linux-kernel - Re: [PATCH v3 bpf-next 00/21] bpf: Sysctl hook

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Date:   Tue, 9 Apr 2019 22:41:44 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Andrey Ignatov <rdna@...com>
Cc:     Network Development <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, guro@...com,
        kernel-team@...com, Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH v3 bpf-next 00/21] bpf: Sysctl hook

On Tue, Apr 9, 2019 at 10:26 PM Andrey Ignatov <rdna@...com> wrote:
> The patch set introduces new BPF hook for sysctl.
>
> It adds new program type BPF_PROG_TYPE_CGROUP_SYSCTL and attach type
> BPF_CGROUP_SYSCTL.
>
> BPF_CGROUP_SYSCTL hook is placed before calling to sysctl's proc_handler so
> that accesses (read/write) to sysctl can be controlled for specific cgroup
> and either allowed or denied, or traced.

Don't look at the credentials of "current" in a read or write handler.
Consider what happens if, for example, someone inside a cgroup opens a
sysctl file and passes the file descriptor to another process outside
the cgroup over a unix domain socket, and that other process then
writes to it. Either do your access check on open, or use the
credentials that were saved during open() in the read/write handler.

> The hook has access to sysctl name, current sysctl value and (on write
> only) to new sysctl value via corresponding helpers. New sysctl value can
> be overridden by program. Both name and values (current/new) are
> represented as strings same way they're visible in /proc/sys/. It is up to
> program to parse these strings.

But even if a filter is installed that prevents all access to a
sysctl, you can still read it by installing your own filter that, when
a read is attempted the next time, dumps the value into a map or
something like that, right?

> To help with parsing the most common kind of sysctl value, vector of
> integers, two new helpers are provided: bpf_strtol and bpf_strtoul with
> semantic similar to user space strtol(3) and strtoul(3).
>
> The hook also provides bpf_sysctl context with two fields:
> * @write indicates whether sysctl is being read (= 0) or written (= 1);
> * @file_pos is sysctl file position to read from or write to, can be
>   overridden.
>
> The hook allows to make better isolation for containerized applications
> that are run as root so that one container can't change a sysctl and affect
> all other containers on a host, make changes to allowed sysctl in a safer
> way and simplify sysctl tracing for cgroups.

Why can't you use a user namespace and isolate things properly that
way? That would be much cleaner, wouldn't it?