ARM: smp: add support for per-task stack canaries

On ARM, we currently only change the value of the stack canary when
switching tasks if the kernel was built for UP. On SMP kernels, this
is impossible since the stack canary value is obtained via a global
symbol reference, which means
a) all running tasks on all CPUs must use the same value
b) we can only modify the value when no kernel stack frames are live
   on any CPU, which is effectively never.

So instead, use a GCC plugin to add a RTL pass that replaces each
reference to the address of the __stack_chk_guard symbol with an
expression that produces the address of the 'stack_canary' field
that is added to struct thread_info. This way, each task will use
its own randomized value.

Cc: Russell King <linux@armlinux.org.uk>
Cc: Kees Cook <keescook@chromium.org>
Cc: Emese Revfy <re.emese@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Laura Abbott <labbott@redhat.com>
Cc: kernel-hardening@lists.openwall.com
Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Kees Cook <keescook@chromium.org>

2 files changed, 107 insertions, 0 deletions

diff --git a/scripts/gcc-plugins/Kconfig b/scripts/gcc-plugins/Kconfig
index 0d5c799688f0..d45f7f36b859 100644
--- a/scripts/gcc-plugins/Kconfig
+++ b/scripts/gcc-plugins/Kconfig
@@ -190,4 +190,8 @@ config STACKLEAK_RUNTIME_DISABLE
 	  runtime to control kernel stack erasing for kernels built with
 	  CONFIG_GCC_PLUGIN_STACKLEAK.
 
+config GCC_PLUGIN_ARM_SSP_PER_TASK
+	bool
+	depends on GCC_PLUGINS && ARM
+
 endif
diff --git a/scripts/gcc-plugins/arm_ssp_per_task_plugin.c b/scripts/gcc-plugins/arm_ssp_per_task_plugin.c
new file mode 100644
index 000000000000..de70b8470971
--- /dev/null
+++ b/scripts/gcc-plugins/arm_ssp_per_task_plugin.c
@@ -0,0 +1,103 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include "gcc-common.h"
+
+__visible int plugin_is_GPL_compatible;
+
+static unsigned int sp_mask, canary_offset;
+
+static unsigned int arm_pertask_ssp_rtl_execute(void)
+{
+	rtx_insn *insn;
+
+	for (insn = get_insns(); insn; insn = NEXT_INSN(insn)) {
+		const char *sym;
+		rtx body;
+		rtx masked_sp;
+
+		/*
+		 * Find a SET insn involving a SYMBOL_REF to __stack_chk_guard
+		 */
+		if (!INSN_P(insn))
+			continue;
+		body = PATTERN(insn);
+		if (GET_CODE(body) != SET ||
+		    GET_CODE(SET_SRC(body)) != SYMBOL_REF)
+			continue;
+		sym = XSTR(SET_SRC(body), 0);
+		if (strcmp(sym, "__stack_chk_guard"))
+			continue;
+
+		/*
+		 * Replace the source of the SET insn with an expression that
+		 * produces the address of the copy of the stack canary value
+		 * stored in struct thread_info
+		 */
+		masked_sp = gen_reg_rtx(Pmode);
+
+		emit_insn_before(gen_rtx_SET(masked_sp,
+					     gen_rtx_AND(Pmode,
+							 stack_pointer_rtx,
+							 GEN_INT(sp_mask))),
+				 insn);
+
+		SET_SRC(body) = gen_rtx_PLUS(Pmode, masked_sp,
+					     GEN_INT(canary_offset));
+	}
+	return 0;
+}
+
+#define PASS_NAME arm_pertask_ssp_rtl
+
+#define NO_GATE
+#include "gcc-generate-rtl-pass.h"
+
+__visible int plugin_init(struct plugin_name_args *plugin_info,
+			  struct plugin_gcc_version *version)
+{
+	const char * const plugin_name = plugin_info->base_name;
+	const int argc = plugin_info->argc;
+	const struct plugin_argument *argv = plugin_info->argv;
+	int tso = 0;
+	int i;
+
+	if (!plugin_default_version_check(version, &gcc_version)) {
+		error(G_("incompatible gcc/plugin versions"));
+		return 1;
+	}
+
+	for (i = 0; i < argc; ++i) {
+		if (!strcmp(argv[i].key, "disable"))
+			return 0;
+
+		/* all remaining options require a value */
+		if (!argv[i].value) {
+			error(G_("no value supplied for option '-fplugin-arg-%s-%s'"),
+			      plugin_name, argv[i].key);
+			return 1;
+		}
+
+		if (!strcmp(argv[i].key, "tso")) {
+			tso = atoi(argv[i].value);
+			continue;
+		}
+
+		if (!strcmp(argv[i].key, "offset")) {
+			canary_offset = atoi(argv[i].value);
+			continue;
+		}
+		error(G_("unknown option '-fplugin-arg-%s-%s'"),
+		      plugin_name, argv[i].key);
+		return 1;
+	}
+
+	/* create the mask that produces the base of the stack */
+	sp_mask = ~((1U << (12 + tso)) - 1);
+
+	PASS_INFO(arm_pertask_ssp_rtl, "expand", 1, PASS_POS_INSERT_AFTER);
+
+	register_callback(plugin_info->base_name, PLUGIN_PASS_MANAGER_SETUP,
+			  NULL, &arm_pertask_ssp_rtl_pass_info);
+
+	return 0;
+}