[PATCH v3 3/6] arm: KVM: Invalidate BTB on guest exit for Cortex-A12/A17

[Marc Zyngier]

In order to avoid aliasing attacks against the branch predictor,
let's invalidate the BTB on guest exit. This is made complicated
by the fact that we cannot take a branch before invalidating the
BTB.

We only apply this to A12 and A17, which are the only two ARM
cores on which this useful.

Signed-off-by: Marc Zyngier <marc.zyngier at arm.com>
---
 arch/arm/include/asm/kvm_asm.h |  2 --
 arch/arm/include/asm/kvm_mmu.h | 13 ++++++++-
 arch/arm/kvm/hyp/hyp-entry.S   | 62 ++++++++++++++++++++++++++++++++++++++++--
 3 files changed, 72 insertions(+), 5 deletions(-)

diff --git a/arch/arm/include/asm/kvm_asm.h b/arch/arm/include/asm/kvm_asm.h
index 36dd2962a42d..df24ed48977d 100644
--- a/arch/arm/include/asm/kvm_asm.h
+++ b/arch/arm/include/asm/kvm_asm.h
@@ -61,8 +61,6 @@ struct kvm_vcpu;
 extern char __kvm_hyp_init[];
 extern char __kvm_hyp_init_end[];
 
-extern char __kvm_hyp_vector[];
-
 extern void __kvm_flush_vm_context(void);
 extern void __kvm_tlb_flush_vmid_ipa(struct kvm *kvm, phys_addr_t ipa);
 extern void __kvm_tlb_flush_vmid(struct kvm *kvm);
diff --git a/arch/arm/include/asm/kvm_mmu.h b/arch/arm/include/asm/kvm_mmu.h
index eb46fc81a440..b47db5b9e407 100644
--- a/arch/arm/include/asm/kvm_mmu.h
+++ b/arch/arm/include/asm/kvm_mmu.h
@@ -37,6 +37,7 @@
 
 #include <linux/highmem.h>
 #include <asm/cacheflush.h>
+#include <asm/cputype.h>
 #include <asm/pgalloc.h>
 #include <asm/stage2_pgtable.h>
 
@@ -223,7 +224,17 @@ static inline unsigned int kvm_get_vmid_bits(void)
 
 static inline void *kvm_get_hyp_vector(void)
 {
-	return kvm_ksym_ref(__kvm_hyp_vector);
+	extern char __kvm_hyp_vector[];
+	extern char __kvm_hyp_vector_bp_inv[];
+
+	switch(read_cpuid_part()) {
+	case ARM_CPU_PART_CORTEX_A12:
+	case ARM_CPU_PART_CORTEX_A17:
+		return kvm_ksym_ref(__kvm_hyp_vector_bp_inv);
+
+	default:
+		return kvm_ksym_ref(__kvm_hyp_vector);
+	}
 }
 
 static inline int kvm_map_vectors(void)
diff --git a/arch/arm/kvm/hyp/hyp-entry.S b/arch/arm/kvm/hyp/hyp-entry.S
index 95a2faefc070..aab6b0c06a19 100644
--- a/arch/arm/kvm/hyp/hyp-entry.S
+++ b/arch/arm/kvm/hyp/hyp-entry.S
@@ -70,6 +70,57 @@ __kvm_hyp_vector:
 	W(b)	hyp_hvc
 	W(b)	hyp_irq
 	W(b)	hyp_fiq
+	
+	.align 5
+__kvm_hyp_vector_bp_inv:
+	.global __kvm_hyp_vector_bp_inv
+
+	/*
+	 * We encode the exception entry in the bottom 3 bits of
+	 * SP, and we have to guarantee to be 8 bytes aligned.
+	 */
+	W(add)	sp, sp, #1	/* Reset 	  7 */
+	W(add)	sp, sp, #1	/* Undef	  6 */
+	W(add)	sp, sp, #1	/* Syscall	  5 */
+	W(add)	sp, sp, #1	/* Prefetch abort 4 */
+	W(add)	sp, sp, #1	/* Data abort	  3 */
+	W(add)	sp, sp, #1	/* HVC		  2 */
+	W(add)	sp, sp, #1	/* IRQ		  1 */
+	W(nop)			/* FIQ		  0 */
+
+	mcr	p15, 0, r0, c7, c5, 6	/* BPIALL */
+	isb
+
+	/*
+	 * Yet another silly hack: Use VPIDR as a temp register.
+	 * Thumb2 is really a pain, as SP cannot be used with most
+	 * of the bitwise instructions. The vect_br macro ensures
+	 * things gets cleaned-up.
+	 */
+	mcr	p15, 4, r0, c0, c0, 0	/* VPIDR */
+	mov	r0, sp
+	and	r0, r0, #7
+	sub	sp, sp, r0
+	push	{r1, r2}
+	mov	r1, r0
+	mrc	p15, 4, r0, c0, c0, 0	/* VPIDR */
+	mrc	p15, 0, r2, c0, c0, 0	/* MIDR  */
+	mcr	p15, 4, r2, c0, c0, 0	/* VPIDR */
+
+.macro vect_br val, targ
+	cmp	r1, #\val
+	popeq	{r1, r2}
+	beq	\targ
+.endm
+
+	vect_br	0, hyp_fiq
+	vect_br	1, hyp_irq
+	vect_br	2, hyp_hvc
+	vect_br	3, hyp_dabt
+	vect_br	4, hyp_pabt
+	vect_br	5, hyp_svc
+	vect_br	6, hyp_undef
+	vect_br	7, hyp_reset
 
 .macro invalid_vector label, cause
 	.align
@@ -149,7 +200,14 @@ hyp_hvc:
 	bx	ip
 
 1:
-	push	{lr}
+	/*
+	 * Pushing r2 here is just a way of keeping the stack aligned to
+	 * 8 bytes on any path that can trigger a HYP exception. Here,
+	 * we may well be about to jump into the guest, and the guest
+	 * exit would otherwise be badly decoded by our fancy
+	 * "decode-exception-without-a-branch" code...
+	 */
+	push	{r2, lr}
 
 	mov	lr, r0
 	mov	r0, r1
@@ -159,7 +217,7 @@ hyp_hvc:
 THUMB(	orr	lr, #1)
 	blx	lr			@ Call the HYP function
 
-	pop	{lr}
+	pop	{r2, lr}
 	eret
 
 guest_trap:
-- 
2.14.2