Re: [PATCH 00/18] bpf: Secure and authenticated preloading of eBPF programs

[Casey Schaufler]

On 4/4/2022 10:20 AM, Roberto Sassu wrote:
> > From: Djalal Harouni [mailto:tixxdz@xxxxxxxxx]
> > Sent: Monday, April 4, 2022 9:45 AM
> > On Sun, Apr 3, 2022 at 5:42 PM KP Singh <kpsingh@xxxxxxxxxx> wrote:
> > > On Sat, Apr 2, 2022 at 1:55 AM Alexei Starovoitov
> > > <alexei.starovoitov@xxxxxxxxx> wrote:
> > ...
> > > > > Pinning
> > > > > them to unreachable inodes intuitively looked the
> > > > > way to go for achieving the stated goal.
> > > > We can consider inodes in bpffs that are not unlinkable by root
> > > > in the future, but certainly not for this use case.
> > > Can this not be already done by adding a BPF_LSM program to the
> > > inode_unlink LSM hook?
> > Also, beside of the inode_unlink... and out of curiosity: making sysfs/bpffs/
> > readonly after pinning, then using bpf LSM hooks
> > sb_mount|remount|unmount...
> > family combining bpf() LSM hook... isn't this enough to:
> > 1. Restrict who can pin to bpffs without using a full MAC
> > 2. Restrict who can delete or unmount bpf filesystem
> >
> > ?
> I'm thinking to implement something like this.
>
> First, I add a new program flag called
> BPF_F_STOP_ONCONFIRM, which causes the ref count
> of the link to increase twice at creation time. In this way,
> user space cannot make the link disappear, unless a
> confirmation is explicitly sent via the bpf() system call.
>
> Another advantage is that other LSMs can decide
> whether or not they allow a program with this flag
> (in the bpf security hook).
>
> This would work regardless of the method used to
> load the eBPF program (user space or kernel space).
>
> Second, I extend the bpf() system call with a new
> subcommand, BPF_LINK_CONFIRM_STOP, which
> decreasres the ref count for the link of the programs
> with the BPF_F_STOP_ONCONFIRM flag. I will also
> introduce a new security hook (something like
> security_link_confirm_stop), so that an LSM has the
> opportunity to deny the stop (the bpf security hook
> would not be sufficient to determine exactly for
> which link the confirmation is given, an LSM should
> be able to deny the stop for its own programs).

Would you please stop referring to a set of eBPF programs
loaded into the BPF LSM as an LSM? Call it a BPF security
module (BSM) if you must use an abbreviation. An LSM is a
provider of security_ hooks. In your case that is BPF. When
you call the set of eBPF programs an LSM it is like calling
an SELinux policy an LSM.

> What do you think?
>
> Thanks
>
> Roberto
>
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