On 05/11, Alexei Starovoitov wrote:
On Mon, May 11, 2020 at 05:12:10PM -0700, sdf@xxxxxxxxxx wrote:
> On 05/08, Alexei Starovoitov wrote:
> > From: Alexei Starovoitov <ast@xxxxxxxxxx>
> [..]
> > @@ -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.
