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Date: Thu, 13 Jan 2022 18:27:15 +0100
From: Florian Weimer <>
To: "Andy Lutomirski" <>
Cc:,  "Linux API" <>,,,,  "the arch/x86 maintainers" <>,,  <>,
  <>,  "Dave Hansen" <>,
  "Kees Cook" <>,  Andrei Vagin <>
Subject: Re: [PATCH v3 1/3] x86: Implement arch_prctl(ARCH_VSYSCALL_CONTROL)
 to disable vsyscall

* Florian Weimer:

> Distributions struggle with changing the default for vsyscall
> emulation because it is a clear break of userspace ABI, something
> that should not happen.
> The legacy vsyscall interface is supposed to be used by libcs only,
> not by applications.  This commit adds a new arch_prctl request,
> ARCH_VSYSCALL_CONTROL, with one argument.  If the argument is 0,
> executing vsyscalls will cause the process to terminate.  Argument 1
> turns vsyscall back on (this is mostly for a largely theoretical
> CRIU use case).
> Newer libcs can use a zero ARCH_VSYSCALL_CONTROL at startup to disable
> vsyscall for the process.  Legacy libcs do not perform this call, so
> vsyscall remains enabled for them.  This approach should achieves
> backwards compatibility (perfect compatibility if the assumption that
> only libcs use vsyscall is accurate), and it provides full hardening
> for new binaries.
> The chosen value of ARCH_VSYSCALL_CONTROL should avoid conflicts
> with other x86-64 arch_prctl requests.  The fact that with
> vsyscall=emulate, reading the vsyscall region is still possible
> even after a zero ARCH_VSYSCALL_CONTROL is considered limitation
> in the current implementation and may change in a future kernel
> version.
> Future arch_prctls requests commonly used at process startup can imply
> ARCH_VSYSCALL_CONTROL with a zero argument, so that a separate system
> call for disabling vsyscall is avoided.
> Signed-off-by: Florian Weimer <>
> Acked-by: Andrei Vagin <>
> ---
> v3: Remove warning log message.  Split out test.
>     for the toggle behavior.  Implement hiding [vsyscall] in
>     /proc/PID/maps and test it.  Various other test fixes cleanups
>     (e.g., fixed missing second argument to gettimeofday).
> arch/x86/entry/vsyscall/vsyscall_64.c | 7 ++++++-
>  arch/x86/include/asm/mmu.h            | 6 ++++++
>  arch/x86/include/uapi/asm/prctl.h     | 2 ++
>  arch/x86/kernel/process_64.c          | 7 +++++++
>  4 files changed, 21 insertions(+), 1 deletion(-)


sorry to bother you again.  What can I do to move this forward?


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