lkdtm: Add CFI_BACKWARD to test ROP mitigations

In order to test various backward-edge control flow integrity methods,
add a test that manipulates the return address on the stack. Currently
only arm64 Pointer Authentication and Shadow Call Stack is supported.

 $ echo CFI_BACKWARD | cat >/sys/kernel/debug/provoke-crash/DIRECT

Under SCS, successful test of the mitigation is reported as:

 lkdtm: Performing direct entry CFI_BACKWARD
 lkdtm: Attempting unchecked stack return address redirection ...
 lkdtm: ok: redirected stack return address.
 lkdtm: Attempting checked stack return address redirection ...
 lkdtm: ok: control flow unchanged.

Under PAC, successful test of the mitigation is reported by the PAC
exception handler:

 lkdtm: Performing direct entry CFI_BACKWARD
 lkdtm: Attempting unchecked stack return address redirection ...
 lkdtm: ok: redirected stack return address.
 lkdtm: Attempting checked stack return address redirection ...
 Unable to handle kernel paging request at virtual address bfffffc0088d0514
 Mem abort info:
   ESR = 0x86000004
   EC = 0x21: IABT (current EL), IL = 32 bits
   SET = 0, FnV = 0
   EA = 0, S1PTW = 0
   FSC = 0x04: level 0 translation fault
 [bfffffc0088d0514] address between user and kernel address ranges
 ...

If the CONFIGs are missing (or the mitigation isn't working), failure
is reported as:

 lkdtm: Performing direct entry CFI_BACKWARD
 lkdtm: Attempting unchecked stack return address redirection ...
 lkdtm: ok: redirected stack return address.
 lkdtm: Attempting checked stack return address redirection ...
 lkdtm: FAIL: stack return address was redirected!
 lkdtm: This is probably expected, since this kernel was built *without* CONFIG_ARM64_PTR_AUTH_KERNEL=y nor CONFIG_SHADOW_CALL_STACK=y
Co-developed-by: Dan Li <ashimida@linux.alibaba.com>
Signed-off-by: Dan Li <ashimida@linux.alibaba.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/lkml/20220416001103.1524653-1-keescook@chromium.org

drivers/misc/lkdtm/cfi.c

@@ -3,6 +3,7 @@
  * This is for all the tests relating directly to Control Flow Integrity.
  */
 #include "lkdtm.h"
+#include <asm/page.h>
 
 static int called_count;
 
@@ -42,8 +43,141 @@ static void lkdtm_CFI_FORWARD_PROTO(void)
 	pr_expected_config(CONFIG_CFI_CLANG);
 }
 
+/*
+ * This can stay local to LKDTM, as there should not be a production reason
+ * to disable PAC && SCS.
+ */
+#ifdef CONFIG_ARM64_PTR_AUTH_KERNEL
+# ifdef CONFIG_ARM64_BTI_KERNEL
+#  define __no_pac             "branch-protection=bti"
+# else
+#  define __no_pac             "branch-protection=none"
+# endif
+# define __no_ret_protection   __noscs __attribute__((__target__(__no_pac)))
+#else
+# define __no_ret_protection   __noscs
+#endif
+
+#define no_pac_addr(addr)      \
+	((__force __typeof__(addr))((__force u64)(addr) | PAGE_OFFSET))
+
+/* The ultimate ROP gadget. */
+static noinline __no_ret_protection
+void set_return_addr_unchecked(unsigned long *expected, unsigned long *addr)
+{
+	/* Use of volatile is to make sure final write isn't seen as a dead store. */
+	unsigned long * volatile *ret_addr = (unsigned long **)__builtin_frame_address(0) + 1;
+
+	/* Make sure we've found the right place on the stack before writing it. */
+	if (no_pac_addr(*ret_addr) == expected)
+		*ret_addr = (addr);
+	else
+		/* Check architecture, stack layout, or compiler behavior... */
+		pr_warn("Eek: return address mismatch! %px != %px\n",
+			*ret_addr, addr);
+}
+
+static noinline
+void set_return_addr(unsigned long *expected, unsigned long *addr)
+{
+	/* Use of volatile is to make sure final write isn't seen as a dead store. */
+	unsigned long * volatile *ret_addr = (unsigned long **)__builtin_frame_address(0) + 1;
+
+	/* Make sure we've found the right place on the stack before writing it. */
+	if (no_pac_addr(*ret_addr) == expected)
+		*ret_addr = (addr);
+	else
+		/* Check architecture, stack layout, or compiler behavior... */
+		pr_warn("Eek: return address mismatch! %px != %px\n",
+			*ret_addr, addr);
+}
+
+static volatile int force_check;
+
+static void lkdtm_CFI_BACKWARD(void)
+{
+	/* Use calculated gotos to keep labels addressable. */
+	void *labels[] = {0, &&normal, &&redirected, &&check_normal, &&check_redirected};
+
+	pr_info("Attempting unchecked stack return address redirection ...\n");
+
+	/* Always false */
+	if (force_check) {
+		/*
+		 * Prepare to call with NULLs to avoid parameters being treated as
+		 * constants in -02.
+		 */
+		set_return_addr_unchecked(NULL, NULL);
+		set_return_addr(NULL, NULL);
+		if (force_check)
+			goto *labels[1];
+		if (force_check)
+			goto *labels[2];
+		if (force_check)
+			goto *labels[3];
+		if (force_check)
+			goto *labels[4];
+		return;
+	}
+
+	/*
+	 * Use fallthrough switch case to keep basic block ordering between
+	 * set_return_addr*() and the label after it.
+	 */
+	switch (force_check) {
+	case 0:
+		set_return_addr_unchecked(&&normal, &&redirected);
+		fallthrough;
+	case 1:
+normal:
+		/* Always true */
+		if (!force_check) {
+			pr_err("FAIL: stack return address manipulation failed!\n");
+			/* If we can't redirect "normally", we can't test mitigations. */
+			return;
+		}
+		break;
+	default:
+redirected:
+		pr_info("ok: redirected stack return address.\n");
+		break;
+	}
+
+	pr_info("Attempting checked stack return address redirection ...\n");
+
+	switch (force_check) {
+	case 0:
+		set_return_addr(&&check_normal, &&check_redirected);
+		fallthrough;
+	case 1:
+check_normal:
+		/* Always true */
+		if (!force_check) {
+			pr_info("ok: control flow unchanged.\n");
+			return;
+		}
+
+check_redirected:
+		pr_err("FAIL: stack return address was redirected!\n");
+		break;
+	}
+
+	if (IS_ENABLED(CONFIG_ARM64_PTR_AUTH_KERNEL)) {
+		pr_expected_config(CONFIG_ARM64_PTR_AUTH_KERNEL);
+		return;
+	}
+	if (IS_ENABLED(CONFIG_SHADOW_CALL_STACK)) {
+		pr_expected_config(CONFIG_SHADOW_CALL_STACK);
+		return;
+	}
+	pr_warn("This is probably expected, since this %s was built *without* %s=y nor %s=y\n",
+		lkdtm_kernel_info,
+		"CONFIG_ARM64_PTR_AUTH_KERNEL", "CONFIG_SHADOW_CALL_STACK");
+}
+
 static struct crashtype crashtypes[] = {
 	CRASHTYPE(CFI_FORWARD_PROTO),
+	CRASHTYPE(CFI_BACKWARD),
 };
 
 struct crashtype_category cfi_crashtypes = {

tools/testing/selftests/lkdtm/tests.txt

@@ -74,6 +74,7 @@ USERCOPY_STACK_BEYOND
 USERCOPY_KERNEL
 STACKLEAK_ERASING OK: the rest of the thread stack is properly erased
 CFI_FORWARD_PROTO
+CFI_BACKWARD call trace:|ok: control flow unchanged
 FORTIFIED_STRSCPY
 FORTIFIED_OBJECT
 FORTIFIED_SUBOBJECT