This adds the capability for seccomp notifier listeners to add file descriptors in response to a seccomp notification. This is useful for syscalls in which the previous capabilities were not sufficient. The current mechanism works well for syscalls that either have side effects that are system / namespace wide (mount), or that operate on a specific set of registers (reboot, mknod), and don't require dereferencing pointers. The problem with derefencing pointers in a supervisor is that it leaves us vulnerable to TOC-TOU [1] style attacks. For syscalls that had a direct effect on file descriptors pidfd_getfd was added, allowing for those file descriptors to be directly operated upon by the supervisor [2]. Unfortunately, this leaves system calls which return file descriptors out of the picture. These are fairly common syscalls, such as openat, socket, and perf_event_open that return file descriptors, and have arguments that are pointers. These require that the supervisor is able to verify the arguments, make the call on behalf of the process on hand, and pass back the resulting file descriptor. This is where addfd comes into play. There is an additional flag that allows you to "set" an FD, rather than add it with an arbitrary number. This has dup2 style semantics, and installs the new file at that file descriptor, and atomically closes the old one if it existed. This is useful for a particular use case that we have, in which we want to swap out AF_INET sockets for AF_UNIX, AF_INET6, and sockets in another namespace when doing "upconversion". My specific usecase at Netflix is to enable our IPv4-IPv6 transition mechanism, in which we our namespaces have no real IPv4 reachability, and when it comes time to do a connect(2), we get a socket from a namespace with global IPv4 reachability. In addition, we intend to use it for our servicemesh, and where our service mesh needs to intercept traffic ingress traffic, the addfd capability will act as a mechanism to do socket activation. Addfd is not implemented as a separate syscall, a la pidfd_getfd, as VFS makes some optimizations in regards to the fdtable, and assumes that they are not modified by external processes. Although a mechanism that scheduled something in the context of the task could work, it is somewhat simpler to do it in the context of the ioctl as we control the task while in kernel. In addition there are not obvious needs for this beyond seccomp notifier. This mechanism leaves a potential issue that if the manager is interrupted while injecting FDs, the child process will be left with leaked / dangling FDs. This may lead to undefined behaviour. A mechanism to work around this is to extend the structure and add a "rollback" mechanism for FDs to be closed if things fail. This introduces a new helper -- file_receive, which is responsible for moving fds across processes. The helper replaces code in SCM_RIGHTS. In SCM_RIGHTS compat codepath there was a bug that resulted in this not being set all. This fixes that bug, and should be cherry-picked into long-term. The file_receive change should probably go into stable. The file_receive code also replaced the receive fd logic in pidfd_getfd. This is somewhat contrary to my original view[5], but I think it is best for the principal of least surprise to adopt it. This should be cherry-picked into stable. I tested this on amd64 with the x86-64 and x32 ABIs. Given there is no testing infrastructure for cgroup v1, I opted to forgo adding new tests there as it is considered deprecated. Changes since v2: * Introducion of the file_receive helper which hoists out logic to manipulate file descriptors outside of seccomp.c to file.c * Small fix that manipulated the socket's cgroup even when the receive failed * seccomp struct layout Changes since v1: * find_notification has been cleaned up slightly, and it replaces a use case in send as well. * Fixes ref counting rules to get / release references in the ioctl side, rather than the seccomp notifier side [3]. * Removes the optional move flag, and opts into SCM_RIGHTS * Rearranges the seccomp_notif_addfd datastructure for greater user clarity [4]. In order to avoid unnamed padding it makes size u64, which is a little bit of a waste of space. * Changes error codes to return ESRCH upon the process going away on notification, and EINPROGRESS is the notification is in an unexpected state (and added tests for this behaviour) [1]: https://lore.kernel.org/lkml/20190918084833.9369-2-christian.brauner@xxxxxxxxxx/ [2]: https://lore.kernel.org/lkml/20200107175927.4558-1-sargun@xxxxxxxxx/ [3]: https://lore.kernel.org/lkml/20200525000537.GB23230@xxxxxxxxxxxxxxxxxx/ [4]: https://lore.kernel.org/lkml/20200525135036.vp2nmmx42y7dfznf@wittgenstein/ [5]: https://lore.kernel.org/lkml/20200107175927.4558-1-sargun@xxxxxxxxx/ Sargun Dhillon (4): fs, net: Standardize on file_receive helper to move fds across processes pid: Use file_receive helper to copy FDs seccomp: Introduce addfd ioctl to seccomp user notifier selftests/seccomp: Test SECCOMP_IOCTL_NOTIF_ADDFD fs/file.c | 56 ++++++ include/linux/file.h | 2 + include/uapi/linux/seccomp.h | 25 +++ kernel/pid.c | 20 +- kernel/seccomp.c | 184 +++++++++++++++++- net/compat.c | 10 +- net/core/scm.c | 14 +- tools/testing/selftests/seccomp/seccomp_bpf.c | 183 +++++++++++++++++ 8 files changed, 467 insertions(+), 27 deletions(-) -- 2.25.1 _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/containers
