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

[Rich Felker]

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.

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.

Rich
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