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