netdev - Re: [PATCH v5 bpf-next 2/3] bpf: implement CAP

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Date:   Tue, 12 May 2020 08:54:11 -0700
From:   sdf@...gle.com
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     davem@...emloft.net, daniel@...earbox.net, netdev@...r.kernel.org,
        bpf@...r.kernel.org, kernel-team@...com,
        linux-security-module@...r.kernel.org, acme@...hat.com,
        jamorris@...ux.microsoft.com, jannh@...gle.com, kpsingh@...gle.com
Subject: Re: [PATCH v5 bpf-next 2/3] bpf: implement CAP_BPF

On 05/11, Alexei Starovoitov wrote:
> On Mon, May 11, 2020 at 05:12:10PM -0700, sdf@...gle.com wrote:
> > On 05/08, Alexei Starovoitov wrote:
> > > From: Alexei Starovoitov <ast@...nel.org>
> > [..]
> > > @@ -3932,7 +3977,7 @@ SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr
> > > __user *, uattr, unsigned int, siz
> > >   	union bpf_attr attr;
> > >   	int err;
> >
> > > -	if (sysctl_unprivileged_bpf_disabled && !capable(CAP_SYS_ADMIN))
> > > +	if (sysctl_unprivileged_bpf_disabled && !bpf_capable())
> > >   		return -EPERM;
> > This is awesome, thanks for reviving the effort!
> >
> > One question I have about this particular snippet:
> > Does it make sense to drop bpf_capable checks for the operations
> > that work on a provided fd?

> Above snippet is for the case when sysctl switches unpriv off.
> It was a big hammer and stays big hammer.
> I certainly would like to improve the situation, but I suspect
> the folks who turn that sysctl knob on are simply paranoid about bpf
> and no amount of reasoning would turn them around.
Yeah, and we do use it unfortunately :-( I suppose we still would
like to keep it that way for a while, but maybe start relaxing
some operations a bit.

> > The use-case I have in mind is as follows:
> > * privileged (CAP_BPF) process loads the programs/maps and pins
> >   them at some known location
> > * unprivileged process opens up those pins and does the following:
> >   * prepares the maps (and will later on read them)
> >   * does SO_ATTACH_BPF/SO_ATTACH_REUSEPORT_EBPF which afaik don't
> >     require any capabilities
> >
> > This essentially pushes some of the permission checks into a fs layer.  
> So
> > whoever has a file descriptor (via unix sock or open) can do BPF  
> operations
> > on the object that represents it.

> cap_bpf doesn't change things in that regard.
> Two cases here:
> sysctl_unprivileged_bpf_disabled==0:
>    Unpriv can load socket_filter prog type and unpriv can attach it
>    via SO_ATTACH_BPF/SO_ATTACH_REUSEPORT_EBPF.
> sysctl_unprivileged_bpf_disabled==1:
>    cap_sys_admin can load socket_filter and unpriv can attach it.
Sorry, I wasn't clear enough, I was talking about unpriv_bpf_disabled=1
case.

> With addition of cap_bpf in the second case cap_bpf process can
> load socket_filter too.
> It doesn't mean that permissions are pushed into fs layer.
> I'm not sure that relaxing of sysctl_unprivileged_bpf_disabled
> will be well received.
> Are you proposing to selectively allow certain bpf syscall commands
> even when sysctl_unprivileged_bpf_disabled==1 ?
> Like allow unpriv to do BPF_OBJ_GET to get an fd from bpffs ?
> And allow unpriv to do map_update ?
Yes, that's the gist of what I'm proposing. Allow the operations that
work on fd even with unpriv_bpf_disabled=1. The assumption that
obtaining fd requires a privileged operation on its own and
should give enough protection.

> It makes complete sense to me, but I'd like to argue about that
> independently from this cap_bpf set.
> We can relax that sysctl later.
Ack, thanks, let me bring it up again later, when we get to the cap_bpf
state.