musl - Re: [PATCH v3 1/3] x86: Implement arch_prctl(ARCH_VSYSCALL

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Date: Thu, 13 Jan 2022 18:27:15 +0100
From: Florian Weimer <fweimer@...hat.com>
To: "Andy Lutomirski" <luto@...nel.org>
Cc: linux-arch@...r.kernel.org,  "Linux API" <linux-api@...r.kernel.org>,
  linux-x86_64@...r.kernel.org,  kernel-hardening@...ts.openwall.com,
  linux-mm@...ck.org,  "the arch/x86 maintainers" <x86@...nel.org>,
  musl@...ts.openwall.com,  <libc-alpha@...rceware.org>,
  <linux-kernel@...r.kernel.org>,  "Dave Hansen" <dave.hansen@...el.com>,
  "Kees Cook" <keescook@...omium.org>,  Andrei Vagin <avagin@...il.com>
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 <fweimer@...hat.com>
> Acked-by: Andrei Vagin <avagin@...il.com>
> ---
> v3: Remove warning log message.  Split out test.
> v2: ARCH_VSYSCALL_CONTROL instead of ARCH_VSYSCALL_LOCKOUT.  New tests
>     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(-)

Hello,

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

Thanks,
Florian

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