Re: [PATCH 2/2] arm64: Branch predictor hardening for Cavium ThunderX2

[Will Deacon <will.deacon@arm.com>]

On Mon, Jan 08, 2018 at 09:19:43AM -0800, Jayachandran C wrote:
> On Mon, Jan 08, 2018 at 04:46:52PM +0000, Will Deacon wrote:
> > On Sun, Jan 07, 2018 at 10:53:36PM -0800, Jayachandran C wrote:
> > > Use PSCI based mitigation for speculative execution attacks targeting
> > > the branch predictor. The approach is similar to the one used for
> > > Cortex-A CPUs, but in case of ThunderX2 we add another SMC call to
> > > test if the firmware supports the capability.
> > > 
> > > If the secure firmware has been updated with the mitigation code to
> > > invalidate the branch target buffer, we use the PSCI version call to
> > > invoke it.
> > > 
> > > Signed-off-by: Jayachandran C <jnair@caviumnetworks.com>
> > > ---
> > >  arch/arm64/kernel/cpu_errata.c | 38 ++++++++++++++++++++++++++++++++++++++
> > >  1 file changed, 38 insertions(+)
> > > 
> > > diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
> > > index cb0fb37..abceb5d 100644
> > > --- a/arch/arm64/kernel/cpu_errata.c
> > > +++ b/arch/arm64/kernel/cpu_errata.c
> > > @@ -124,6 +124,7 @@ static void  install_bp_hardening_cb(const struct arm64_cpu_capabilities *entry,
> > >  	__install_bp_hardening_cb(fn, hyp_vecs_start, hyp_vecs_end);
> > >  }
> > >  
> > > +#include <linux/arm-smccc.h>
> > >  #include <linux/psci.h>
> > >  
> > >  static int enable_psci_bp_hardening(void *data)
> > > @@ -138,6 +139,33 @@ static int enable_psci_bp_hardening(void *data)
> > >  
> > >  	return 0;
> > >  }
> > > +
> > > +#define CAVIUM_TX2_SIP_SMC_CALL		0xC200FF00
> > > +#define CAVIUM_TX2_BTB_HARDEN_CAP	0xB0A0
> > > +
> > > +static int enable_tx2_psci_bp_hardening(void *data)
> > > +{
> > > +	const struct arm64_cpu_capabilities *entry = data;
> > > +	struct arm_smccc_res res;
> > > +
> > > +	if (!entry->matches(entry, SCOPE_LOCAL_CPU))
> > > +		return;
> > > +
> > > +	arm_smccc_smc(CAVIUM_TX2_SIP_SMC_CALL, CAVIUM_TX2_BTB_HARDEN_CAP, 0, 0, 0, 0, 0, 0, &res);
> > 
> > One thing to be aware of here is that if somebody configures qemu to emulate
> > a TX2, this may actually disappear into EL3 and not return. You're better
> > off sticking with PSCI GET_VERSION in terms of portability, but it's your
> > call -- I'd expect you to deal with any breakage reports on the list due
> > to the SMC above. Fair?
> 
> I don't like having a custom SMC here either. But Overloading PSCI get version
> is the problem as I wrote earlier - there is no way to check if the firmware
> implements BTB hardening with overloading. There is a good chance that users
> with old firmware will just fail without any warning.

That's true, but there is precedent for this elsewhere. For example, CPU
errata that require a firmware change are often not probable. Also, your SMC
call won't always work (see the qemu comment below). Note that I'm not
saying I won't take this code, just that you need to be aware of what
you're doing.

> Is there a reason for overloading PSCI get version? Allocating a new standard
> SMC number would make checking for existance and usage much simpler.

PSCI get version is what we have today. We're working on extending PSCI to
allocate a new standard SMC number, but we need something that can be used
with existing firmware too and standardisation doesn't happen overnight.

> I did not quite understand the possible issue in qemu, unimplemented SMC calls
> are expected to return an error code. What am I missing here?

Qemu will inject them into EL3, and there might not be anything there.

Will