linux-kernel - Re: [PATCH] x86/retpoline: Fill RSB on context switch for affected CPUs

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Date:   Fri, 12 Jan 2018 18:56:18 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Andrew Cooper <andrew.cooper3@...rix.com>,
        Andi Kleen <ak@...ux.intel.com>
Cc:     Paul Turner <pjt@...gle.com>, LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>, tglx@...utronix.de,
        Kees Cook <keescook@...gle.com>,
        Rik van Riel <riel@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        Jiri Kosina <jikos@...nel.org>, gnomes@...rguk.ukuu.org.uk,
        x86@...nel.org, thomas.lendacky@....com,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH] x86/retpoline: Fill RSB on context switch for affected
 CPUs

On Fri, 2018-01-12 at 18:05 +0000, Andrew Cooper wrote:
> 
> If you unconditionally fill the RSB on every entry to supervisor mode,
> then there are never guest-controlled RSB values to be found.
> 
> With that property (and IBRS to protect Skylake+), you shouldn't need
> RSB filling anywhere in the middle.

Yes, that's right.

We have a choice — we can do it on kernel entry (in the interrupt and
syscall and NMI paths), and that's nice and easy and really safe
because we know there's *never* a bad RSB entry lurking while we're in
the kernel.

The alternative, which is what we seem to be learning towards now in
the latest tables from Dave (https://goo.gl/pXbvBE and
https://goo.gl/Grbuhf), is to do it on context switch when we might be
switching from a shallow call stack to a deeper one. Which has much
better performance characteristics for processes which make non-
sleeping syscalls.

The caveat with the latter approach is that we do depend on the fact
that context switches are the only imbalance in the kernel. But that's
OK — we don't have a longjmp or anything else like that. Especially
that goes into a *deeper* call stack. Do we?
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