kernel: core_hook: screw path_umount backport, call sys_umount directly

I am repasting here what I posted on the source code originally:

/*
 * turns out path_umount backport is completely unneeded
 * we copy the trick used on strncpy_from_unsafe_user / strncpy_from_user_nofault
 * https://elixir.bootlin.com/linux/v4.4.302/source/mm/maccess.c#L184
 * basically
 *
 *     mm_segment_t old_fs = get_fs();     // remember original fs segment
 *     set_fs(USER_DS);                    // or KERNEL_DS *
 *     do_whatever_in_userspace();
 *     set_fs(old_fs);                     // restore fs segment
 *
 *  * kernel -> user, KERNEL_DS, user -> kernel, USER_DS
 *
 * so yes, we can try to straight up call a syscall from kernel space
 *
 * NOTE: on newer kernels you can use force_uaccess_begin + force_uaccess_end
 * ref: https://elixir.bootlin.com/linux/v5.10.237/source/mm/maccess.c#L250
 *
 */

path_umount backport now optional — neat trick, werks, what can I say.

Backports? Nah, we’re good.

Signed-off-by: backslashxx <[email protected]>

Author: backslashxx

kernel/core_hook.c

@@ -16,6 +16,9 @@
 #include <linux/mount.h>
 #include <linux/fs.h>
 #include <linux/namei.h>
+#ifndef KSU_HAS_PATH_UMOUNT
+#include <linux/syscalls.h> // sys_umount
+#endif
 
 #include "allowlist.h"
 #include "core_hook.h"
@@ -506,13 +509,26 @@ static bool should_umount(struct path *path)
 	return false;
 }
 
+#ifdef KSU_HAS_PATH_UMOUNT
 static void ksu_umount_mnt(struct path *path, int flags)
 {
 	int err = path_umount(path, flags);
 	if (err) {
 		pr_info("umount %s failed: %d\n", path->dentry->d_iname, err);
 	}
 }
+#else
+static void ksu_sys_umount(const char *mnt, int flags)
+{
+	char __user *usermnt = (char __user *)mnt;
+
+	mm_segment_t old_fs = get_fs();
+	set_fs(KERNEL_DS);
+	long ret = sys_umount(usermnt, flags); // cuz asmlinkage long sys##name
+	set_fs(old_fs);
+	pr_info("%s: path: %s code: %d \n", __func__, mnt, ret);
+}
+#endif // KSU_HAS_PATH_UMOUNT
 
 static void try_umount(const char *mnt, bool check_mnt, int flags)
 {
@@ -532,7 +548,11 @@ static void try_umount(const char *mnt, bool check_mnt, int flags)
 		return;
 	}
 
+#ifdef KSU_HAS_PATH_UMOUNT
 	ksu_umount_mnt(&path, flags);
+#else
+	ksu_sys_umount(mnt, flags);
+#endif
 }
 
 int ksu_handle_setuid(struct cred *new, const struct cred *old)