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


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


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