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

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Message-ID: <87sftqtp5z.fsf@oldenburg.str.redhat.com>
Date:   Fri, 14 Jan 2022 14:36:24 +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

* Andy Lutomirski:

> Is there a reason you didn't just change the check earlier in the
> function to:
>
> if (vsyscall_mode == NONE || current->mm->context.vsyscall_disabled)

Andrei requested that I don't print anything if vsyscall was disabled.

The original patch used a different message for better diagnostics.

> Also, I still think the prctl should not be available if
> vsyscall=emulate.  Either we should fully implement it or we should
> not implement.  We could even do:
>
> pr_warn_once("userspace vsyscall hardening request ignored because you
> have vsyscall=emulate.  Unless you absolutely need vsyscall=emulate, 
> update your system to use vsyscall=xonly.\n");
>
> and thus encourage good behavior.

I think there is still some hardening applied even with
vsyscall=emulate.  The question is what is more important: the
additional hardening, or clean, easily described behavior of the
interface.

Maybe ARCH_VSYSCALL_CONTROL could return different values based on to
what degree it could disable vsyscall?

The pr_warn_once does not seem particularly useful.  Anyone who upgrades
glibc and still uses vsyscall=emulate will see that, with no way to
disable it.

Thanks,
Florian