[PATCH v4 2/2] lkdtm: Add Shadow Call Stack tests

Dan Li
2022-03-14T14:09:40Z

Add tests for SCS (Shadow Call Stack) based backward CFI.

Signed-off-by: Dan Li <ashimida@linux.alibaba.com>
---
arch/arm64/include/asm/compiler.h | 18 ++++++
drivers/misc/lkdtm/cfi.c | 84 +++++++++++++++++++++++++
drivers/misc/lkdtm/core.c | 1 +
drivers/misc/lkdtm/lkdtm.h | 1 +
include/linux/compiler-clang.h | 1 +
include/linux/compiler-gcc.h | 2 +
include/linux/compiler_types.h | 4 ++
tools/testing/selftests/lkdtm/tests.txt | 1 +
8 files changed, 112 insertions(+)

diff --git a/arch/arm64/include/asm/compiler.h b/arch/arm64/include/asm/compiler.h
index dc3ea4080e2e..96590fb4a8de 100644
--- a/arch/arm64/include/asm/compiler.h
+++ b/arch/arm64/include/asm/compiler.h
@@ -8,6 +8,24 @@
#define ARM64_ASM_PREAMBLE
#endif

+#ifndef __ASSEMBLY__
+#ifdef __KERNEL__
+
+#ifdef CONFIG_ARM64_PTR_AUTH_KERNEL
+
+#ifdef CONFIG_ARM64_BTI_KERNEL
+# define __no_ptrauth __attribute__((target("branch-protection=bti")))
+#elif defined(CONFIG_CC_HAS_BRANCH_PROT_PAC_RET)
+# define __no_ptrauth __attribute__((target("branch-protection=none")))
+#elif defined(CONFIG_CC_HAS_SIGN_RETURN_ADDRESS)
+# define __no_ptrauth __attribute__((target("sign-return-address=none")))
+#endif
+
+#endif /* CONFIG_ARM64_PTR_AUTH_KERNEL */
+
+#endif /* __KERNEL__ */
+#endif /* __ASSEMBLY__ */
+
/*
* The EL0/EL1 pointer bits used by a pointer authentication code.
* This is dependent on TBI0/TBI1 being enabled, or bits 63:56 would also apply.
diff --git a/drivers/misc/lkdtm/cfi.c b/drivers/misc/lkdtm/cfi.c
index c9aeddef1044..468ba2f26f74 100644
--- a/drivers/misc/lkdtm/cfi.c
+++ b/drivers/misc/lkdtm/cfi.c
@@ -41,3 +41,87 @@ void lkdtm_CFI_FORWARD_PROTO(void)
pr_err("FAIL: survived mismatched prototype function call!\n");
pr_expected_config(CONFIG_CFI_CLANG);
}
+
+#ifdef CONFIG_ARM64
+/*
+ * This function is used to modify its return address. The PAC needs to be turned
+ * off here to ensure that the modification of the return address will not be blocked.
+ */
+static noinline __no_ptrauth
+void lkdtm_scs_set_lr(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 (*ret_addr == expected)
+ *ret_addr = addr;
+}
+
+/* Function with __noscs attribute attempts to modify its return address. */
+static noinline __no_ptrauth __noscs
+void lkdtm_noscs_set_lr(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 (*ret_addr == expected)
+ *ret_addr = addr;
+}
+#else
+static inline void lkdtm_noscs_set_lr(unsigned long *expected, unsigned long *addr) { }
+static inline void lkdtm_scs_set_lr(unsigned long *expected, unsigned long *addr) { }
+#endif
+
+static volatile unsigned int force_label;
+
+/*
+ * This first checks whether a function with the __noscs attribute under
+ * the current platform can directly modify its return address, and if so,
+ * checks whether scs takes effect.
+ */
+void __no_optimize lkdtm_CFI_BACKWARD_SHADOW(void)
+{
+ void *array[] = {&&unexpected, &&expected, &&good_scs, &&bad_scs};
+
+ if (force_label && (force_label < sizeof(array))) {
+ /*
+ * Call them with "NULL" first to avoid
+ * arguments being treated as constants in -02.
+ */
+ lkdtm_noscs_set_lr(NULL, NULL);
+ lkdtm_scs_set_lr(NULL, NULL);
+ goto *array[force_label];
+ }
+
+ /* Keep labels in scope to avoid compiler warnings. */
+ do {
+ /* Verify the "normal" condition of LR corruption working. */
+ pr_info("Trying to corrupt lr in a function without scs protection ...\n");
+ lkdtm_noscs_set_lr(&&unexpected, &&expected);
+
+unexpected:
+ /*
+ * If lr cannot be modified, the following check is meaningless,
+ * returns directly.
+ */
+ pr_err("XPASS: Unexpectedly survived lr corruption without scs?!\n");
+ break;
+
+expected:
+ pr_info("ok: lr corruption redirected without scs.\n");
+
+ /* Verify that SCS is in effect. */
+ pr_info("Trying to corrupt lr in a function with scs protection ...\n");
+ lkdtm_scs_set_lr(&&good_scs, &&bad_scs);
+
+good_scs:
+ pr_info("ok: scs takes effect.\n");
+ break;
+
+bad_scs:
+ pr_err("FAIL: return address rewritten!\n");
+ pr_expected_config(CONFIG_SHADOW_CALL_STACK);
+ } while (0);
+}
diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
index f69b964b9952..7af7268b82e4 100644
--- a/drivers/misc/lkdtm/core.c
+++ b/drivers/misc/lkdtm/core.c
@@ -178,6 +178,7 @@ static const struct crashtype crashtypes[] = {
CRASHTYPE(USERCOPY_KERNEL),
CRASHTYPE(STACKLEAK_ERASING),
CRASHTYPE(CFI_FORWARD_PROTO),
+ CRASHTYPE(CFI_BACKWARD_SHADOW),
CRASHTYPE(FORTIFIED_OBJECT),
CRASHTYPE(FORTIFIED_SUBOBJECT),
CRASHTYPE(FORTIFIED_STRSCPY),
diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h
index d6137c70ebbe..a66fba949ab5 100644
--- a/drivers/misc/lkdtm/lkdtm.h
+++ b/drivers/misc/lkdtm/lkdtm.h
@@ -157,6 +157,7 @@ void lkdtm_STACKLEAK_ERASING(void);

/* cfi.c */
void lkdtm_CFI_FORWARD_PROTO(void);
+void lkdtm_CFI_BACKWARD_SHADOW(void);

/* fortify.c */
void lkdtm_FORTIFIED_OBJECT(void);
diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
index 3c4de9b6c6e3..2db37db36651 100644
--- a/include/linux/compiler-clang.h
+++ b/include/linux/compiler-clang.h
@@ -68,3 +68,4 @@

#define __nocfi __attribute__((__no_sanitize__("cfi")))
#define __cficanonical __attribute__((__cfi_canonical_jump_table__))
+#define __no_optimize __attribute__((optnone))
diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index deff5b308470..28d1b0ec6656 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -162,3 +162,5 @@
#if GCC_VERSION < 90100
#undef __alloc_size__
#endif
+
+#define __no_optimize __attribute__((optimize("-O0")))
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 3c1795fdb568..f5ad83f7ea2f 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -257,6 +257,10 @@ struct ftrace_likely_data {
# define __nocfi
#endif

+#ifndef __no_ptrauth
+# define __no_ptrauth
+#endif
+
#ifndef __cficanonical
# define __cficanonical
#endif
diff --git a/tools/testing/selftests/lkdtm/tests.txt b/tools/testing/selftests/lkdtm/tests.txt
index 6b36b7f5dcf9..12df67a3b419 100644
--- a/tools/testing/selftests/lkdtm/tests.txt
+++ b/tools/testing/selftests/lkdtm/tests.txt
@@ -73,6 +73,7 @@ USERCOPY_STACK_BEYOND
USERCOPY_KERNEL
STACKLEAK_ERASING OK: the rest of the thread stack is properly erased
CFI_FORWARD_PROTO
+CFI_BACKWARD_SHADOW ok: scs takes effect
FORTIFIED_STRSCPY
FORTIFIED_OBJECT
FORTIFIED_SUBOBJECT
--
2.17.1