[PATCH bpf-next 03/29] bpf: introduce BPF token object

[Christian Brauner]

On Wed, Jan 03, 2024 at 02:20:08PM -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 at kernel.org>
> ---

Acked-by: Christian Brauner <brauner at kernel.org>