linux-kernel - Re: [PATCH] x86/speculation: Use Indirect Branch Prediction Barrier in context switch

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Message-ID: <20180131035907.sye4x7f3e77wnroh@treble>
Date:   Tue, 30 Jan 2018 21:59:07 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     David Woodhouse <dwmw@...zon.co.uk>, arjan@...ux.intel.com,
        tglx@...utronix.de, karahmed@...zon.de, x86@...nel.org,
        linux-kernel@...r.kernel.org, bp@...en8.de, peterz@...radead.org,
        pbonzini@...hat.com, ak@...ux.intel.com,
        torvalds@...ux-foundation.org, gregkh@...ux-foundation.org,
        mingo@...nel.org, luto@...nel.org, linux@...inikbrodowski.net
Subject: Re: [PATCH] x86/speculation: Use Indirect Branch Prediction Barrier
 in context switch

On Tue, Jan 30, 2018 at 01:23:17PM -0800, Tim Chen wrote:
> On 01/30/2018 09:48 AM, Josh Poimboeuf wrote:
> > On Mon, Jan 29, 2018 at 10:04:47PM +0000, David Woodhouse wrote:
> >> From: Tim Chen <tim.c.chen@...ux.intel.com>
> >>
> >> Flush indirect branches when switching into a process that marked itself
> >> non dumpable. This protects high value processes like gpg better,
> >> without having too high performance overhead.
> > 
> > I wonder what the point of this patch is.  An audit of my laptop shows
> > only a single user of PR_SET_DUMPABLE: systemd-coredump.
> 
> This is an opt in approach.  For processes who need extra
> security, it set itself as non-dumpable.  Then it can
> ensure that it doesn't see any poisoned BTB.  

I don't want other users reading my applications' memory.

I don't want other containers reading my containers' memory.

I don't want *any* user tasks reading root daemons' memory.

Those are not unreasonable expectations.

So now I have to go and modify all my containers and applications to set
PR_SET_DUMPABLE?  That seems highly impractical and unlikely.

Plus, I happen to *like* core dumps.

The other option is to rebuild the entire userland with retpolines, but
again, that would make this patch completely pointless.

> > [ And yes, I have gpg-agent running.  Also, a grep of the gnupg source
> > doesn't show any evidence of it being used there.  So the gpg thing
> > seems to be a myth. ]
> 
> I'm less familiar with gpg-agent.  Dave was the one who
> put in comments about gpg-agent in this patch so perhaps
> he can comment.
> 
> > 
> > But also, I much preferred the original version of the patch which only
> > skipped IBPB when 'prev' could ptrace 'next'.
> 
> For the A->kernel thread->B scenario, you will need context of A
> to decide if you need IBPB when switching to B.  You need to
> worry about whether the context of A has been released ... etc if
> you want to use ptrace.

Is that why the ptrace approach was abandoned?  Surely that's a solvable
problem?  We have some smart people on lkml.  And anyway I didn't see it
discussed anywhere.  In the worst case we could just always do IBPB when
switching between kernel and user tasks.

-- 
Josh