linux-kernel - Re: [PATCH] x86/vsyscall/64: Drop "native" vsyscalls

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Date:   Wed, 7 Mar 2018 20:31:10 +0100
From:   Dominik Brodowski <linux@...inikbrodowski.net>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH] x86/vsyscall/64: Drop "native" vsyscalls

On Wed, Mar 07, 2018 at 11:21:46AM -0800, Kees Cook wrote:
> On Wed, Mar 7, 2018 at 11:12 AM, Andy Lutomirski <luto@...nel.org> wrote:
> > Since Linux 3.2, vsyscalls have been deprecated and slow.  From 3.2
> > on, Linux had three vsyscall modes: "native", "emulate", and "none".
> >
> > "emulate" is the default.  All known user programs work correctly in
> > emulate mode, but vsyscalls turn into page faults and are emulated.
> > This is very slow.  In "native" mode, the vsyscall page is easily
> > usable as an exploit gadget, but vsyscalls are a bit faster -- they
> > turn into normal syscalls.  (This is in contrast to vDSO functions,
> > which can be much faster than syscalls.)  In "none" mode, there are
> > no vsyscalls.
> >
> > For all practical purposes, "native" was really just a chicken bit
> > in case something went wrong with the emulation.  It's been over six
> > years, and nothing has gone wrong.  Delete it.
> >
> > Signed-off-by: Andy Lutomirski <luto@...nel.org>
> 
> Acked-by: Kees Cook <keescook@...omium.org>
> 
> I related news, I wonder how long before we can switch from EMULATE to
> NONE as the default? glibc 2.15 (which stopped using vsyscall) is (not
> coincidentally) 6 years old too...

Or, in the meantime, add a warning

        if (vsyscall_mode == EMULATE) {
                warn_bad_vsyscall(KERN_INFO, regs,
                                  "vsyscalls are deprecated -- use vDSO instead");
        }

Otherwise, the patch looks good.

Thanks,
	Dominik