xen/arm32: Invalidate BTB on guest exit for Cortex A17 and 12

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

This is based on the fourth version posted by Marc Zyngier on Linux-arm
mailing list (see [1]).

This is part of XSA-254.

[1] https://www.spinics.net/lists/arm-kernel/msg632062.html

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Julien Grall <julien.grall@linaro.org>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

xen/arch/arm/arm32/entry.S

@@ -160,6 +160,44 @@ GLOBAL(hyp_traps_vector)
         b trap_irq                      /* 0x18 - IRQ */
         b trap_fiq                      /* 0x1c - FIQ */
 
+#ifdef CONFIG_HARDEN_BRANCH_PREDICTOR
+
+        .align 5
+GLOBAL(hyp_traps_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.
+         */
+        add sp, sp, #1                  /* Reset            7 */
+        add sp, sp, #1                  /* Undef            6 */
+        add sp, sp, #1                  /* Hypervisor Call  5 */
+        add sp, sp, #1                  /* Prefetch abort   4 */
+        add sp, sp, #1                  /* Data abort       3 */
+        add sp, sp, #1                  /* Hypervisor       2 */
+        add sp, sp, #1                  /* IRQ              1 */
+        nop                             /* FIQ              0 */
+
+        mcr	p15, 0, r0, c7, c5, 6	    /* BPIALL */
+        isb
+
+.macro vect_br val, targ
+        eor     sp, sp, #\val
+        tst     sp, #7
+        eorne   sp, sp, #\val
+        beq     \targ
+.endm
+
+        vect_br 0, trap_fiq
+        vect_br 1, trap_irq
+        vect_br 2, trap_guest_sync
+        vect_br 3, trap_data_abort
+        vect_br 4, trap_prefetch_abort
+        vect_br 5, trap_hypervisor_call
+        vect_br 6, trap_undefined_instruction
+        vect_br 7, trap_reset
+
+#endif /* CONFIG_HARDEN_BRANCH_PREDICTOR */
+
 DEFINE_TRAP_ENTRY(reset)
 DEFINE_TRAP_ENTRY(undefined_instruction)
 DEFINE_TRAP_ENTRY(hypervisor_call)

xen/arch/arm/cpuerrata.c

@@ -198,6 +198,13 @@ install_bp_hardening_vecs(const struct arm_cpu_capabilities *entry,
     this_cpu(bp_harden_vecs) = hyp_vecs;
 }
 
+static int enable_bp_inv_hardening(void *data)
+{
+    install_bp_hardening_vecs(data, hyp_traps_vector_bp_inv,
+                              "execute BPIALL");
+    return 0;
+}
+
 #endif
 
 #define MIDR_RANGE(model, min, max)     \
@@ -283,6 +290,18 @@ static const struct arm_cpu_capabilities arm_errata[] = {
         MIDR_ALL_VERSIONS(MIDR_CORTEX_A75),
         .enable = enable_psci_bp_hardening,
     },
+#endif
+#ifdef CONFIG_ARM32_HARDEN_BRANCH_PREDICTOR
+    {
+        .capability = ARM_HARDEN_BRANCH_PREDICTOR,
+        MIDR_ALL_VERSIONS(MIDR_CORTEX_A12),
+        .enable = enable_bp_inv_hardening,
+    },
+    {
+        .capability = ARM_HARDEN_BRANCH_PREDICTOR,
+        MIDR_ALL_VERSIONS(MIDR_CORTEX_A17),
+        .enable = enable_bp_inv_hardening,
+    },
 #endif
     {},
 };