arm64/kvm: hide ptrauth from guests

In subsequent patches we're going to expose ptrauth to the host kernel
and userspace, but things are a bit trickier for guest kernels. For the
time being, let's hide ptrauth from KVM guests.

Regardless of how well-behaved the guest kernel is, guest userspace
could attempt to use ptrauth instructions, triggering a trap to EL2,
resulting in noise from kvm_handle_unknown_ec(). So let's write up a
handler for the PAC trap, which silently injects an UNDEF into the
guest, as if the feature were really missing.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Kristina Martsenko <kristina.martsenko@arm.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Christoffer Dall <christoffer.dall@arm.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: kvmarm@lists.cs.columbia.edu
Signed-off-by: Will Deacon <will.deacon@arm.com>

diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
index 35a81be..ab35929 100644 (file)
--- a/arch/arm64/kvm/handle_exit.c
+++ b/arch/arm64/kvm/handle_exit.c
@@ -173,6 +173,23 @@ static int handle_sve(struct kvm_vcpu *vcpu, struct kvm_run *run)
        return 1;
 }
 
+/*
+ * Guest usage of a ptrauth instruction (which the guest EL1 did not turn into
+ * a NOP).
+ */
+static int kvm_handle_ptrauth(struct kvm_vcpu *vcpu, struct kvm_run *run)
+{
+       /*
+        * We don't currently support ptrauth in a guest, and we mask the ID
+        * registers to prevent well-behaved guests from trying to make use of
+        * it.
+        *
+        * Inject an UNDEF, as if the feature really isn't present.
+        */
+       kvm_inject_undefined(vcpu);
+       return 1;
+}
+
 static exit_handle_fn arm_exit_handlers[] = {
        [0 ... ESR_ELx_EC_MAX]  = kvm_handle_unknown_ec,
        [ESR_ELx_EC_WFx]        = kvm_handle_wfx,
@@ -195,6 +212,7 @@ static exit_handle_fn arm_exit_handlers[] = {
        [ESR_ELx_EC_BKPT32]     = kvm_handle_guest_debug,
        [ESR_ELx_EC_BRK64]      = kvm_handle_guest_debug,
        [ESR_ELx_EC_FP_ASIMD]   = handle_no_fpsimd,
+       [ESR_ELx_EC_PAC]        = kvm_handle_ptrauth,
 };
 
 static exit_handle_fn kvm_get_exit_handler(struct kvm_vcpu *vcpu)

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 22fbbdb..1ca592d 100644 (file)
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -1040,6 +1040,14 @@ static u64 read_id_reg(struct sys_reg_desc const *r, bool raz)
                        kvm_debug("SVE unsupported for guests, suppressing\n");
 
                val &= ~(0xfUL << ID_AA64PFR0_SVE_SHIFT);
+       } else if (id == SYS_ID_AA64ISAR1_EL1) {
+               const u64 ptrauth_mask = (0xfUL << ID_AA64ISAR1_APA_SHIFT) |
+                                        (0xfUL << ID_AA64ISAR1_API_SHIFT) |
+                                        (0xfUL << ID_AA64ISAR1_GPA_SHIFT) |
+                                        (0xfUL << ID_AA64ISAR1_GPI_SHIFT);
+               if (val & ptrauth_mask)
+                       kvm_debug("ptrauth unsupported for guests, suppressing\n");
+               val &= ~ptrauth_mask;
        } else if (id == SYS_ID_AA64MMFR1_EL1) {
                if (val & (0xfUL << ID_AA64MMFR1_LOR_SHIFT))
                        kvm_debug("LORegions unsupported for guests, suppressing\n");