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


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.

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

This sounds more plausible. gettimeofday actually writes to memory
pointed to by its arguments. The others look benign.

> > 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.

I thought the idea of the old vsyscall was that you always call it
rather than using a syscall instruction and it decides whether it can
do it in userspace or needs to make a real syscall. But if it was only
called from certain places, then yes, I think you're right that my
approach doesn't work.

Rich


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