linux-kernel - Re: [PATCH v2 6/8] x86/vsyscall: Change the default vsyscall mode to xonly

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Message-ID: <201906271029.584EC8319@keescook>
Date:   Thu, 27 Jun 2019 10:30:01 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     x86@...nel.org, LKML <linux-kernel@...r.kernel.org>,
        Florian Weimer <fweimer@...hat.com>,
        Jann Horn <jannh@...gle.com>, Borislav Petkov <bp@...en8.de>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 6/8] x86/vsyscall: Change the default vsyscall mode to
 xonly

On Wed, Jun 26, 2019 at 09:45:07PM -0700, Andy Lutomirski wrote:
> The use case for full emulation over xonly is very esoteric.  Let's
> change the default to the safer xonly mode.
> 
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Kernel Hardening <kernel-hardening@...ts.openwall.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Andy Lutomirski <luto@...nel.org>

I still think it'd be valuable to describe any known esoteric cases
here, but regardless:

Reviewed-by: Kees Cook <keescook@...omium.org>

-Kees

> ---
>  arch/x86/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 0182d2c67590..32028edc1b0e 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -2285,7 +2285,7 @@ config COMPAT_VDSO
>  choice
>  	prompt "vsyscall table for legacy applications"
>  	depends on X86_64
> -	default LEGACY_VSYSCALL_EMULATE
> +	default LEGACY_VSYSCALL_XONLY
>  	help
>  	  Legacy user code that does not know how to find the vDSO expects
>  	  to be able to issue three syscalls by calling fixed addresses in
> -- 
> 2.21.0
> 

-- 
Kees Cook