Re: [PATCH RFC v3 08/10] net, pidfs, coredump: only allow coredumping tasks to connect to coredump socket

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Jann Horn <jannh@google.com>
Date: Mon, 5 May 2025 21:10:28 +0200
> On Mon, May 5, 2025 at 8:41 PM Kuniyuki Iwashima <kuniyu@amazon.com> wrote:
> > From: Christian Brauner <brauner@kernel.org>
> > Date: Mon, 5 May 2025 16:06:40 +0200
> > > On Mon, May 05, 2025 at 03:08:07PM +0200, Jann Horn wrote:
> > > > On Mon, May 5, 2025 at 1:14 PM Christian Brauner <brauner@kernel.org> wrote:
> > > > > Make sure that only tasks that actually coredumped may connect to the
> > > > > coredump socket. This restriction may be loosened later in case
> > > > > userspace processes would like to use it to generate their own
> > > > > coredumps. Though it'd be wiser if userspace just exposed a separate
> > > > > socket for that.
> > > >
> > > > This implementation kinda feels a bit fragile to me... I wonder if we
> > > > could instead have a flag inside the af_unix client socket that says
> > > > "this is a special client socket for coredumping".
> > >
> > > Should be easily doable with a sock_flag().
> >
> > This restriction should be applied by BPF LSM.
> 
> I think we shouldn't allow random userspace processes to connect to
> the core dump handling service and provide bogus inputs; that
> unnecessarily increases the risk that a crafted coredump can be used
> to exploit a bug in the service. So I think it makes sense to enforce
> this restriction in the kernel.

It's already restricted by /proc/sys/kernel/core_pattern.
We don't need a duplicated logic.

Even when the process holding the listener dies, you can
still avoid such a leak.

e.g.

1. Set up a listener
2. Put the socket into a bpf map
3. Attach LSM at connect()

Then, the LSM checks if the destination socket is

  * listening on the name specified in /proc/sys/kernel/core_pattern
  * exists in the associated BPF map

So, if the socket is dies and a malicious user tries to hijack
the core_pattern name, LSM still rejects such connect().

Later, the admin can restart the program with different core_pattern.


> 
> My understanding is that BPF LSM creates fairly tight coupling between
> userspace and the kernel implementation, and it is kind of unwieldy
> for userspace. (I imagine the "man 5 core" manpage would get a bit
> longer and describe more kernel implementation detail if you tried to
> show how to write a BPF LSM that is capable of detecting unix domain
> socket connections to a specific address that are not initiated by
> core dumping.) I would like to keep it possible to implement core
> userspace functionality in a best-practice way without needing eBPF.

I think the untrusted user scenario is paranoia in most cases,
and the man page just says "if you really care, use BPF LSM".

If someone can listen on a name AND set it to core_pattern, most
likely something worse already happened.


> 
> > It's hard to loosen such a default restriction as someone might
> > argue that's unexpected and regression.
> 
> If userspace wants to allow other processes to connect to the core
> dumping service, that's easy to implement - userspace can listen on a
> separate address that is not subject to these restrictions.
>