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

[Tim Chen]

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@xxxxxxxxxxxxxxx>
>>
>> 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.  

> 
> [ 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.

> 
> If performance is a concern, let's look at that in more detail.  But I
> don't see how the solution to a performance issue could possibly be
> "leave (almost) all tasks vulnerable by default."
> 

Thanks.

Tim