From: Linus Torvalds Date: Mon, 26 May 2025 17:30:02 +0000 (-0700) Subject: Merge tag 'vfs-6.16-rc1.pidfs' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs X-Git-Tag: block-6.16-20250606~48 X-Git-Url: https://git.kernel.dk/?a=commitdiff_plain;h=7d7a103d299eb5b95d67873c5ea7db419eaaebc0;p=linux-block.git Merge tag 'vfs-6.16-rc1.pidfs' of git://git./linux/kernel/git/vfs/vfs Pull pidfs updates from Christian Brauner: "Features: - Allow handing out pidfds for reaped tasks for AF_UNIX SO_PEERPIDFD socket option SO_PEERPIDFD is a socket option that allows to retrieve a pidfd for the process that called connect() or listen(). This is heavily used to safely authenticate clients in userspace avoiding security bugs due to pid recycling races (dbus, polkit, systemd, etc.) SO_PEERPIDFD currently doesn't support handing out pidfds if the sk->sk_peer_pid thread-group leader has already been reaped. In this case it currently returns EINVAL. Userspace still wants to get a pidfd for a reaped process to have a stable handle it can pass on. This is especially useful now that it is possible to retrieve exit information through a pidfd via the PIDFD_GET_INFO ioctl()'s PIDFD_INFO_EXIT flag Another summary has been provided by David Rheinsberg: > A pidfd can outlive the task it refers to, and thus user-space > must already be prepared that the task underlying a pidfd is > gone at the time they get their hands on the pidfd. For > instance, resolving the pidfd to a PID via the fdinfo must be > prepared to read `-1`. > > Despite user-space knowing that a pidfd might be stale, several > kernel APIs currently add another layer that checks for this. In > particular, SO_PEERPIDFD returns `EINVAL` if the peer-task was > already reaped, but returns a stale pidfd if the task is reaped > immediately after the respective alive-check. > > This has the unfortunate effect that user-space now has two ways > to check for the exact same scenario: A syscall might return > EINVAL/ESRCH/... *or* the pidfd might be stale, even though > there is no particular reason to distinguish both cases. This > also propagates through user-space APIs, which pass on pidfds. > They must be prepared to pass on `-1` *or* the pidfd, because > there is no guaranteed way to get a stale pidfd from the kernel. > > Userspace must already deal with a pidfd referring to a reaped > task as the task may exit and get reaped at any time will there > are still many pidfds referring to it In order to allow handing out reaped pidfd SO_PEERPIDFD needs to ensure that PIDFD_INFO_EXIT information is available whenever a pidfd for a reaped task is created by PIDFD_INFO_EXIT. The uapi promises that reaped pidfds are only handed out if it is guaranteed that the caller sees the exit information: TEST_F(pidfd_info, success_reaped) { struct pidfd_info info = { .mask = PIDFD_INFO_CGROUPID | PIDFD_INFO_EXIT, }; /* * Process has already been reaped and PIDFD_INFO_EXIT been set. * Verify that we can retrieve the exit status of the process. */ ASSERT_EQ(ioctl(self->child_pidfd4, PIDFD_GET_INFO, &info), 0); ASSERT_FALSE(!!(info.mask & PIDFD_INFO_CREDS)); ASSERT_TRUE(!!(info.mask & PIDFD_INFO_EXIT)); ASSERT_TRUE(WIFEXITED(info.exit_code)); ASSERT_EQ(WEXITSTATUS(info.exit_code), 0); } To hand out pidfds for reaped processes we thus allocate a pidfs entry for the relevant sk->sk_peer_pid at the time the sk->sk_peer_pid is stashed and drop it when the socket is destroyed. This guarantees that exit information will always be recorded for the sk->sk_peer_pid task and we can hand out pidfds for reaped processes - Hand a pidfd to the coredump usermode helper process Give userspace a way to instruct the kernel to install a pidfd for the crashing process into the process started as a usermode helper. There's still tricky race-windows that cannot be easily or sometimes not closed at all by userspace. There's various ways like looking at the start time of a process to make sure that the usermode helper process is started after the crashing process but it's all very very brittle and fraught with peril The crashed-but-not-reaped process can be killed by userspace before coredump processing programs like systemd-coredump have had time to manually open a PIDFD from the PID the kernel provides them, which means they can be tricked into reading from an arbitrary process, and they run with full privileges as they are usermode helper processes Even if that specific race-window wouldn't exist it's still the safest and cleanest way to let the kernel provide the pidfd directly instead of requiring userspace to do it manually. In parallel with this commit we already have systemd adding support for this in [1] When the usermode helper process is forked we install a pidfd file descriptor three into the usermode helper's file descriptor table so it's available to the exec'd program Since usermode helpers are either children of the system_unbound_wq workqueue or kthreadd we know that the file descriptor table is empty and can thus always use three as the file descriptor number Note, that we'll install a pidfd for the thread-group leader even if a subthread is calling do_coredump(). We know that task linkage hasn't been removed yet and even if this @current isn't the actual thread-group leader we know that the thread-group leader cannot be reaped until @current has exited - Allow telling when a task has not been found from finding the wrong task when creating a pidfd We currently report EINVAL whenever a struct pid has no tasked attached anymore thereby conflating two concepts: (1) The task has already been reaped (2) The caller requested a pidfd for a thread-group leader but the pid actually references a struct pid that isn't used as a thread-group leader This is causing issues for non-threaded workloads as in where they expect ESRCH to be reported, not EINVAL So allow userspace to reliably distinguish between (1) and (2) - Make it possible to detect when a pidfs entry would outlive the struct pid it pinned - Add a range of new selftests Cleanups: - Remove unneeded NULL check from pidfd_prepare() for passed struct pid - Avoid pointless reference count bump during release_task() Fixes: - Various fixes to the pidfd and coredump selftests - Fix error handling for replace_fd() when spawning coredump usermode helper" * tag 'vfs-6.16-rc1.pidfs' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs: pidfs: detect refcount bugs coredump: hand a pidfd to the usermode coredump helper coredump: fix error handling for replace_fd() pidfs: move O_RDWR into pidfs_alloc_file() selftests: coredump: Raise timeout to 2 minutes selftests: coredump: Fix test failure for slow machines selftests: coredump: Properly initialize pointer net, pidfs: enable handing out pidfds for reaped sk->sk_peer_pid pidfs: get rid of __pidfd_prepare() net, pidfs: prepare for handing out pidfds for reaped sk->sk_peer_pid pidfs: register pid in pidfs net, pidfd: report EINVAL for ESRCH release_task: kill the no longer needed get/put_pid(thread_pid) pidfs: ensure consistent ENOENT/ESRCH reporting exit: move wake_up_all() pidfd waiters into __unhash_process() selftest/pidfd: add test for thread-group leader pidfd open for thread pidfd: improve uapi when task isn't found pidfd: remove unneeded NULL check from pidfd_prepare() selftests/pidfd: adapt to recent changes --- 7d7a103d299eb5b95d67873c5ea7db419eaaebc0