linux-kernel - Re: [PATCH 2/5] x86/vsyscall: Add a new vsyscall=xonly mode

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Date:   Thu, 13 Jun 2019 12:08:38 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        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 2/5] x86/vsyscall: Add a new vsyscall=xonly mode

On Mon, Jun 10, 2019 at 1:43 PM Kees Cook <keescook@...omium.org> wrote:
>
> On Mon, Jun 10, 2019 at 01:25:28PM -0700, Andy Lutomirski wrote:
> > With vsyscall emulation on, we still expose a readable vsyscall page
> > that contains syscall instructions that validly implement the
> > vsyscalls.  We need this because certain dynamic binary
> > instrumentation tools attempt to read the call targets of call
> > instructions in the instrumented code.  If the instrumented code
> > uses vsyscalls, then the vsyscal page needs to contain readable
> > code.
> >
> > Unfortunately, leaving readable memory at a deterministic address
> > can be used to help various ASLR bypasses, so we gain some hardening
> > value if we disallow vsyscall reads.
> >
> > Given how rarely the vsyscall page needs to be readable, add a
> > mechanism to make the vsyscall page be execute only.
>
> Should the commit log mention that the VVAR portion goes away under
> xonly? (Since it's not executable.)

No, because vsyscall VVAR is long gone no matter what.  Even the old
vsyscall=native didn't have it.