arm64: ftrace: Ensure module ftrace trampoline is coherent with I-side

The initial support for dynamic ftrace trampolines in modules made use
of an indirect branch which loaded its target from the beginning of
a special section (e71a4e1beb ("arm64: ftrace: add support for far
branches to dynamic ftrace")). Since no instructions were being patched,
no cache maintenance was needed. However, later in be0f272bfc ("arm64:
ftrace: emit ftrace-mod.o contents through code") this code was reworked
to output the trampoline instructions directly into the PLT entry but,
unfortunately, the necessary cache maintenance was overlooked.

Add a call to __flush_icache_range() after writing the new trampoline
instructions but before patching in the branch to the trampoline.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: James Morse <james.morse@arm.com>
Cc: <stable@vger.kernel.org>
Fixes: be0f272bfc ("arm64: ftrace: emit ftrace-mod.o contents through code")
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/ftrace.c

@@ -73,7 +73,7 @@ int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
 
 	if (offset < -SZ_128M || offset >= SZ_128M) {
 #ifdef CONFIG_ARM64_MODULE_PLTS
-		struct plt_entry trampoline;
+		struct plt_entry trampoline, *dst;
 		struct module *mod;
 
 		/*
@@ -106,23 +106,27 @@ int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
 		 * to check if the actual opcodes are in fact identical,
 		 * regardless of the offset in memory so use memcmp() instead.
 		 */
-		trampoline = get_plt_entry(addr, mod->arch.ftrace_trampoline);
-		if (memcmp(mod->arch.ftrace_trampoline, &trampoline,
-			   sizeof(trampoline))) {
-			if (plt_entry_is_initialized(mod->arch.ftrace_trampoline)) {
+		dst = mod->arch.ftrace_trampoline;
+		trampoline = get_plt_entry(addr, dst);
+		if (memcmp(dst, &trampoline, sizeof(trampoline))) {
+			if (plt_entry_is_initialized(dst)) {
 				pr_err("ftrace: far branches to multiple entry points unsupported inside a single module\n");
 				return -EINVAL;
 			}
 
 			/* point the trampoline to our ftrace entry point */
 			module_disable_ro(mod);
-			*mod->arch.ftrace_trampoline = trampoline;
+			*dst = trampoline;
 			module_enable_ro(mod, true);
 
-			/* update trampoline before patching in the branch */
-			smp_wmb();
+			/*
+			 * Ensure updated trampoline is visible to instruction
+			 * fetch before we patch in the branch.
+			 */
+			__flush_icache_range((unsigned long)&dst[0],
+					     (unsigned long)&dst[1]);
 		}
-		addr = (unsigned long)(void *)mod->arch.ftrace_trampoline;
+		addr = (unsigned long)dst;
 #else /* CONFIG_ARM64_MODULE_PLTS */
 		return -EINVAL;
 #endif /* CONFIG_ARM64_MODULE_PLTS */