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

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

[Andy Lutomirski]

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.

--Andy

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

[Ian Lance Taylor]

On Tue, Sep 1, 2015 at 5:51 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>
> 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.

What sets up the vsyscall page, and what information does it have
before doing so?

I'm guessing it's the kernel that sets it up, and that all it can see
at that point is the program headers.

We could pass information using an appropriate note section.  My
recollection is that the linkers will turn an SHF_ALLOC note section
into a PT_NOTE program header.

Ian

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

[Andy Lutomirski]

On Sep 1, 2015 6:12 PM, "Ian Lance Taylor" <iant@google.com> wrote:
>
> On Tue, Sep 1, 2015 at 5:51 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> >
> > 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.
>
> What sets up the vsyscall page, and what information does it have
> before doing so?
>
> I'm guessing it's the kernel that sets it up, and that all it can see
> at that point is the program headers.

Currently it's global and nothing thinks about it per-process at all.
The kernel can do whatever it likes going forward, subject to
backwards compatibility.  Doing something at ELF load time is probably
the right approach.

>
> We could pass information using an appropriate note section.  My
> recollection is that the linkers will turn an SHF_ALLOC note section
> into a PT_NOTE program header.

Oh, interesting.  I'll check that.  Glibc and competitors could add
notes to their statically-linked bits.

The unpleasant case is a new dynamic binary linked against an old
libc, but that might be irrelevant in practice.  After all, I think
that a lot of libc competitors never supported the vsyscall page at
all, and even glibc isn't really backwards compatible that way.

We could also require that both the binary and interpreter have the
note, which would more or less solve the backwards compatibility
issue.

--Andy

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

[Brian Gerst]

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

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

[Andy Lutomirski]

On Sep 1, 2015 6:53 PM, "Brian Gerst" <brgerst@gmail.com> wrote:
>
> On Tue, Sep 1, 2015 at 8:51 PM, Andy Lutomirski <luto@amacapital.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.

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.

--Andy

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

[Brian Gerst]

On Tue, Sep 1, 2015 at 10:21 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Sep 1, 2015 6:53 PM, "Brian Gerst" <brgerst@gmail.com> wrote:
>>
>> On Tue, Sep 1, 2015 at 8:51 PM, Andy Lutomirski <luto@amacapital.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.
>
> 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.

--
Brian Gerst

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

[Andy Lutomirski]

On Sep 2, 2015 6:57 AM, "Brian Gerst" <brgerst@gmail.com> wrote:
>
> On Tue, Sep 1, 2015 at 10:21 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> > On Sep 1, 2015 6:53 PM, "Brian Gerst" <brgerst@gmail.com> wrote:
> >>
> >> On Tue, Sep 1, 2015 at 8:51 PM, Andy Lutomirski <luto@amacapital.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.
> >
> > 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.

True, but I don't think that means that the mapping has to be readable
in all processes.  Once it's the only user-readable mapping in the top
512 GB, we can turn off user access to the whole top 512 GB.

The only other user accessible thing in the top 512GB (and the only
other user accessible thing in a kernel address at all) is the KVM
pvclock mapping.  We should turn that off, too, because it's
exploitable in more or less the same way as the vsyscall page.

--Andy

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

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

[Andy Lutomirski]

On Tue, Sep 1, 2015 at 7:54 PM, Rich Felker <dalias@libc.org> 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:

http://googleprojectzero.blogspot.com/2015/08/three-bypasses-and-fix-for-one-of.html

I'm also waiting for someone to find an exploit that uses one of the
vsyscalls as a ROP gadget.

>
> 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.

Unfortunately, I don't think this will work.  It's never been possible
to use the vsyscalls for anything other than gettimeofday, time, or
getcpu, so I doubt we can detect affected glibc versions that way.

--Andy

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

[Rich Felker]

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@libc.org> 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:
> 
> http://googleprojectzero.blogspot.com/2015/08/three-bypasses-and-fix-for-one-of.html

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.

> I'm also waiting for someone to find an exploit that uses one of the
> vsyscalls as a ROP gadget.

This sounds more plausible. gettimeofday actually writes to memory
pointed to by its arguments. The others look benign.

> > 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.
> 
> Unfortunately, I don't think this will work.  It's never been possible
> to use the vsyscalls for anything other than gettimeofday, time, or
> getcpu, so I doubt we can detect affected glibc versions that way.

I thought the idea of the old vsyscall was that you always call it
rather than using a syscall instruction and it decides whether it can
do it in userspace or needs to make a real syscall. But if it was only
called from certain places, then yes, I think you're right that my
approach doesn't work.

Rich

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

[Andy Lutomirski]

On Tue, Sep 1, 2015 at 9:18 PM, Rich Felker <dalias@libc.org> 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@libc.org> 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:
>>
>> http://googleprojectzero.blogspot.com/2015/08/three-bypasses-and-fix-for-one-of.html
>
> 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
to put a literal mov $NR, %rax; syscall; ret sequence at the syscall
address but to mark the whole page NX so that any attempt to call it
traps.  The trap gets fixed up if the call looks valid (properly
aligned, etc) and the process gets SIGSEGV if not.

This caught me by surprise when I implemented vsyscall emulation the first time.

>> I'm also waiting for someone to find an exploit that uses one of the
>> vsyscalls as a ROP gadget.
>
> This sounds more plausible. gettimeofday actually writes to memory
> pointed to by its arguments. The others look benign.
>
>> > 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.
>>
>> Unfortunately, I don't think this will work.  It's never been possible
>> to use the vsyscalls for anything other than gettimeofday, time, or
>> getcpu, so I doubt we can detect affected glibc versions that way.
>
> I thought the idea of the old vsyscall was that you always call it
> rather than using a syscall instruction and it decides whether it can
> do it in userspace or needs to make a real syscall. But if it was only
> called from certain places, then yes, I think you're right that my
> approach doesn't work.

No, it's actually just three separate functions, one for each of
gettimeofday, time, and getcpu.

--Andy

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

[Rich Felker]

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@libc.org> 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@libc.org> 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:
> >>
> >> http://googleprojectzero.blogspot.com/2015/08/three-bypasses-and-fix-for-one-of.html
> >
> > 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.

Rich

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

[Andy Lutomirski]

On Tue, Sep 1, 2015 at 9:55 PM, Rich Felker <dalias@libc.org> 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@libc.org> 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@libc.org> 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:
>> >>
>> >> http://googleprojectzero.blogspot.com/2015/08/three-bypasses-and-fix-for-one-of.html
>> >
>> > 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.

--Andy

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

[Rich Felker]

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@libc.org> 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@libc.org> 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@libc.org> 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:
> >> >>
> >> >> http://googleprojectzero.blogspot.com/2015/08/three-bypasses-and-fix-for-one-of.html
> >> >
> >> > 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
   deprecated.

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 libc.so.

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.

Rich

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

[Austin S Hemmelgarn]

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On 2015-09-02 00:32, Andy Lutomirski wrote:
> On Tue, Sep 1, 2015 at 9:18 PM, Rich Felker <dalias@libc.org> 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@libc.org> wrote:
>>>> 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.
>>>
>>> Unfortunately, I don't think this will work.  It's never been possible
>>> to use the vsyscalls for anything other than gettimeofday, time, or
>>> getcpu, so I doubt we can detect affected glibc versions that way.
>>
>> I thought the idea of the old vsyscall was that you always call it
>> rather than using a syscall instruction and it decides whether it can
>> do it in userspace or needs to make a real syscall. But if it was only
>> called from certain places, then yes, I think you're right that my
>> approach doesn't work.
>
> No, it's actually just three separate functions, one for each of
> gettimeofday, time, and getcpu.
Did the old versions of glibc always use vsyscall calling for these 
syscalls?  If they did, then we could (probably) safely disable the 
vsyscall stuff the first time we see any of these called through the 
normal syscall paths.



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