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Date: Thu, 25 Jan 2018 13:53:53 -0800
From: Linus Torvalds <>
To: Dan Williams <>
Cc: Andy Lutomirski <>, "the arch/x86 maintainers" <>, LKML <>, 
	Greg Kroah-Hartman <>, Alan Cox <>, 
	Jann Horn <>, Samuel Neves <>, 
	Kernel Hardening <>, Borislav Petkov <>
Subject: Re: [PATCH] x86/retpoline/entry: Disable the entire SYSCALL64 fast
 path with retpolines on

On Thu, Jan 25, 2018 at 1:39 PM, Dan Williams <> wrote:
> If you're rejiggering, can we also put in a mechanism for detecting
> which registers to clear so that userspace can't inject useful values
> into speculation paths?

That actually becomes trivial with just the "no fastpath" patch I sent
out. You can just clear all of them.

Sure, then do_syscall_64() will reload the six first ones, but since
those are the argument registers anyway, and since they are
caller-clobbered, they have very short lifetimes. So it would
effectively not really be an issue.

But yes, SYSCALL_DEFINEx() rejiggery would close even that tiny hole.


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