ksud: Address pagefault in ksu_handle_execveat_ksud (#662)

* ksud: Address pagefault in ksu_handle_execveat_ksud

As pointed out by @backslashxx, when strncpy pagefaults, it causes
the first_arg to be completely NULL in some systems. This causes
second_stage initialization to fail hence causing SU to be
non-functional.

This patch copies ksu_strncpy_from_user_retry from @backslashxx's
commit:
e2fe25e485

This adds a fallback to perform a normal strncpy_from_user when nofault
fails which allows us to get the first_arg in such cases.

Co-authored-by: backslashxx <118538522+backslashxx@users.noreply.github.com>
Signed-off-by: Edrick Sinsuan <evcsinsuan@gmail.com>

* Revert "ksud: Add second_stage init variant (#653)"

This reverts commit c6b60a24e8.

---------

Signed-off-by: Edrick Sinsuan <evcsinsuan@gmail.com>
Co-authored-by: backslashxx <118538522+backslashxx@users.noreply.github.com>

kernel/kernel_compat.c

@@ -173,3 +173,26 @@ long ksu_strncpy_from_user_nofault(char *dst, const void __user *unsafe_addr,
 	return ret;
 }
 #endif
+
+static inline int ksu_access_ok(const void *addr, unsigned long size)
+{
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(5,0,0)
+	return access_ok(addr, size);
+#else
+	return access_ok(VERIFY_READ, addr, size);
+#endif
+}
+
+long ksu_strncpy_from_user_retry(char *dst, const void __user *unsafe_addr,
+				   long count)
+{
+	long ret = ksu_strncpy_from_user_nofault(dst, unsafe_addr, count);
+	if (likely(ret >= 0))
+		return ret;
+
+	// we faulted! fallback to slow path
+	if (unlikely(!ksu_access_ok(unsafe_addr, count)))
+		return -EFAULT;
+
+	return strncpy_from_user(dst, unsafe_addr, count);
+}

kernel/kernel_compat.h

@@ -23,6 +23,9 @@
 extern long ksu_strncpy_from_user_nofault(char *dst,
 					  const void __user *unsafe_addr,
 					  long count);
+extern long ksu_strncpy_from_user_retry(char *dst,
+					  const void __user *unsafe_addr,
+					  long count);
 
 #if LINUX_VERSION_CODE < KERNEL_VERSION(4, 10, 0) || defined(CONFIG_IS_HW_HISI) || defined(CONFIG_KSU_ALLOWLIST_WORKAROUND)
 extern struct key *init_session_keyring;

kernel/ksud.c

@@ -197,12 +197,11 @@ int ksu_handle_execveat_ksud(int *fd, struct filename **filename_ptr,
 			const char __user *p = get_user_arg_ptr(*argv, 1);
 			if (p && !IS_ERR(p)) {
 				char first_arg[16];
-				ksu_strncpy_from_user_nofault(
+				ksu_strncpy_from_user_retry(
 					first_arg, p, sizeof(first_arg));
 				pr_info("/system/bin/init first arg: %s\n",
 					first_arg);
-				if (!strcmp(first_arg, "second_stage") || 
+				if (!strcmp(first_arg, "second_stage")) {
-				    (argc == 2 && !strcmp(first_arg, ""))) {
 					pr_info("/system/bin/init second_stage executed\n");
 					apply_kernelsu_rules();
 					init_second_stage_executed = true;
@@ -223,7 +222,7 @@ int ksu_handle_execveat_ksud(int *fd, struct filename **filename_ptr,
 			const char __user *p = get_user_arg_ptr(*argv, 1);
 			if (p && !IS_ERR(p)) {
 				char first_arg[16];
-				ksu_strncpy_from_user_nofault(
+				ksu_strncpy_from_user_retry(
 					first_arg, p, sizeof(first_arg));
 				pr_info("/init first arg: %s\n", first_arg);
 				if (!strcmp(first_arg, "--second-stage")) {
@@ -248,7 +247,7 @@ int ksu_handle_execveat_ksud(int *fd, struct filename **filename_ptr,
 					}
 					char env[256];
 					// Reading environment variable strings from user space
-					if (ksu_strncpy_from_user_nofault(
+					if (ksu_strncpy_from_user_retry(
 						    env, p, sizeof(env)) < 0)
 						continue;
 					// Parsing environment variable names and values