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. There is an additional flag (move) that was added to enable cgroup v1 controllers (netprio, classid), and moving sockets, as a socket can only be associated with one cgroup at a time. [1]: https://lore.kernel.org/lkml/20190918084833.9369-2-christian.brauner@xxxxxxxxxx/ [2]: https://lore.kernel.org/lkml/20200107175927.4558-1-sargun@xxxxxxxxx/ Sargun Dhillon (5): seccomp: Add find_notification helper seccomp: Introduce addfd ioctl to seccomp user notifier selftests/seccomp: Test SECCOMP_IOCTL_NOTIF_ADDFD seccomp: Add SECCOMP_ADDFD_FLAG_MOVE flag to add fd ioctl selftests/seccomp: Add test for addfd move semantics include/uapi/linux/seccomp.h | 33 +++ kernel/seccomp.c | 228 +++++++++++++++-- tools/testing/selftests/seccomp/seccomp_bpf.c | 235 ++++++++++++++++++ 3 files changed, 479 insertions(+), 17 deletions(-) -- 2.25.1
