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Re: RFC: adding Linux vsyscall-disable and similar backwards-incompatibility flags to ELF headers?
- From: Brian Gerst <brgerst at gmail dot com>
- To: Andy Lutomirski <luto at amacapital dot net>
- Cc: "musl at lists dot openwall dot com" <musl at lists dot openwall dot com>, Kees Cook <keescook at chromium dot org>, gcc at gcc dot gnu dot org, libc-alpha <libc-alpha at sourceware dot org>, "linux-kernel at vger dot kernel dot org" <linux-kernel at vger dot kernel dot org>, Binutils <binutils at sourceware dot org>
- Date: Wed, 2 Sep 2015 09:57:26 -0400
- Subject: Re: RFC: adding Linux vsyscall-disable and similar backwards-incompatibility flags to ELF headers?
- Authentication-results: sourceware.org; auth=none
- References: <CALCETrUzU5UVe_2eWuMCOgHTs=5mnor5m0RO0STTi3K5FzdNvQ at mail dot gmail dot com> <CAMzpN2gQq7TbmjgHYut53gNTLCtVQ197akKZ69ZoM1BHHMo8cQ at mail dot gmail dot com> <CALCETrUO0VJ_BO+3gRQvfHq=MkNVeip55N5JZ9vcoxGaen_VoQ at mail dot gmail dot com>
On Tue, Sep 1, 2015 at 10:21 PM, Andy Lutomirski <firstname.lastname@example.org> wrote:
> On Sep 1, 2015 6:53 PM, "Brian Gerst" <email@example.com> wrote:
>> On Tue, Sep 1, 2015 at 8:51 PM, Andy Lutomirski <firstname.lastname@example.org> 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
> 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.
Sure, you can turn off emulation per-process. But the page mapping
will be the same for every process because it is in the kernel part of
the page tables which is shared by all processes.