Re: [PATCH stable v4.9 v2] arm64: entry: Place an SB sequence following an ERET instruction

[Will Deacon]

On Fri, Aug 07, 2020 at 03:14:29PM +0200, Greg KH wrote:
> On Thu, Aug 06, 2020 at 01:00:54PM -0700, Florian Fainelli wrote:
> > 
> > 
> > On 7/20/2020 11:26 AM, Florian Fainelli wrote:
> > > On 7/20/20 6:04 AM, Greg KH wrote:
> > >> On Thu, Jul 09, 2020 at 12:50:23PM -0700, Florian Fainelli wrote:
> > >>> From: Will Deacon <will.deacon@xxxxxxx>
> > >>>
> > >>> commit 679db70801da9fda91d26caf13bf5b5ccc74e8e8 upstream
> > >>>
> > >>> Some CPUs can speculate past an ERET instruction and potentially perform
> > >>> speculative accesses to memory before processing the exception return.
> > >>> Since the register state is often controlled by a lower privilege level
> > >>> at the point of an ERET, this could potentially be used as part of a
> > >>> side-channel attack.
> > >>>
> > >>> This patch emits an SB sequence after each ERET so that speculation is
> > >>> held up on exception return.
> > >>>
> > >>> Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
> > >>> [florian: Adjust hyp-entry.S to account for the label
> > >>>  added change to hyp/entry.S]
> > >>> Signed-off-by: Florian Fainelli <f.fainelli@xxxxxxxxx>
> > >>> ---
> > >>> Changes in v2:
> > >>>
> > >>> - added missing hunk in hyp/entry.S per Will's feedback
> > >>
> > >> What about 4.19.y and 4.14.y trees?  I can't take something for 4.9.y
> > >> and then have a regression if someone moves to a newer release, right?
> > > 
> > > Sure, send you candidates for 4.14 and 4.19.
> > 
> > Greg, did you have a chance to queue those changes for 4.9, 4.14 and 4.19?
> > 
> > https://lore.kernel.org/linux-arm-kernel/20200720182538.13304-1-f.fainelli@xxxxxxxxx/
> > https://lore.kernel.org/linux-arm-kernel/20200720182937.14099-1-f.fainelli@xxxxxxxxx/
> > https://lore.kernel.org/linux-arm-kernel/20200709195034.15185-1-f.fainelli@xxxxxxxxx/
> 
> Nope, I was waiting for Will's "ack" for these.

This patch doesn't even build for me (the 'sb' macro is not defined in 4.9),
and I really wonder why we bother backporting it at all. Nobody's ever shown
it to be a problem in practice, and it's clear that this is just being
submitted to tick a box rather than anything else (otherwise it would build,
right?).

So I'm not going to Ack any of them. As with a lot of this side-channel
stuff the cure is far worse than the disease.

Will