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From:   Andy Lutomirski <luto@kernel.org>
Date:   Fri, 18 Sep 2020 17:11:50 -0700
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Subject: Re: [PATCH v12 8/8] x86: Disallow vsyscall emulation when CET is enabled
To:     Yu-cheng Yu <yu-cheng.yu@intel.com>
Cc:     X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
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On Fri, Sep 18, 2020 at 12:23 PM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
>
> Emulation of the legacy vsyscall page is required by some programs
> built before 2013.  Newer programs after 2013 don't use it.
> Disable vsyscall emulation when Control-flow Enforcement (CET) is
> enabled to enhance security.
>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> ---
> v12:
> - Disable vsyscall emulation only when it is attempted (vs. at compile time).
>
>  arch/x86/entry/vsyscall/vsyscall_64.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c b/arch/x86/entry/vsyscall/vsyscall_64.c
> index 44c33103a955..3196e963e365 100644
> --- a/arch/x86/entry/vsyscall/vsyscall_64.c
> +++ b/arch/x86/entry/vsyscall/vsyscall_64.c
> @@ -150,6 +150,15 @@ bool emulate_vsyscall(unsigned long error_code,
>
>         WARN_ON_ONCE(address != regs->ip);
>
> +#ifdef CONFIG_X86_INTEL_CET
> +       if (current->thread.cet.shstk_size ||
> +           current->thread.cet.ibt_enabled) {
> +               warn_bad_vsyscall(KERN_INFO, regs,
> +                                 "vsyscall attempted with cet enabled");
> +               return false;
> +       }

Nope, try again.  Having IBT on does *not* mean that every library in
the process knows that we have indirect branch tracking.  The legacy
bitmap exists for a reason.  Also, I want a way to flag programs as
not using the vsyscall page, but that flag should not be called CET.
And a process with vsyscalls off should not be able to read the
vsyscall page, and /proc/self/maps should be correct.

So you have some choices:

1. Drop this patch and make it work.

2. Add a real per-process vsyscall control.  Either make it depend on
vsyscall=xonly and wire it up correctly or actually make it work
correctly with vsyscall=emulate.

NAK to any hacks in this space.  Do it right or don't do it at all.