Re: [PATCH v11 bpf-next 03/17] bpf: introduce BPF token object

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, Nov 30, 2023 at 6:27 AM Christian Brauner <brauner@kernel.org> wrote:
>
> On Tue, Nov 28, 2023 at 04:05:36PM -0800, Andrii Nakryiko wrote:
> > On Mon, Nov 27, 2023 at 11:06 AM Andrii Nakryiko <andrii@kernel.org> 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>
> > > ---
> > >  include/linux/bpf.h            |  41 +++++++
> > >  include/uapi/linux/bpf.h       |  37 ++++++
> > >  kernel/bpf/Makefile            |   2 +-
> > >  kernel/bpf/inode.c             |  17 ++-
> > >  kernel/bpf/syscall.c           |  17 +++
> > >  kernel/bpf/token.c             | 209 +++++++++++++++++++++++++++++++++
> > >  tools/include/uapi/linux/bpf.h |  37 ++++++
> > >  7 files changed, 350 insertions(+), 10 deletions(-)
> > >  create mode 100644 kernel/bpf/token.c
> > >
> >
> > [...]
> >
> > > +int bpf_token_create(union bpf_attr *attr)
> > > +{
> > > +       struct bpf_mount_opts *mnt_opts;
> > > +       struct bpf_token *token = NULL;
> > > +       struct user_namespace *userns;
> > > +       struct inode *inode;
> > > +       struct file *file;
> > > +       struct path path;
> > > +       struct fd f;
> > > +       umode_t mode;
> > > +       int err, fd;
> > > +
> > > +       f = fdget(attr->token_create.bpffs_fd);
> > > +       if (!f.file)
> > > +               return -EBADF;
> > > +
> > > +       path = f.file->f_path;
> > > +       path_get(&path);
> > > +       fdput(f);
> > > +
> > > +       if (path.dentry != path.mnt->mnt_sb->s_root) {
> > > +               err = -EINVAL;
> > > +               goto out_path;
> > > +       }
> > > +       if (path.mnt->mnt_sb->s_op != &bpf_super_ops) {
> > > +               err = -EINVAL;
> > > +               goto out_path;
> > > +       }
> > > +       err = path_permission(&path, MAY_ACCESS);
> > > +       if (err)
> > > +               goto out_path;
> > > +
> > > +       userns = path.dentry->d_sb->s_user_ns;
> > > +       /*
> > > +        * Enforce that creators of BPF tokens are in the same user
> > > +        * namespace as the BPF FS instance. This makes reasoning about
> > > +        * permissions a lot easier and we can always relax this later.
> > > +        */
> > > +       if (current_user_ns() != userns) {
> > > +               err = -EPERM;
> > > +               goto out_path;
> > > +       }
> >
> > Hey Christian,
> >
> > I've added stricter userns check as discussed on previous revision,
> > and a few lines above fixed BPF FS root check (path.dentry !=
> > path.mnt->mnt_sb->s_root). Hopefully that addresses the remaining
> > concerns you've had.
> >
> > I'd appreciate it if you could take another look to double check if
> > I'm not messing anything up, and if it all looks good, can I please
> > get an ack from you? Thank you!
>
> Please enforce that in order to use a token the caller must be in the
> same user namespace as the token as well. IOW, we don't want to yet make
> it possible to use a token created in an ancestor user namespace to load
> or attach bpf programs in a descendant user namespace. Let's be as
> restrictive as we can: tokens are only valid within the user namespace
> they were created in.

Ok, I will add the check to bpf_token_allow_cmd() and bpf_token_capable().

Thanks a lot for the reviews!