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seccomp fixes for v5.13-rc4

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- Fix addfd notification race condition (Sargun Dhillon)
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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
This commit is contained in:
Linus Torvalds 2021-05-29 18:16:09 -10:00
commit 9a76c0ee3a
2 changed files with 24 additions and 22 deletions
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16
Documentation/userspace-api/seccomp_filter.rst
View file

@ -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

30
kernel/seccomp.c
View file

@ -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 */