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@google.com wrote:

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On 05/08, Alexei Starovoitov wrote:

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From: Alexei Starovoitov <ast@kernel.org>

[..]

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@@ -3932,7 +3977,7 @@ SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr

__user *, uattr, unsigned int, siz
  	union bpf_attr attr;
  	int err;

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-	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.

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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

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whoever has a file descriptor (via unix sock or open) can do BPF  

operations

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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.