Re: [PATCH 07/10] isolation: Replace drop_caps() with a version that actually does something

On Thu, Oct 13, 2022 at 06:37:05PM +0200, Stefano Brivio wrote:

On Thu, 13 Oct 2022 15:08:02 +0200 Stefano Brivio wrote:

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On Thu, 13 Oct 2022 06:01:19 +0200 Stefano Brivio wrote:

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On Tue, 11 Oct 2022 16:40:15 +1100 David Gibson wrote:

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@@ -251,7 +275,19 @@ int isolate_prefork(struct ctx *c) return -errno; }

- drop_caps(); /* Relative to the new user namespace this time. */ + /* Drop capabilites in our new userns */ + if (c->mode == MODE_PASTA) { + /* Keep CAP_SYS_ADMIN, so that we can setns() to the + * netns when we need to act upon it + */ + ns_caps |= 1UL << CAP_SYS_ADMIN; + /* Keep CAP_NET_BIND_SERVICE, so we can splice + * outbound connections to low port numbers + */ + ns_caps |= 1UL << CAP_NET_BIND_SERVICE; + } + + drop_caps_ep_except(ns_caps);

Hmm, I didn't really look into this yet, but there seems to be an issue with filesystem-bound network namespaces now. Running something like:

pasta --config-net --netns /run/user/1000/netns/netns-6466ff4b-1efc-2b58-685b-cbc12feb9ccc

(from Podman), this happens:

[...]

[pid 1763223] setns(7, CLONE_NEWNET) = -1 EPERM (Operation not permitted)

Ah, "of course". Podman calls us with UID 0 in the user namespace it just created, so if we drop CAP_SYS_ADMIN in isolate_initial() we can't join the network namespace, and if we drop CAP_NET_ADMIN we can't configure it.

So for that case (and only for that, I suppose), we need something like (tested):

diff --git a/isolation.c b/isolation.c index 1769180..fee6dbd 100644 --- a/isolation.c +++ b/isolation.c @@ -190,7 +190,7 @@ void isolate_initial(void) * namespace if we have it, so that we can forward low ports * into the guest/namespace */ - drop_caps_ep_except((1UL << CAP_NET_BIND_SERVICE)); + drop_caps_ep_except(BIT(CAP_SYS_ADMIN) | BIT(CAP_NET_ADMIN)); }

...which is a bit pointless. Better than *any* capability, but not by far.

So, if we make this totally independent from configuration, we need those two capabilities.

We could add a "postconf" stage and cover a tiny bit more of conf.c.

Or we could have a special path in isolate_initial() for the case we know we're not in the init namespace.

I'm not sure. If you have a specific preference/strong opinion I would actually be happier. :)

Further on, if we are started as root, we'll fail to drop to 'nobody' or any other user, if we lose CAP_SETUID and CAP_SETGID here. I have tested this version of isolate_initial():

drop_caps_ep_except(BIT(CAP_SYS_ADMIN) | BIT(CAP_NET_ADMIN) | BIT(CAP_SETGID) | BIT(CAP_SETUID) | BIT(CAP_NET_BIND_SERVICE));

for any use case I can reasonably think of. Yes, it's a lot -- we should make it really clear that those are not the capabilities we actually use at "run time".

Ah, right. I didn't think through the --netns-only case. I think what we need to do in the short term at least is: * Add all those caps and a comment in isolate_initial() (or even just remove the drop_caps there) * In isolate user drop the caps we can at that point (SETGID and SETUID) - whether or not we've joined a new userns * Remove the rest of the "runtime" caps in isolate_prefork() like we do now. I'll rework accordingly. -- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson