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

From: Rich Felker
Date: Wed Sep 02 2015 - 01:22:19 EST

On Tue, Sep 01, 2015 at 10:03:27PM -0700, Andy Lutomirski wrote:
> On Tue, Sep 1, 2015 at 9:55 PM, Rich Felker <dalias@xxxxxxxx> wrote:
> > On Tue, Sep 01, 2015 at 09:32:22PM -0700, Andy Lutomirski wrote:
> >> On Tue, Sep 1, 2015 at 9:18 PM, Rich Felker <dalias@xxxxxxxx> wrote:
> >> > 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@xxxxxxxx> 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:
> >> >>
> >> >>
> >> >
> >> > 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.
> >> >
> >>
> >> Oddly, that causes a compatibility problem. There's a program called
> >> pin that does dynamic instrumentation and actually expects to be able
> >> to read the targets of calls. The way that Linux handles this now is
> >
> > Um, do people seriously need to do this dynamic instrumentation on
> > ancient obsolete binaries? This sounds to me like confused
> > requirements.
> Unclear. They certainly did, and I got a bug report, the first time
> around. That was a couple years ago.
> I suppose we could have a sysctl that you need to set to enable that
> use case. OTOH, I think that, as long as we have a way to distinguish
> new and old binaries, it's not that much harder to twiddle vsyscall
> readability per process than it is to twiddle vsyscall executability
> per process.

But we don't have a (reasonable) way to distinguish new and old
binaries, at least not at the right point in history. If we're adding
a new header or whatnot, only bleeding-edge binaries will benefit from
it. All existing binaries from the past N years that don't need the
vsyscall nonsense will still get it unnecessarily, and still be
subject to the risks.

This has me wondering if there's any point in trying to solve the
problem on the granularity of individual programs. Users running
all-new binaries (that would benefit from a header flag) can just
remove vsyscall support entirely from their kernels. Users with a mix
binaries of various ages will likely still have the vsyscall risk in
most programs, _including_ many newer binaries that have no use for
vsyscall but lack the new header.

BTW, since the only calls to vsyscall are from glibc, it seems to me
that the only ways vsyscall can be needed are:

1. The user is running old glibc, in which case all dynamic-linked
programs need it.

2. The user is running old static-linked glibc binaries. Almost nobody
does this. During the era of vsyscall, static linking was all but

3. The user is running old binaries using a custom library path with
old glibc in it. This is almost certainly just a bogus setup since
glibc's symbol versioning is supposed to make old binaries run fine
with a newer

None of these seem to be use cases that we should be engineering
complex solutions for. For case 1, the solution wouldn't help anyway
since all programs need vsyscall. For cases 2 and 3, if the user wants
to harden their system so that newer binaries are not affected by
vsyscall, they should just remove vsyscall and fix their old
binaries/libraries. In case 2, in particular, you can assume the
ability to re-link with an updated glibc; otherwise, there's an LGPL
violation going on.

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