Merge tag 'seccomp-fixes-v5.13-rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux

Pull seccomp fixes from Kees Cook:
 "This fixes a hard-to-hit race condition in the addfd user_notif
  feature of seccomp, visible since v5.9.

  And a small documentation fix"

* tag 'seccomp-fixes-v5.13-rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux:
  seccomp: Refactor notification handler to prepare for new semantics
  Documentation: seccomp: Fix user notification documentation

Author: torvalds

Documentation/userspace-api/seccomp_filter.rst

@@ -250,14 +250,14 @@ Users can read via ``ioctl(SECCOMP_IOCTL_NOTIF_RECV)``  (or ``poll()``) on a
 seccomp notification fd to receive a ``struct seccomp_notif``, which contains
 five members: the input length of the structure, a unique-per-filter ``id``,
 the ``pid`` of the task which triggered this request (which may be 0 if the
-task is in a pid ns not visible from the listener's pid namespace), a ``flags``
-member which for now only has ``SECCOMP_NOTIF_FLAG_SIGNALED``, representing
-whether or not the notification is a result of a non-fatal signal, and the
-``data`` passed to seccomp. Userspace can then make a decision based on this
-information about what to do, and ``ioctl(SECCOMP_IOCTL_NOTIF_SEND)`` a
-response, indicating what should be returned to userspace. The ``id`` member of
-``struct seccomp_notif_resp`` should be the same ``id`` as in ``struct
-seccomp_notif``.
+task is in a pid ns not visible from the listener's pid namespace). The
+notification also contains the ``data`` passed to seccomp, and a filters flag.
+The structure should be zeroed out prior to calling the ioctl.
+
+Userspace can then make a decision based on this information about what to do,
+and ``ioctl(SECCOMP_IOCTL_NOTIF_SEND)`` a response, indicating what should be
+returned to userspace. The ``id`` member of ``struct seccomp_notif_resp`` should
+be the same ``id`` as in ``struct seccomp_notif``.
 
 It is worth noting that ``struct seccomp_data`` contains the values of register
 arguments to the syscall, but does not contain pointers to memory. The task's

kernel/seccomp.c

@@ -1105,28 +1105,30 @@ static int seccomp_do_user_notification(int this_syscall,
 
 	up(&match->notif->request);
 	wake_up_poll(&match->wqh, EPOLLIN | EPOLLRDNORM);
-	mutex_unlock(&match->notify_lock);
 
 	/*
 	 * This is where we wait for a reply from userspace.
 	 */
-wait:
-	err = wait_for_completion_interruptible(&n.ready);
-	mutex_lock(&match->notify_lock);
-	if (err == 0) {
-		/* Check if we were woken up by a addfd message */
+	do {
+		mutex_unlock(&match->notify_lock);
+		err = wait_for_completion_interruptible(&n.ready);
+		mutex_lock(&match->notify_lock);
+		if (err != 0)
+			goto interrupted;
+
 		addfd = list_first_entry_or_null(&n.addfd,
 						 struct seccomp_kaddfd, list);
-		if (addfd && n.state != SECCOMP_NOTIFY_REPLIED) {
+		/* Check if we were woken up by a addfd message */
+		if (addfd)
 			seccomp_handle_addfd(addfd);
-			mutex_unlock(&match->notify_lock);
-			goto wait;
-		}
-		ret = n.val;
-		err = n.error;
-		flags = n.flags;
-	}
 
+	}  while (n.state != SECCOMP_NOTIFY_REPLIED);
+
+	ret = n.val;
+	err = n.error;
+	flags = n.flags;
+
+interrupted:
 	/* If there were any pending addfd calls, clear them out */
 	list_for_each_entry_safe(addfd, tmp, &n.addfd, list) {
 		/* The process went away before we got a chance to handle it */