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Date: Wed, 26 Jun 2019 08:21:05 -0700
From: Andy Lutomirski <>
To: Florian Weimer <>
Cc: Andy Lutomirski <>,
 Thomas Gleixner <>,
 Linux API <>,
 Kernel Hardening <>,, linux-arch <>,
 Kees Cook <>, Carlos O'Donell <>,
 X86 ML <>
Subject: Re: Detecting the availability of VSYSCALL

> On Jun 26, 2019, at 8:00 AM, Florian Weimer <> wrote:
> * Andy Lutomirski:
>> I didn’t add a flag because the vsyscall page was thoroughly obsolete
>> when all this happened, and I wanted to encourage all new code to just
>> parse the vDSO instead of piling on the hacks.
> It turned out that the thorny cases just switched to system calls
> instead.  I think we finally completed the transition in glibc upstream
> in 2018 (for x86).
>> Anyway, you may be the right person to ask: is there some credible way
>> that the kernel could detect new binaries that don’t need vsyscalls?
>> Maybe a new ELF note on a static binary or on the ELF interpreter? We
>> can dynamically switch it in principle.
> For this kind of change, markup similar to PT_GNU_STACK would have been
> appropriate, I think: Old kernels and loaders would have ignored the
> program header and loaded the program anyway, but the vsyscall page
> still existed, so that would have been fine. The kernel would have
> needed to check the program interpreter or the main executable (without
> a program interpreter, i.e., the statically linked case).  Due the way
> the vsyscalls are concentrated in glibc, a dynamically linked executable
> would not have needed checking (or re-linking).  I don't think we would
> have implemented the full late enablement after dlopen we did for
> executable stacks.  In theory, any code could have jumped to the
> vsyscall area, but in practice, it's just dynamically linked glibc and
> static binaries.
> But nowadays, unmarked glibcs which do not depend on vsyscall vastly
> outnumber unmarked glibcs which requrie it.  Therefore, markup of
> binaries does not seem to be reasonable to day.  I could imagine a
> personality flag you can set (if yoy have CAP_SYS_ADMIN) that re-enables
> vsyscall support for new subprocesses.  And a container runtime would do
> this based on metadata found in the image.  This way, the container host
> itself could be protected, and you could still run legacy images which
> require vsyscall.
> For the non-container case, if you know that you'll run legacy
> workloads, you'd still have the boot parameter.  But I think it could
> default to vsyscall=none in many more cases.

I’m wondering if we can still do it: add a note or other ELF indicator that says “I don’t need vsyscalls.”  Then we can change the default mode to “no vsyscalls if the flag is there, else execute-only vsyscalls”.

Would glibc go along with this?  Would enterprise distros consider backporting such a thing?


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