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Date: Thu, 16 Dec 2021 19:31:19 +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, "Dave Hansen via Libc-alpha"
<libc-alpha@...rceware.org>, "Linux Kernel Mailing List"
<linux-kernel@...r.kernel.org>, "Dave Hansen" <dave.hansen@...el.com>,
"Kees Cook" <keescook@...omium.org>
Subject: Re: [PATCH] x86: Implement arch_prctl(ARCH_VSYSCALL_LOCKOUT) to
disable vsyscall
* Andy Lutomirski:
> This could possibly be much more generic: have a mask of legacy
> features to disable and a separate mask of lock bits.
Is that really necessary? Adding additional ARCH_* constants does not
seem to be particularly onerous and helps with detection of kernel
support.
>> I can turn this into a toggle, and we could probably default our builds
>> to vsyscalls=xonly. Given the userspace ABI impact, we'd still have to
>> upstream the toggle. Do you see a chance of a patch a long these lines
>> going in at all, given that it's an incomplete solution for
>> vsyscall=emulate?
>
> There is basically no reason for anyone to use vsyscall=emulate any
> more. I'm aware of exactly one use case, and it's quite bizarre and
> involves instrumenting an outdated binary with an outdated
> instrumentation tool. If either one is recent (last few years),
> vsyscall=xonly is fine.
Yeah, we plan to stick to vsyscall=xonly. This means that the toggle is
easier to implement, of course.
>> Hmm. But only for vsyscall=xonly, right? With vsyscall=emulate,
>> reading at those addresses will still succeed.
>
> IMO if vsyscall is disabled for a process, reads and executes should
> both fail. This is trivial in xonly mode.
Right, I'll document this as a glitch for now.
I've got a v2 (with the toggle rather than pure lockout) and will sent
it out shortly.
Thanks,
Florian
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