linux-kernel - Re: RFC: adding Linux vsyscall-disable and similar backwards-incompatibility flags to ELF headers?

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Message-ID: <CAMzpN2gQq7TbmjgHYut53gNTLCtVQ197akKZ69ZoM1BHHMo8cQ@mail.gmail.com>
Date:	Tue, 1 Sep 2015 21:53:12 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Kees Cook <keescook@...omium.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	libc-alpha <libc-alpha@...rceware.org>,
	"musl@...ts.openwall.com" <musl@...ts.openwall.com>,
	gcc@....gnu.org, Binutils <binutils@...rceware.org>
Subject: Re: RFC: adding Linux vsyscall-disable and similar
 backwards-incompatibility flags to ELF headers?

On Tue, Sep 1, 2015 at 8:51 PM, Andy Lutomirski <luto@...capital.net> 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.

--
Brian Gerst
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