Re: [PATCH v2] KVM: arm64: Prevent the host from using an smc with imm16 != 0

[Marc Zyngier <maz@kernel.org>]

On Wed, 25 Mar 2026 11:41:17 +0000,
Sebastian Ene <sebastianene@google.com> wrote:
> 
> On Wed, Mar 25, 2026 at 11:35:18AM +0000, Vincent Donnefort wrote:
> > On Wed, Mar 25, 2026 at 11:31:38AM +0000, Sebastian Ene wrote:
> > > The ARM Service Calling Convention (SMCCC) specifies that the function
> > > identifier and parameters should be passed in registers, leaving the
> > > 16-bit immediate field of the SMC instruction un-handled.
> > > Currently, our pKVM handler ignores the immediate value, which could lead
> > > to non-compliant software relying on implementation-defined behavior.
> > > Enforce the host kernel running under pKVM to use an immediate value
> > > of 0 by decoding the ISS from the ESR_EL2 and return a not supported
> > > error code back to the caller.
> > > 
> > > Signed-off-by: Sebastian Ene <sebastianene@google.com>
> > > ---
> > > v1 -> v2:
> > > 
> > >  - Dropped injecting an UNDEF and return an error instead
> > >    (SMCCC_RET_NOT_SUPPORTED)
> > >  - Used the mask ESR_ELx_xVC_IMM_MASK instead of masking with U16_MAX
> > >  - Updated the title of the commit message from:
> > >    "[PATCH] KVM: arm64: Inject UNDEF when host is executing an
> > >     smc with imm16 != 0
> > 
> > > ---
> > >  arch/arm64/kvm/hyp/nvhe/hyp-main.c | 6 ++++++
> > >  1 file changed, 6 insertions(+)
> > > 
> > > diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> > > index e7790097db93..4ffe30fd8707 100644
> > > --- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> > > +++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> > > @@ -762,6 +762,12 @@ void handle_trap(struct kvm_cpu_context *host_ctxt)
> > >  		handle_host_hcall(host_ctxt);
> > >  		break;
> > >  	case ESR_ELx_EC_SMC64:
> > > +		if (ESR_ELx_xVC_IMM_MASK & esr) {
> > > +			cpu_reg(host_ctxt, 0) = SMCCC_RET_NOT_SUPPORTED;
> > > +			kvm_skip_host_instr();
> > > +			break;
> > > +		}
> > > +
> > 
> > I wonder if it isn't better to move that into handle_host_smc() as this is part
> > of how we handle the SMC after all? (and it calls that kvm_skip_host_instr()
> > already)
> > 
> 
> I was thinking of doing that as well but I prefer this since we don't
> have to look again at the esr in the callee.

I don't see that obtaining ESR_EL2 is that hard or expensive. Given
that everything is in the same compilation unit, the compiler will
probably optimise it.

But if I have to state the obvious: you are *handling* an SMC
instruction. Surely handle_host_smc() is the correct spot for that
job?

	M.

-- 
Without deviation from the norm, progress is not possible.