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 10:06:27 +0200
> On Mon, May 05, 2025 at 09:10:28PM +0200, Jann Horn wrote:
> > 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.
> > 
> > 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.
> > 
> > > 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.
> 
> I think Kuniyuki's point is defensible. And I did discuss this with
> Lennart when I wrote the patch and he didn't see a point in preventing
> other processes from connecting to the core dump socket. He actually
> would like this to be possible because there's some userspace programs
> out there that generate their own coredumps (Python?) and he wanted them
> to use the general coredump socket to send them to.
> 
> I just found it more elegant to simply guarantee that only connections
> are made to that socket come from coredumping tasks.
> 
> But I should note there are two ways to cleanly handle this in
> userspace. I had already mentioned the bpf LSM in the contect of
> rate-limiting in an earlier posting:
> 
> (1) complex:
> 
>     Use a bpf LSM to intercept the connection request via
>     security_unix_stream_connect() in unix_stream_connect().
> 
>     The bpf program can simply check:
> 
>     current->signal->core_state
> 
>     and reject any connection if it isn't set to NULL.
> 
>     The big downside is that bpf (and security) need to be enabled.
>     Neither is guaranteed and there's quite a few users out there that
>     don't enable bpf.
> 
> (2) simple (and supported in this series):
> 
>     Userspace accepts a connection. It has to get SO_PEERPIDFD anyway.
>     It then needs to verify:
> 
>     struct pidfd_info info = {
>             info.mask = PIDFD_INFO_EXIT | PIDFD_INFO_COREDUMP,
>     };
> 
>     ioctl(pidfd, PIDFD_GET_INFO, &info);
>     if (!(info.mask & PIDFD_INFO_COREDUMP)) {
>             // Can't be from a coredumping task so we can close the
> 	    // connection without reading.
> 	    close(coredump_client_fd);
> 	    return;
>     }
> 
>     /* This has to be set and is only settable by do_coredump(). */
>     if (!(info.coredump_mask & PIDFD_COREDUMPED)) {
>             // Can't be from a coredumping task so we can close the
> 	    // connection without reading.
> 	    close(coredump_client_fd);
> 	    return;
>     }
> 
>     // Ok, this is a connection from a task that has coredumped, let's
>     // handle it.
> 
>     The crux is that the series guarantees that by the time the
>     connection is made the info whether the task/thread-group did
>     coredump is guaranteed to be available via the pidfd.
>  
> I think if we document that most coredump servers have to do (2) then
> this is fine. But I wouldn't mind a nod from Jann on this.

I like this approach (2) allowing users to filter the right client.
This way we can extend the application flexibly for another coredump
service.