[PATCH v5 09/37] KVM: arm64: Extend masking facility to arbitrary registers

Marc Zyngier
2024-10-23T14:53:53Z

We currently only use the masking (RES0/RES1) facility for VNCR
registers, as they are memory-based and thus easy to sanitise.

But we could apply the same thing to other registers if we:

- split the sanitisation from __VNCR_START__
- apply the sanitisation when reading from a HW register

This involves a new "marker" in the vcpu_sysreg enum, which
defines the point at which the sanitisation applies (the VNCR
registers being of course after this marker).

Whle we are at it, rename kvm_vcpu_sanitise_vncr_reg() to
kvm_vcpu_apply_reg_masks(), which is vaguely more explicit,
and harden set_sysreg_masks() against setting masks for
random registers...

Signed-off-by: Marc Zyngier <maz@kernel.org>
---
arch/arm64/include/asm/kvm_host.h | 19 +++++++++++++------
arch/arm64/kvm/nested.c | 12 ++++++++----
arch/arm64/kvm/sys_regs.c | 3 +++
3 files changed, 24 insertions(+), 10 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 1adf68971bb17..7f409dfc5cd4a 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -367,7 +367,7 @@ struct kvm_arch {

u64 ctr_el0;

- /* Masks for VNCR-baked sysregs */
+ /* Masks for VNCR-backed and general EL2 sysregs */
struct kvm_sysreg_masks *sysreg_masks;

/*
@@ -401,6 +401,9 @@ struct kvm_vcpu_fault_info {
r = __VNCR_START__ + ((VNCR_ ## r) / 8), \
__after_##r = __MAX__(__before_##r - 1, r)

+#define MARKER(m) \
+ m, __after_##m = m - 1
+
enum vcpu_sysreg {
__INVALID_SYSREG__, /* 0 is reserved as an invalid value */
MPIDR_EL1, /* MultiProcessor Affinity Register */
@@ -487,7 +490,11 @@ enum vcpu_sysreg {
CNTHV_CTL_EL2,
CNTHV_CVAL_EL2,

- __VNCR_START__, /* Any VNCR-capable reg goes after this point */
+ /* Anything from this can be RES0/RES1 sanitised */
+ MARKER(__SANITISED_REG_START__),
+
+ /* Any VNCR-capable reg goes after this point */
+ MARKER(__VNCR_START__),

VNCR(SCTLR_EL1),/* System Control Register */
VNCR(ACTLR_EL1),/* Auxiliary Control Register */
@@ -547,7 +554,7 @@ struct kvm_sysreg_masks {
struct {
u64 res0;
u64 res1;
- } mask[NR_SYS_REGS - __VNCR_START__];
+ } mask[NR_SYS_REGS - __SANITISED_REG_START__];
};

struct kvm_cpu_context {
@@ -995,13 +1002,13 @@ static inline u64 *___ctxt_sys_reg(const struct kvm_cpu_context *ctxt, int r)

#define ctxt_sys_reg(c,r) (*__ctxt_sys_reg(c,r))

-u64 kvm_vcpu_sanitise_vncr_reg(const struct kvm_vcpu *, enum vcpu_sysreg);
+u64 kvm_vcpu_apply_reg_masks(const struct kvm_vcpu *, enum vcpu_sysreg, u64);
#define __vcpu_sys_reg(v,r) \
(*({ \
const struct kvm_cpu_context *ctxt = &(v)->arch.ctxt; \
u64 *__r = __ctxt_sys_reg(ctxt, (r)); \
- if (vcpu_has_nv((v)) && (r) >= __VNCR_START__) \
- *__r = kvm_vcpu_sanitise_vncr_reg((v), (r)); \
+ if (vcpu_has_nv((v)) && (r) >= __SANITISED_REG_START__) \
+ *__r = kvm_vcpu_apply_reg_masks((v), (r), *__r);\
__r; \
}))

diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
index f9e30dd34c7a1..b20b3bfb9caec 100644
--- a/arch/arm64/kvm/nested.c
+++ b/arch/arm64/kvm/nested.c
@@ -908,15 +908,15 @@ static void limit_nv_id_regs(struct kvm *kvm)
kvm_set_vm_id_reg(kvm, SYS_ID_AA64DFR0_EL1, val);
}

-u64 kvm_vcpu_sanitise_vncr_reg(const struct kvm_vcpu *vcpu, enum vcpu_sysreg sr)
+u64 kvm_vcpu_apply_reg_masks(const struct kvm_vcpu *vcpu,
+ enum vcpu_sysreg sr, u64 v)
{
- u64 v = ctxt_sys_reg(&vcpu->arch.ctxt, sr);
struct kvm_sysreg_masks *masks;

masks = vcpu->kvm->arch.sysreg_masks;

if (masks) {
- sr -= __VNCR_START__;
+ sr -= __SANITISED_REG_START__;

v &= ~masks->mask[sr].res0;
v |= masks->mask[sr].res1;
@@ -927,7 +927,11 @@ u64 kvm_vcpu_sanitise_vncr_reg(const struct kvm_vcpu *vcpu, enum vcpu_sysreg sr)

static void set_sysreg_masks(struct kvm *kvm, int sr, u64 res0, u64 res1)
{
- int i = sr - __VNCR_START__;
+ int i = sr - __SANITISED_REG_START__;
+
+ BUILD_BUG_ON(!__builtin_constant_p(sr));
+ BUILD_BUG_ON(sr < __SANITISED_REG_START__);
+ BUILD_BUG_ON(sr >= NR_SYS_REGS);

kvm->arch.sysreg_masks->mask[i].res0 = res0;
kvm->arch.sysreg_masks->mask[i].res1 = res1;
diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 932d2fb7a52a0..d9c20563cae93 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -189,6 +189,9 @@ u64 vcpu_read_sys_reg(const struct kvm_vcpu *vcpu, int reg)

/* Get the current version of the EL1 counterpart. */
WARN_ON(!__vcpu_read_sys_reg_from_cpu(el1r, &val));
+ if (reg >= __SANITISED_REG_START__)
+ val = kvm_vcpu_apply_reg_masks(vcpu, reg, val);
+
return val;
}

--
2.39.2