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authorKees Cook <keescook@chromium.org>2022-03-11 12:00:42 -0800
committerKees Cook <keescook@chromium.org>2022-04-16 13:57:23 -0700
commit2e53b877dc1258d4ac3de98f496bb88ec3bf5e25 (patch)
tree7ea3bac63c7f3f686da2eba687ea8a8ac142773e
parent73f62e60d80c2d74491933ca49006b9e70563738 (diff)
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
-rw-r--r--drivers/misc/lkdtm/cfi.c134
-rw-r--r--tools/testing/selftests/lkdtm/tests.txt1
2 files changed, 135 insertions, 0 deletions
diff --git a/drivers/misc/lkdtm/cfi.c b/drivers/misc/lkdtm/cfi.c
index e88f778be0d5..804965a480b7 100644
--- a/drivers/misc/lkdtm/cfi.c
+++ b/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 = {
diff --git a/tools/testing/selftests/lkdtm/tests.txt b/tools/testing/selftests/lkdtm/tests.txt
index 243c781f0780..9dace01dbf15 100644
--- a/tools/testing/selftests/lkdtm/tests.txt
+++ b/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