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Date: Tue, 1 Sep 2015 19:21:01 -0700
From: Andy Lutomirski <>
To: Brian Gerst <>
Cc: "" <>, Kees Cook <>,, 
	libc-alpha <>, 
	"" <>, Binutils <>
Subject: Re: RFC: adding Linux vsyscall-disable and similar
 backwards-incompatibility flags to ELF headers?

On Sep 1, 2015 6:53 PM, "Brian Gerst" <> wrote:
> On Tue, Sep 1, 2015 at 8:51 PM, 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.
> The vsyscall page is mapped in the fixmap region, which is shared
> between all processes.  You can't turn it off for an individual
> process.

Why not?

We already emulate all attempts to execute it, and that's trivial to
turn of per process.  Project Zero pointed out that read access is a
problem, too, but we can flip the U/S bit in the pgd once we evict
pvclock from the fixmap.

And we definitely need to evict pvclock from the fixmap regardless.


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