to ELF headers?


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

I'm also waiting for someone to find an exploit that uses one of the
vsyscalls as a ROP gadget.

>
> If this is not the case, I have what sounds like an elegant solution,
> if it works: presumably affected versions of glibc that used this used
> it for all syscalls, so if the process has made any normal syscalls
> before using the vsyscall addresses, you can assume it's a bug/attack
> and and just raise SIGSEGV. If there are corner cases this doesn't
> cover, maybe the approach can still be adapted to work; it's cleaner
> than introducing header cruft, IMO.

Unfortunately, I don't think this will work.  It's never been possible
to use the vsyscalls for anything other than gettimeofday, time, or
getcpu, so I doubt we can detect affected glibc versions that way.

--Andy


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