Re: [PATCH v2 11/13] KVM: arm64: nv: Add emulation for ERETAx instructions

[Marc Zyngier <maz@kernel.org>]

On Thu, 07 Mar 2024 13:39:12 +0000,
Joey Gouly <joey.gouly@arm.com> wrote:
> 
> Note that this is my first time looking at PAuth.

I'm sorry! ;-)

> 
> On Mon, Feb 26, 2024 at 10:05:59AM +0000, Marc Zyngier wrote:
> > FEAT_NV has the interesting property of relying on ERET being
> > trapped. An added complexity is that it also traps ERETAA and
> > ERETAB, meaning that the Pointer Authentication aspect of these
> > instruction must be emulated.
> > 
> > Add an emulation of Pointer Authentication, limited to ERETAx
> > (always using SP_EL2 as the modifier and ELR_EL2 as the pointer),
> > using the Generic Authentication instructions.
> > 
> > The emulation, however small, is placed in its own compilation
> > unit so that it can be avoided if the configuration doesn't
> > include it (or the toolchan in not up to the task).
> > 
> > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > ---
> >  arch/arm64/include/asm/kvm_nested.h    |  12 ++
> >  arch/arm64/include/asm/pgtable-hwdef.h |   1 +
> >  arch/arm64/kvm/Makefile                |   1 +
> >  arch/arm64/kvm/pauth.c                 | 196 +++++++++++++++++++++++++
> >  4 files changed, 210 insertions(+)
> >  create mode 100644 arch/arm64/kvm/pauth.c
> > 
> > diff --git a/arch/arm64/include/asm/kvm_nested.h b/arch/arm64/include/asm/kvm_nested.h
> > index dbc4e3a67356..5e0ab0596246 100644
> > --- a/arch/arm64/include/asm/kvm_nested.h
> > +++ b/arch/arm64/include/asm/kvm_nested.h
> > @@ -64,4 +64,16 @@ extern bool forward_smc_trap(struct kvm_vcpu *vcpu);
> >  
> >  int kvm_init_nv_sysregs(struct kvm *kvm);
> >  
> > +#ifdef CONFIG_ARM64_PTR_AUTH
> > +bool kvm_auth_eretax(struct kvm_vcpu *vcpu, u64 *elr);
> > +#else
> > +static inline bool kvm_auth_eretax(struct kvm_vcpu *vcpu, u64 *elr)
> > +{
> > +	/* We really should never execute this... */
> > +	WARN_ON_ONCE(1);
> > +	*elr = 0xbad9acc0debadbad;
> > +	return false;
> > +}
> > +#endif
> > +
> >  #endif /* __ARM64_KVM_NESTED_H */
> > diff --git a/arch/arm64/include/asm/pgtable-hwdef.h b/arch/arm64/include/asm/pgtable-hwdef.h
> > index e4944d517c99..bb88e9ef6296 100644
> > --- a/arch/arm64/include/asm/pgtable-hwdef.h
> > +++ b/arch/arm64/include/asm/pgtable-hwdef.h
> > @@ -277,6 +277,7 @@
> >  #define TCR_TBI1		(UL(1) << 38)
> >  #define TCR_HA			(UL(1) << 39)
> >  #define TCR_HD			(UL(1) << 40)
> > +#define TCR_TBID0		(UL(1) << 51)
> >  #define TCR_TBID1		(UL(1) << 52)
> >  #define TCR_NFD0		(UL(1) << 53)
> >  #define TCR_NFD1		(UL(1) << 54)
> > diff --git a/arch/arm64/kvm/Makefile b/arch/arm64/kvm/Makefile
> > index c0c050e53157..04882b577575 100644
> > --- a/arch/arm64/kvm/Makefile
> > +++ b/arch/arm64/kvm/Makefile
> > @@ -23,6 +23,7 @@ kvm-y += arm.o mmu.o mmio.o psci.o hypercalls.o pvtime.o \
> >  	 vgic/vgic-its.o vgic/vgic-debug.o
> >  
> >  kvm-$(CONFIG_HW_PERF_EVENTS)  += pmu-emul.o pmu.o
> > +kvm-$(CONFIG_ARM64_PTR_AUTH)  += pauth.o
> >  
> >  always-y := hyp_constants.h hyp-constants.s
> >  
> > diff --git a/arch/arm64/kvm/pauth.c b/arch/arm64/kvm/pauth.c
> > new file mode 100644
> > index 000000000000..a3a5c404375b
> > --- /dev/null
> > +++ b/arch/arm64/kvm/pauth.c
> > @@ -0,0 +1,196 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +/*
> > + * Copyright (C) 2024 - Google LLC
> > + * Author: Marc Zyngier <maz@kernel.org>
> > + *
> > + * Primitive PAuth emulation for ERETAA/ERETAB.
> > + *
> > + * This code assumes that is is run from EL2, and that it is part of
> > + * the emulation of ERETAx for a guest hypervisor. That's a lot of
> > + * baked-in assumptions and shortcuts.
> > + *
> > + * Do no reuse for anything else!
> > + */
> > +
> > +#include <linux/kvm_host.h>
> > +
> > +#include <asm/kvm_emulate.h>
> > +#include <asm/pointer_auth.h>
> > +
> > +static u64 compute_pac(struct kvm_vcpu *vcpu, u64 ptr,
> > +		       struct ptrauth_key ikey)
> > +{
> > +	struct ptrauth_key gkey;
> > +	u64 mod, pac = 0;
> > +
> > +	preempt_disable();
> > +
> > +	if (!vcpu_get_flag(vcpu, SYSREGS_ON_CPU))
> > +		mod = __vcpu_sys_reg(vcpu, SP_EL2);
> > +	else
> > +		mod = read_sysreg(sp_el1);
> > +
> > +	gkey.lo = read_sysreg_s(SYS_APGAKEYLO_EL1);
> > +	gkey.hi = read_sysreg_s(SYS_APGAKEYHI_EL1);
> > +
> > +	__ptrauth_key_install_nosync(APGA, ikey);
> > +	isb();
> > +
> > +	asm volatile(ARM64_ASM_PREAMBLE ".arch_extension pauth\n"
> > +		     "pacga %0, %1, %2" : "=r" (pac) : "r" (ptr), "r" (mod));
> 
> To use `pacga`, we require that the Address authentication and Generic
> authentication use the same algorithm, right?

Indeed. It is a strong requirement, and if we don't have that, nothing
works.

Or rather, emulating ERETAx becomes so bloody complicated it isn't
funny: you need to somehow to replay the auth in the context of a
guest so that all of TCR_EL2, SP_EL2, SCTLR_EL2, HCR_EL2, ELR_EL2 are
correctly set, get the value back, and compare it to the one you are
trying to authenticate.

Not happening! Mark and I talked about it ages ago and concluded it
was absolutely insane. PACGA allows us to sidestep the whole thing and
reconstruct the PAC like the pseudocode does using the ComputePAC()
function.

> There doesn't seem to be a check for that up front. There is kinda a check for
> that if the PAC doesn't match, (by kvm_has_pauth()).

Indeed. The main issue is that it is really hard to prevent that
unless we forbid it for all KVM guests, not just NV guests. It is also
that I don't know of any HW that would implement two different auth
methods (which would be really bizarre from an implementation
perspective).

I'm happy to harden system_has_full_ptr_auth() in that case, which
would do the trick.

Thanks,

	M.

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

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