linux-kernel - Re: [PATCH bpf-next v1 7/9] bpf: Lift permission check in __sys

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Date:   Wed, 2 Mar 2022 12:01:55 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Hao Luo <haoluo@...gle.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        KP Singh <kpsingh@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Joe Burton <jevburton.kernel@...il.com>,
        Tejun Heo <tj@...nel.org>, joshdon@...gle.com, sdf@...gle.com,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v1 7/9] bpf: Lift permission check in __sys_bpf
 when called from kernel.

On Fri, Feb 25, 2022 at 03:43:37PM -0800, Hao Luo wrote:
> After we introduced sleepable tracing programs, we now have an
> interesting problem. There are now three execution paths that can
> reach bpf_sys_bpf:
> 
>  1. called from bpf syscall.
>  2. called from kernel context (e.g. kernel modules).
>  3. called from bpf programs.
> 
> Ideally, capability check in bpf_sys_bpf is necessary for the first two
> scenarios. But it may not be necessary for the third case.

Well, it's unnecessary for the first two as well.
When called from the kernel lskel it's a pointless check.
The kernel module can do anything regardless.
When called from bpf syscall program it's not quite correct either.
When CAP_BPF was introduced we've designed it to enforce permissions
at prog load time. The prog_run doesn't check permissions.
So syscall progs don't need this secondary permission check.
Please add "case BPF_PROG_TYPE_SYSCALL:" to is_perfmon_prog_type()
and combine it with this patch.

That would be the best. The alternative below is less appealing.

> An alternative of lifting this permission check would be introducing an
> 'unpriv' version of bpf_sys_bpf, which doesn't check the current task's
> capability. If the owner of the tracing prog wants it to be exclusively
> used by root users, they can use the 'priv' version of bpf_sys_bpf; if
> the owner wants it to be usable for non-root users, they can use the
> 'unpriv' version.

...

> -	if (sysctl_unprivileged_bpf_disabled && !bpf_capable())
> +	if (sysctl_unprivileged_bpf_disabled && !bpf_capable() && !uattr.is_kernel)

This is great idea. If I could think of this I would went with it when prog_syscall
was introduced.