KVM: arm64: Set HCR_EL2.PTW to prevent AT taking synchronous exception

AT instructions do a translation table walk and return the result, or
the fault in PAR_EL1. KVM uses these to find the IPA when the value is
not provided by the CPU in HPFAR_EL1.

If a translation table walk causes an external abort it is taken as an
exception, even if it was due to an AT instruction. (DDI0487F.a's D5.2.11
"Synchronous faults generated by address translation instructions")

While we previously made KVM resilient to exceptions taken due to AT
instructions, the device access causes mismatched attributes, and may
occur speculatively. Prevent this, by forbidding a walk through memory
described as device at stage2. Now such AT instructions will report a
stage2 fault.

Such a fault will cause KVM to restart the guest. If the AT instructions
always walk the page tables, but guest execution uses the translation cached
in the TLB, the guest can't make forward progress until the TLB entry is
evicted. This isn't a problem, as since commit 5dcd0fdbb492 ("KVM: arm64:
Defer guest entry when an asynchronous exception is pending"), KVM will
return to the host to process IRQs allowing the rest of the system to keep
running.

Cc: Marc Zyngier <maz@kernel.org>
Cc: stable@vger.kernel.org # <v5.3: 5dcd0fdbb492 ("KVM: arm64: Defer guest entry when an asynchronous exception is pending")
Signed-off-by: James Morse <james.morse@arm.com>
---

If KVM guests have access to MMIO that returns an abort for an aligned
64 bit access, this would be exploitable by a guest. I don't know if
this is a realistic deployment.

5dcd0fdbb492 can't be backported to v4.4 as in that epoch KVM was all
written in assembly. For v4.14 and v4.9, 5dcd0fdbb492 needs the RAS
extension alternative removing.
diff --git a/arch/arm64/include/asm/kvm_arm.h b/arch/arm64/include/asm/kvm_arm.h
index ddf9d76..a4ffd9b 100644
--- a/arch/arm64/include/asm/kvm_arm.h
+++ b/arch/arm64/include/asm/kvm_arm.h
@@ -72,11 +72,12 @@
  * IMO:		Override CPSR.I and enable signaling with VI
  * FMO:		Override CPSR.F and enable signaling with VF
  * SWIO:	Turn set/way invalidates into set/way clean+invalidate
+ * PTW:		Take a stage2 fault if a stage1 walk steps in device memory
  */
 #define HCR_GUEST_FLAGS (HCR_TSC | HCR_TSW | HCR_TWE | HCR_TWI | HCR_VM | \
 			 HCR_TVM | HCR_BSU_IS | HCR_FB | HCR_TAC | \
 			 HCR_AMO | HCR_SWIO | HCR_TIDCP | HCR_RW | HCR_TLOR | \
-			 HCR_FMO | HCR_IMO)
+			 HCR_FMO | HCR_IMO | HCR_PTW )
 #define HCR_VIRT_EXCP_MASK (HCR_VSE | HCR_VI | HCR_VF)
 #define HCR_HOST_NVHE_FLAGS (HCR_RW | HCR_API | HCR_APK)
 #define HCR_HOST_VHE_FLAGS (HCR_RW | HCR_TGE | HCR_E2H)