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

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Christian Brauner <brauner@kernel.org>
Date: Tue, 6 May 2025 17:16:13 +0200
> On Tue, May 06, 2025 at 04:51:25PM +0200, Jann Horn wrote:
> > On Tue, May 6, 2025 at 9:39 AM Christian Brauner <brauner@kernel.org> wrote:
> > > > ("a kernel socket" is not necessarily the same as "a kernel socket
> > > > intended for core dumping")
> > >
> > > Indeed. The usermodehelper is a kernel protocol. Here it's the task with
> > > its own credentials that's connecting to a userspace socket. Which makes
> > > this very elegant because it's just userspace IPC. No one is running
> > > around with kernel credentials anywhere.
> > 
> > To be clear: I think your current patch is using special kernel
> > privileges in one regard, because kernel_connect() bypasses the
> > security_socket_connect() security hook.

Precisely, whether LSM ignores kernel sockets or not depends on LSM.

When we create a socket, kern=0/1 is passed to security_socket_create().
Currently, SELinux always ignore the kernel socket, and AppArmor depends
on another condition.  Other LSM doesn't care.  Especially, BPF LSM is
just a set of functions to attach BPF programs, so it can enfoce whatever.


> I think it is a good thing
> > that it bypasses security hooks in this way; I think we wouldn't want
> > LSMs to get in the way of this special connect(), since the task in
> > whose context the connect() call happens is not in control of this
> > connection; the system administrator is the one who decided that this
> > connect() should happen on core dumps. It is kind of inconsistent
> > though that that separate security_unix_stream_connect() LSM hook will
> > still be invoked in this case, and we might have to watch out to make
> > sure that LSMs won't end up blocking such connections... which I think
> 
> Right, it is the same as for the usermode helper. It calls
> kernel_execve() which invokes at least security_bprm_creds_for_exec()
> and security_bprm_check(). Both of which can be used to make the
> usermodehelper execve fail.
> 
> Fwiw, it's even the case for dumping directly to a file as in that case
> it's subject to all kinds of lookup and open security hooks like
> security_file_open() and then another round in do_truncate().
> 
> All of that happens fully in the task's context as well via
> file_open()/file_open_root() or do_truncate().
> 
> So there's nothing special here.