[PATCH stable 4.9] arm64: entry: Place an SB sequence following an ERET instruction

[Will Deacon]

On Tue, Jun 23, 2020 at 11:46:37AM -0700, Florian Fainelli wrote:
> On 6/11/20 9:42 PM, Florian Fainelli wrote:
> > From: Will Deacon <will.deacon at arm.com>
> > 
> > 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 at arm.com>
> > [florian: Adjust hyp-entry.S to account for the label]
> > Signed-off-by: Florian Fainelli <f.fainelli at gmail.com>
> > ---
> > Will,
> > 
> > Can you confirm that for 4.9 these are the only places that require
> > patching? Thank you!
> 
> Hi Will, Catalin,
> 
> Does this look good to you for a 4.9 backport? I would like to see this
> included at some point since this pertains to CVE-2020-13844.

I think you're missing one of the ERET instructions in hyp/entry.S

Will