Messages in this thread | | | Date | Tue, 30 Jan 2018 21:59:07 -0600 | From | Josh Poimboeuf <> | Subject | Re: [PATCH] x86/speculation: Use Indirect Branch Prediction Barrier in context switch |
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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@linux.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
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