kernel: sucompat: increase reliability of execve_sucompat

On plain ARMv8.0 devices (A53,A57,A73), strncpy_from_user_nofault() sometimes
fails to copy `filename_user` string correctly. This breaks su ofc, breaking
some apps like Termux (Play Store ver), ZArchiver and Root Explorer.

This does NOT seem to affect newer ARMv8.2+ CPUs (A75/A76 and newer)

My speculation? ARMv8.0 has weak speculation :)

here we replace `strncpy_from_user_nofault()` with another routine:
 - access_ok() to validate the pointer
 - strncpy_from_user() to copy and validate string
 - manual null-termination just in case, as strncpy_from_user_nofault also does it
 - remove that memset, seems useless as it is an strncpy, not strncat

Kind of mimicking _nofault, but yes with this one we allow pagefaults.

Tested on:
- ARMv8.0 A73.a53, A57.a53, A53.a53
- ARMv8.2 A76.a55

Tested-by: iDead XD <rafifirdaus12bb@gmail.com>
Signed-off-by: backslashxx <118538522+backslashxx@users.noreply.github.com>

kernel/sucompat.c

@@ -161,8 +161,25 @@ int ksu_handle_execve_sucompat(int *fd, const char __user **filename_user,
 	if (unlikely(!filename_user))
 		return 0;
 
-	memset(path, 0, sizeof(path));
-	ksu_strncpy_from_user_nofault(path, *filename_user, sizeof(path));
+	/*
+	 * nofault variant fails silently due to pagefault_disable
+	 * some cpus dont really have that good speculative execution
+	 * access_ok to substitute set_fs, we check if pointer is accessible
+	 */
+#if LINUX_VERSION_CODE < KERNEL_VERSION(5,0,0)
+	if (!access_ok(VERIFY_READ, *filename_user, sizeof(path)))
+		return 0;
+#else
+	if (!access_ok(*filename_user, sizeof(path)))
+		return 0;
+#endif
+	// success = returns number of bytes and should be less than path
+	long len = strncpy_from_user(path, *filename_user, sizeof(path));
+	if (len <= 0)
+		return 0;
+
+	// strncpy_from_user_nofault does this too
+	path[sizeof(path) - 1] = '\0';
 
 	if (likely(memcmp(path, su, sizeof(su))))
 		return 0;