Date:   Tue, 12 Jul 2022 12:47:05 -0400
From:   "Benjamin Coddington" <bcodding@...hat.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     "David Howells" <dhowells@...hat.com>,
        linux-kernel@...r.kernel.org, "Ian Kent" <raven@...maw.net>,
        "Trond Myklebust" <trond.myklebust@...merspace.com>,
        linux-nfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        "Linux Containers" <containers@...ts.linux.dev>
Subject: Re: [RFC PATCH 0/2] Keyagents: another call_usermodehelper approach
 for namespaces

On 12 Jul 2022, at 10:16, Eric W. Biederman wrote:

> Adding the containers list to the discussion so more interested people
> have a chance of seeing this.
>
> Benjamin Coddington <bcodding@...hat.com> writes:
>
>> A persistent unsolved problem exists: how can the kernel find and/or 
>> create
>> the appropriate "container" within which to execute a userspace 
>> program to
>> construct keys or satisfy users of call_usermodehelper()?
>>
>> I believe the latest serious attempt to solve this problem was 
>> David's "Make
>> containers kernel objects":
>> https://lore.kernel.org/lkml/149547014649.10599.12025037906646164347.stgit@warthog.procyon.org.uk/
>>
>> Over in NFS' space, we've most recently pondered this issue while 
>> looking at
>> ways to pass a kernel socket to userspace in order to handle TLS 
>> events:
>> https://lore.kernel.org/linux-nfs/E2BF9CFF-9361-400B-BDEE-CF5E0AFDCA63@redhat.com/
>>
>> The problem is that containers are not kernel objects, rather a 
>> collection
>> of namespaces, cgroups, etc.  Attempts at making the kernel aware of
>> containers have been mired in discussion and problems.  It has been
>> suggested that the best representation of a "container" from the 
>> kernel's
>> perspective is a process.
>>
>> Keyagents are processes represented by a key.  If a keyagent's key is 
>> linked
>> to a session_keyring, it can be sent a realtime signal when a calling
>> process requests a matching key_type.  That signal will dispatch the 
>> process
>> to construct the desired key within the keyagent process context.  
>> Keyagents
>> are similar to ssh-agents.  To use a keyagent, one must execute a 
>> keyagent
>> process in the desired context, and then link the keyagent's key onto 
>> other
>> process' session_keyrings.
>>
>> This method of linking keyagent keys to session_keyrings can be used 
>> to
>> construct the various mappings of callers to keyagents that 
>> containers may
>> need.  A single keyagent process can answer request-key upcalls 
>> across
>> container boundaries, or upcalls can be restricted to specific 
>> containers.
>>
>> I'm aware that building on realtime signals may not be a popular 
>> choice, but
>> using realtime signals makes this work simple and ensures delivery.  
>> Realtime
>> signals are able to convey everything needed to construct keys in 
>> userspace:
>> the under-construction key's serial number.
>>
>> This work is not complete; it has security implications, it needs
>> documentation, it has not been reviewed by anyone.  Thanks for 
>> reading this
>> RFC.  I wish to collect criticism and validate this approach.
>
> At a high level I do agree that we need to send a message to a 
> userspace
> process and that message should contain enough information to start 
> the
> user mode helper.
>
> Then a daemon or possibly the container init can receive the message
> and dispatch the user mode helper.
>
> Fundamentally that design solves all of the container issues, and I
> think solves a few of the user mode helper issues as well.
>
> The challenge with this design is that it requires someone standing up 
> a
> daemon to receive the messages and call a user mode helper to retain
> compatibility with current systems.

Yes..

> I would prefer to see a file descriptor rather than a signal used to
> deliver the message.  Signals suck for many many reasons and a file
> descriptor based notification potentially can be much simpler.

In the example keyagent on userspace side, signal handling is done with
signalfd(2), which greatly simplifies things.

> One of those many reasons is that by not following the common pattern
> for filling in kernel_siginfo you have left uninitialized padding in
> your structure that will be copied to userspace thus creating a kernel
> information leak.  Similarly your code doesn't fill in about half the
> fields that are present in the siginfo union for the _rt case.

Yes, I just used the stack and only filled in the bare minimum.

> I think a file descriptor based design could additionally address the
> back and forth your design needs with keys to figure out what event 
> has
> happened and what user mode helper to invoke.

The keys have already built out a fairly rich interface for accepting
authorization keys, and instantiating partially-constructed keys.  I 
think
the only communication needed (currently) is to dispatch and pass the 
key
serial value.

If we used file descriptors instead of rt signals, there'd be some 
protocol
engineering to do.

> Ideally I would also like to see a design less tied to keys.  So that 
> we
> could use this for the other user mode helper cases as well.   That 
> said
> solving request_key appears to be the truly important part, there 
> aren't
> many other user mode helpers.  Still it would be nice if in theory the
> design could be used to dispatch the coredump helper as well.

What if there was a key_type "usermode_helper"?  Requesting a key of 
that
type executes the binary specified in the callout info.  A keyagent 
could
satisfy the creation of this key, which would allow the usermode_helper
process to execute in the context of a container.  If no keyagent, fall 
back
to the legacy call_usermode_helper.

Thanks for the look,
Ben

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