linux-kernel - Re: [PATCH] x86/bugs: protect against userspace-userspace spectreRSB

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Message-ID: <nycvar.YFH.7.76.1807251547550.997@cbobk.fhfr.pm>
Date:   Wed, 25 Jul 2018 15:50:44 +0200 (CEST)
From:   Jiri Kosina <jikos@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Borislav Petkov <bp@...e.de>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Peter Zijlstra <peterz@...radead.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86/bugs: protect against userspace-userspace
 spectreRSB

On Wed, 25 Jul 2018, Josh Poimboeuf wrote:

> > The article "Spectre Returns! Speculation Attacks using the Return Stack
> > Buffer" [1] describes two new (sub-)variants of spectrev2-like attack,
> > making use solely of the RSB contents even on CPUs that don't fallback to
> > BTB on RSB underflow (Skylake+).
> > 
> > Mitigate userspace-userspace attacks by always unconditionally filling RSB on
> > context switch when generic spectrev2 mitigation has been enabled.
> > 
> > [1] https://arxiv.org/pdf/1807.07940.pdf
> > 
> > Signed-off-by: Jiri Kosina <jkosina@...e.cz>
> 
> While I generally agree with this patch, isn't it odd that we would do
> RSB filling on every context switch, but almost never do IBPB?

Yeah, I have actually been wondering exactly the same, but that's what we 
have been doing so far on SKL+, so I didn't really want to mix this aspect 
in.

I actually believe that in the name of consistency we should've been doing 
the RSB fills under the same conditions we're issuing IBPB even on SKL+; I 
can resend a patch that re-adjusts that, if that's the consensus.

Thanks,

-- 
Jiri Kosina
SUSE Labs