xen/arm32: Invalidate icache on guest exist for Cortex-A15

In order to avoid aliasing attacks against the branch predictor on
Cortex A-15, let's invalidate the BTB on guest exit, which can only be
done by invalidating the icache (with ACTLR[0] being set).

We use the same hack as for A12/A17 to perform the vector decoding.

This is based on Linux patch from the kpti branch in [1].

[1] https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git

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

@@ -162,6 +162,26 @@ GLOBAL(hyp_traps_vector)
 
 #ifdef CONFIG_HARDEN_BRANCH_PREDICTOR
 
+        .align 5
+GLOBAL(hyp_traps_vector_ic_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, 0       /* ICIALLU */
+        isb
+
+        b decode_vectors
+
         .align 5
 GLOBAL(hyp_traps_vector_bp_inv)
         /*
@@ -180,6 +200,7 @@ GLOBAL(hyp_traps_vector_bp_inv)
         mcr	p15, 0, r0, c7, c5, 6	    /* BPIALL */
         isb
 
+decode_vectors:
 .macro vect_br val, targ
         eor     sp, sp, #\val
         tst     sp, #7

xen/arch/arm/cpuerrata.c

@@ -180,6 +180,7 @@ static int enable_psci_bp_hardening(void *data)
 DEFINE_PER_CPU_READ_MOSTLY(const char *, bp_harden_vecs);
 
 extern char hyp_traps_vector_bp_inv[];
+extern char hyp_traps_vector_ic_inv[];
 
 static void __maybe_unused
 install_bp_hardening_vecs(const struct arm_cpu_capabilities *entry,
@@ -205,6 +206,13 @@ static int enable_bp_inv_hardening(void *data)
     return 0;
 }
 
+static int enable_ic_inv_hardening(void *data)
+{
+    install_bp_hardening_vecs(data, hyp_traps_vector_ic_inv,
+                              "execute ICIALLU");
+    return 0;
+}
+
 #endif
 
 #define MIDR_RANGE(model, min, max)     \
@@ -302,6 +310,11 @@ static const struct arm_cpu_capabilities arm_errata[] = {
         MIDR_ALL_VERSIONS(MIDR_CORTEX_A17),
         .enable = enable_bp_inv_hardening,
     },
+    {
+        .capability = ARM_HARDEN_BRANCH_PREDICTOR,
+        MIDR_ALL_VERSIONS(MIDR_CORTEX_A15),
+        .enable = enable_ic_inv_hardening,
+    },
 #endif
     {},
 };