Re: [PATCH v12 bpf-next 03/17] bpf: introduce BPF token object

[Christian Brauner <brauner@kernel.org>]

On Fri, Dec 08, 2023 at 02:39:56PM -0800, Andrii Nakryiko wrote:
> On Fri, Dec 8, 2023 at 5:41 AM Christian Brauner <brauner@kernel.org> wrote:
> >
> > On Thu, Nov 30, 2023 at 10:52:15AM -0800, Andrii Nakryiko wrote:
> > > Add new kind of BPF kernel object, BPF token. BPF token is meant to
> > > allow delegating privileged BPF functionality, like loading a BPF
> > > program or creating a BPF map, from privileged process to a *trusted*
> > > unprivileged process, all while having a good amount of control over which
> > > privileged operations could be performed using provided BPF token.
> > >
> > > This is achieved through mounting BPF FS instance with extra delegation
> > > mount options, which determine what operations are delegatable, and also
> > > constraining it to the owning user namespace (as mentioned in the
> > > previous patch).
> > >
> > > BPF token itself is just a derivative from BPF FS and can be created
> > > through a new bpf() syscall command, BPF_TOKEN_CREATE, which accepts BPF
> > > FS FD, which can be attained through open() API by opening BPF FS mount
> > > point. Currently, BPF token "inherits" delegated command, map types,
> > > prog type, and attach type bit sets from BPF FS as is. In the future,
> > > having an BPF token as a separate object with its own FD, we can allow
> > > to further restrict BPF token's allowable set of things either at the
> > > creation time or after the fact, allowing the process to guard itself
> > > further from unintentionally trying to load undesired kind of BPF
> > > programs. But for now we keep things simple and just copy bit sets as is.
> > >
> > > When BPF token is created from BPF FS mount, we take reference to the
> > > BPF super block's owning user namespace, and then use that namespace for
> > > checking all the {CAP_BPF, CAP_PERFMON, CAP_NET_ADMIN, CAP_SYS_ADMIN}
> > > capabilities that are normally only checked against init userns (using
> > > capable()), but now we check them using ns_capable() instead (if BPF
> > > token is provided). See bpf_token_capable() for details.
> > >
> > > Such setup means that BPF token in itself is not sufficient to grant BPF
> > > functionality. User namespaced process has to *also* have necessary
> > > combination of capabilities inside that user namespace. So while
> > > previously CAP_BPF was useless when granted within user namespace, now
> > > it gains a meaning and allows container managers and sys admins to have
> > > a flexible control over which processes can and need to use BPF
> > > functionality within the user namespace (i.e., container in practice).
> > > And BPF FS delegation mount options and derived BPF tokens serve as
> > > a per-container "flag" to grant overall ability to use bpf() (plus further
> > > restrict on which parts of bpf() syscalls are treated as namespaced).
> > >
> > > Note also, BPF_TOKEN_CREATE command itself requires ns_capable(CAP_BPF)
> > > within the BPF FS owning user namespace, rounding up the ns_capable()
> > > story of BPF token.
> > >
> > > Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> > > ---
> >
> > Same concerns as in the other mail. For the bpf_token_create() code,
> > Acked-by: Christian Brauner <brauner@kernel.org>
> 
> This patch set has landed in bpf-next and there are a bunch of other
> patches after it, so I presume it will be a bit problematic to add ack
> after the fact. But thanks for taking another look and acking!

Yeah, I don't mind. :)