Re: [PATCH RFC v3 00/10] coredump: add coredump socket

["Mickaël Salaün" <mic@digikod.net>]

On Mon, May 05, 2025 at 04:59:41PM +0200, Jann Horn wrote:
> On Mon, May 5, 2025 at 4:41 PM Mickaël Salaün <mic@digikod.net> wrote:
> > On Mon, May 05, 2025 at 01:13:38PM +0200, Christian Brauner wrote:
> > > 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 core_pattern shown above systemd-coredump is spawned 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 series adds a new mode:
> > >
> > > (3) Dumping into an abstract AF_UNIX socket.
> > >
> > > Userspace can set /proc/sys/kernel/core_pattern to:
> > >
> > >         @linuxafsk/coredump_socket
> > >
> > > The "@" at the beginning indicates to the kernel that the abstract
> > > AF_UNIX coredump socket will be used to process coredumps.
> > >
> > > The coredump socket uses the fixed address "linuxafsk/coredump.socket"
> > > for now.
> > >
> > > The coredump socket is located in the initial network namespace. To bind
> > > the coredump socket userspace must hold CAP_SYS_ADMIN in the initial
> > > user namespace. Listening and reading can happen from whatever
> > > unprivileged context is necessary to safely process coredumps.
> > >
> > > When a task coredumps it opens a client socket in the initial network
> > > namespace and connects to the coredump socket. For now only tasks that
> > > are acctually coredumping are allowed to connect to the initial coredump
> > > socket.
> >
> > I think we should avoid using abstract UNIX sockets, especially for new
> > interfaces, because it is hard to properly control such access.  Can we
> > create new dedicated AF_UNIX protocols instead?  One could be used by a
> > privileged process in the initial namespace to create a socket to
> > collect coredumps, and the other could be dedicatde to coredumped
> > proccesses.  Such (coredump collector) file descriptor or new (proxy)
> > socketpair ones could be passed to containers.
> 
> I would agree with you if we were talking about designing a pure
> userspace thing; but I think the limits that Christian added on bind()
> and connect() to these special abstract names in this series
> effectively make it behave as if they were dedicated AF_UNIX
> protocols, and prevent things like random unprivileged userspace
> processes bind()ing to them.

OK, so why not create a proper protocol?  That should also simplify the
interface.