su should not chdir to home
--- busybox-1.19.4/include/libbb.h
+++ busybox-1.19.4/include/libbb.h
@@ -1274,6 +1274,7 @@
 #define SETUP_ENV_CHANGEENV (1 << 0)
 #define SETUP_ENV_CLEARENV  (1 << 1)
 #define SETUP_ENV_TO_TMP    (1 << 2)
+#define SETUP_ENV_NO_CHDIR  (1 << 4)
 extern void setup_environment(const char *shell, int flags, const struct passwd *pw) FAST_FUNC;
 extern int correct_password(const struct passwd *pw) FAST_FUNC;
 /* Returns a malloced string */

--- busybox-1.19.4/libbb/setup_environment.c
+++ busybox-1.19.4/libbb/setup_environment.c
@@ -37,9 +37,11 @@
 
 	/* Change the current working directory to be the home directory
 	 * of the user */
-	if (chdir(pw->pw_dir)) {
-		xchdir((flags & SETUP_ENV_TO_TMP) ? "/tmp" : "/");
-		bb_error_msg("can't chdir to home directory '%s'", pw->pw_dir);
+	if ((flags & SETUP_ENV_NO_CHDIR) == 0) {
+		if (chdir(pw->pw_dir)) {
+			xchdir((flags & SETUP_ENV_TO_TMP) ? "/tmp" : "/");
+			bb_error_msg("can't chdir to home directory '%s'", pw->pw_dir);
+		}
 	}
 
 	if (flags & SETUP_ENV_CLEARENV) {

--- busybox-1.19.4/loginutils/su.c
+++ busybox-1.19.4/loginutils/su.c
@@ -131,7 +131,8 @@
 	change_identity(pw);
 	setup_environment(opt_shell,
 			((flags & SU_OPT_l) / SU_OPT_l * SETUP_ENV_CLEARENV)
-			+ (!(flags & SU_OPT_mp) * SETUP_ENV_CHANGEENV),
+			+ (!(flags & SU_OPT_mp) * SETUP_ENV_CHANGEENV)
+			+ (!(flags & SU_OPT_l) * SETUP_ENV_NO_CHDIR),
 			pw);
 	IF_SELINUX(set_current_security_context(NULL);)