Merge tag 'vfs-6.16-rc1.coredump' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs

Pull coredump updates from Christian Brauner: "This adds support for sending coredumps over an AF_UNIX socket. It also makes (implicit) use of the new SO_PEERPIDFD ability to hand out pidfds for reaped peer tasks The new coredump socket will allow userspace to not have to rely on usermode helpers for processing coredumps and provides a saf way to handle them instead of relying on super privileged coredumping helpers This will also be significantly more lightweight since the kernel doens't have to do a fork()+exec() for each crashing process to spawn a usermodehelper. Instead the kernel just connects to the AF_UNIX socket and userspace can process it concurrently however it sees fit. Support for userspace is incoming starting with systemd-coredump There's more work coming in that direction next cycle. The rest below goes into some details and background Coredumping currently supports two modes: (1) Dumping directly into a file somewhere on the filesystem. (2) Dumping into a pipe connected to a usermode helper process spawned as a child of the system_unbound_wq or kthreadd For simplicity I'm mostly ignoring (1). There's probably still some users of (1) out there but processing coredumps in this way can be considered adventurous especially in the face of set*id binaries The most common option should be (2) by now. It works by allowing userspace to put a string into /proc/sys/kernel/core_pattern like: |/usr/lib/systemd/systemd-coredump %P %u %g %s %t %c %h The "|" at the beginning indicates to the kernel that a pipe must be used. The path following the pipe indicator is a path to a binary that will be spawned as a usermode helper process. Any additional parameters pass information about the task that is generating the coredump to the binary that processes the coredump In the example the core_pattern shown causes the kernel to spawn systemd-coredump as a usermode helper. There's various conceptual consequences of this (non-exhaustive list): - systemd-coredump is spawned with file descriptor number 0 (stdin) connected to the read-end of the pipe. All other file descriptors are closed. That specifically includes 1 (stdout) and 2 (stderr). This has already caused bugs because userspace assumed that this cannot happen (Whether or not this is a sane assumption is irrelevant) - systemd-coredump will be spawned as a child of system_unbound_wq. So it is not a child of any userspace process and specifically not a child of PID 1. It cannot be waited upon and is in a weird hybrid upcall which are difficult for userspace to control correctly - systemd-coredump is spawned with full kernel privileges. This necessitates all kinds of weird privilege dropping excercises in userspace to make this safe - A new usermode helper has to be spawned for each crashing process This adds a new mode: (3) Dumping into an AF_UNIX socket Userspace can set /proc/sys/kernel/core_pattern to: @/path/to/coredump.socket The "@" at the beginning indicates to the kernel that an AF_UNIX coredump socket will be used to process coredumps The coredump socket must be located in the initial mount namespace. When a task coredumps it opens a client socket in the initial network namespace and connects to the coredump socket: - The coredump server uses SO_PEERPIDFD to get a stable handle on the connected crashing task. The retrieved pidfd will provide a stable reference even if the crashing task gets SIGKILLed while generating the coredump. That is a huge attack vector right now - By setting core_pipe_limit non-zero userspace can guarantee that the crashing task cannot be reaped behind it's back and thus process all necessary information in /proc/<pid>. The SO_PEERPIDFD can be used to detect whether /proc/<pid> still refers to the same process The core_pipe_limit isn't used to rate-limit connections to the socket. This can simply be done via AF_UNIX socket directly - The pidfd for the crashing task will contain information how the task coredumps. The PIDFD_GET_INFO ioctl gained a new flag PIDFD_INFO_COREDUMP which can be used to retreive the coredump information If the coredump gets a new coredump client connection the kernel guarantees that PIDFD_INFO_COREDUMP information is available. Currently the following information is provided in the new @coredump_mask extension to struct pidfd_info: * PIDFD_COREDUMPED is raised if the task did actually coredump * PIDFD_COREDUMP_SKIP is raised if the task skipped coredumping (e.g., undumpable) * PIDFD_COREDUMP_USER is raised if this is a regular coredump and doesn't need special care by the coredump server * PIDFD_COREDUMP_ROOT is raised if the generated coredump should be treated as sensitive and the coredump server should restrict access to the generated coredump to sufficiently privileged users" * tag 'vfs-6.16-rc1.coredump' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs: mips, net: ensure that SOCK_COREDUMP is defined selftests/coredump: add tests for AF_UNIX coredumps selftests/pidfd: add PIDFD_INFO_COREDUMP infrastructure coredump: validate socket name as it is written coredump: show supported coredump modes pidfs, coredump: add PIDFD_INFO_COREDUMP coredump: add coredump socket coredump: reflow dump helpers a little coredump: massage do_coredump() coredump: massage format_corename()

LTLinus Torvalds committed 1 year ago