seccomp: hoist out filter resolving logic

Hoist out the nth filter resolving logic that ptrace uses into a new
function. We'll use this in the next patch to implement the new
PTRACE_SECCOMP_GET_FILTER_FLAGS command.
Signed-off-by: Tycho Andersen <tycho@docker.com>
CC: Kees Cook <keescook@chromium.org>
CC: Andy Lutomirski <luto@amacapital.net>
CC: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

kernel/seccomp.c

@@ -978,49 +978,68 @@ long prctl_set_seccomp(unsigned long seccomp_mode, char __user *filter)
 }
 
 #if defined(CONFIG_SECCOMP_FILTER) && defined(CONFIG_CHECKPOINT_RESTORE)
-long seccomp_get_filter(struct task_struct *task, unsigned long filter_off,
-			void __user *data)
+static struct seccomp_filter *get_nth_filter(struct task_struct *task,
+					     unsigned long filter_off)
 {
-	struct seccomp_filter *filter;
-	struct sock_fprog_kern *fprog;
-	long ret;
-	unsigned long count = 0;
-
-	if (!capable(CAP_SYS_ADMIN) ||
-	    current->seccomp.mode != SECCOMP_MODE_DISABLED) {
-		return -EACCES;
-	}
+	struct seccomp_filter *orig, *filter;
+	unsigned long count;
 
+	/*
+	 * Note: this is only correct because the caller should be the (ptrace)
+	 * tracer of the task, otherwise lock_task_sighand is needed.
+	 */
 	spin_lock_irq(&task->sighand->siglock);
+
 	if (task->seccomp.mode != SECCOMP_MODE_FILTER) {
-		ret = -EINVAL;
-		goto out;
+		spin_unlock_irq(&task->sighand->siglock);
+		return ERR_PTR(-EINVAL);
 	}
 
-	filter = task->seccomp.filter;
-	while (filter) {
-		filter = filter->prev;
+	orig = task->seccomp.filter;
+	__get_seccomp_filter(orig);
+	spin_unlock_irq(&task->sighand->siglock);
+
+	count = 0;
+	for (filter = orig; filter; filter = filter->prev)
 		count++;
-	}
 
 	if (filter_off >= count) {
-		ret = -ENOENT;
+		filter = ERR_PTR(-ENOENT);
 		goto out;
 	}
-	count -= filter_off;
 
-	filter = task->seccomp.filter;
-	while (filter && count > 1) {
-		filter = filter->prev;
+	count -= filter_off;
+	for (filter = orig; filter && count > 1; filter = filter->prev)
 		count--;
-	}
 
 	if (WARN_ON(count != 1 || !filter)) {
-		/* The filter tree shouldn't shrink while we're using it. */
-		ret = -ENOENT;
+		filter = ERR_PTR(-ENOENT);
 		goto out;
 	}
 
+	__get_seccomp_filter(filter);
+
+out:
+	__put_seccomp_filter(orig);
+	return filter;
+}
+
+long seccomp_get_filter(struct task_struct *task, unsigned long filter_off,
+			void __user *data)
+{
+	struct seccomp_filter *filter;
+	struct sock_fprog_kern *fprog;
+	long ret;
+
+	if (!capable(CAP_SYS_ADMIN) ||
+	    current->seccomp.mode != SECCOMP_MODE_DISABLED) {
+		return -EACCES;
+	}
+
+	filter = get_nth_filter(task, filter_off);
+	if (IS_ERR(filter))
+		return PTR_ERR(filter);
+
 	fprog = filter->prog->orig_prog;
 	if (!fprog) {
 		/* This must be a new non-cBPF filter, since we save
@@ -1035,17 +1054,11 @@ long seccomp_get_filter(struct task_struct *task, unsigned long filter_off,
 	if (!data)
 		goto out;
 
-	__get_seccomp_filter(filter);
-	spin_unlock_irq(&task->sighand->siglock);
-
 	if (copy_to_user(data, fprog->filter, bpf_classic_proglen(fprog)))
 		ret = -EFAULT;
 
-	__put_seccomp_filter(filter);
-	return ret;
-
 out:
-	spin_unlock_irq(&task->sighand->siglock);
+	__put_seccomp_filter(filter);
 	return ret;
 }
 #endif