coredump: validate socket name as it is written

In contrast to other parameters written into
/proc/sys/kernel/core_pattern that never fail we can validate enabling
the new AF_UNIX support. This is obviously racy as hell but it's always
been that way.

Link: https://lore.kernel.org/20250516-work-coredump-socket-v8-7-664f3caf2516@kernel.org
Acked-by: Luca Boccassi <luca.boccassi@gmail.com>
Reviewed-by: Jann Horn <jannh@google.com>
Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/coredump.c

@@ -1236,13 +1236,44 @@ void validate_coredump_safety(void)
 	}
 }
 
+static inline bool check_coredump_socket(void)
+{
+	if (core_pattern[0] != '@')
+		return true;
+
+	/*
+	 * Coredump socket must be located in the initial mount
+	 * namespace. Don't give the impression that anything else is
+	 * supported right now.
+	 */
+	if (current->nsproxy->mnt_ns != init_task.nsproxy->mnt_ns)
+		return false;
+
+	/* Must be an absolute path. */
+	if (*(core_pattern + 1) != '/')
+		return false;
+
+	return true;
+}
+
 static int proc_dostring_coredump(const struct ctl_table *table, int write,
 		  void *buffer, size_t *lenp, loff_t *ppos)
 {
-	int error = proc_dostring(table, write, buffer, lenp, ppos);
+	int error;
+	ssize_t retval;
+	char old_core_pattern[CORENAME_MAX_SIZE];
 
-	if (!error)
+	retval = strscpy(old_core_pattern, core_pattern, CORENAME_MAX_SIZE);
-		validate_coredump_safety();
+
+	error = proc_dostring(table, write, buffer, lenp, ppos);
+	if (error)
+		return error;
+	if (!check_coredump_socket()) {
+		strscpy(core_pattern, old_core_pattern, retval + 1);
+		return -EINVAL;
+	}
+
+	validate_coredump_safety();
 	return error;
 }