Re: [RFC 04/10] x86/mm: Only flush indirect branches when switching into non dumpable process

[David Woodhouse]

> On Sat, Jan 20, 2018 at 08:22:55PM +0100, KarimAllah Ahmed 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.
>
> So if I understand it right, this is only needed if the 'other'
> executable itself is susceptible to spectre. If say someone audited gpg
> for spectre-v1 and build it with retpoline, it would be safe to not
> issue the IBPB, right?


Spectre V2 not v1. V1 is separate.
For V2 retpoline is enough... as long as all the libraries have it too.

> So would it make sense to provide an ELF flag / personality thing such
> that userspace can indicate its spectre-safe?

Yes, Arjan and I were pondering that yesterday; it probably does make
sense. Also for allowing a return to userspace after vmexit, if the army
process itself is so marked.

> I realize that this is all future work, because so far auditing for v1
> is a lot of pain (we need better tools), but would it be something that
> makes sense in the longer term?

It's *only* retpoline so it isn't actually that much. Although I'm wary of
Cc'ing HJ on such thoughts because he seems to never sleep and always
respond promptly with "OK I did that... " :)

If we did systematically do this in userspace we'd probably want to do
external thunks there too, and a flag in the auxvec to tell it not to
bother (for IBRS_ALL etc.).

-- 
dwmw2