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From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Jordan Glover <Golden_Miller83@protonmail.ch>,
	Andy Lutomirski <luto@kernel.org>,
	Daniel Colascione <dancol@google.com>,
	Song Liu <songliubraving@fb.com>,
	Kees Cook <keescook@chromium.org>,
	Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Kernel Team <Kernel-team@fb.com>,
	Lorenz Bauer <lmb@cloudflare.com>, Jann Horn <jannh@google.com>,
	Greg KH <gregkh@linuxfoundation.org>,
	Linux API <linux-api@vger.kernel.org>,
	LSM List <linux-security-module@vger.kernel.org>
Subject: Re: [PATCH v2 bpf-next 1/4] bpf: unprivileged BPF access via /dev/bpf
Date: Sat, 17 Aug 2019 08:02:47 -0700
Message-ID: <20190817150245.xxzxqjpvgqsxmloe@ast-mbp> (raw)
In-Reply-To: <alpine.DEB.2.21.1908162211270.1923@nanos.tec.linutronix.de>

On Fri, Aug 16, 2019 at 10:28:29PM +0200, Thomas Gleixner wrote:
> Alexei,
> 
> On Fri, 16 Aug 2019, Alexei Starovoitov wrote:
> > It's both of the above when 'systemd' is not taken literally.
> > To earlier Thomas's point: the use case is not only about systemd.
> > There are other containers management systems.
> 
> <SNIP>
> 
> > These daemons need to drop privileges to make the system safer == less
> > prone to corruption due to bugs in themselves. Not necessary security
> > bugs.
> 
> Let's take a step back.
> 
> While real usecases are helpful to understand a design decision, the design
> needs to be usecase independent.
> 
> The kernel provides mechanisms, not policies. My impression of this whole
> discussion is that it is policy driven. That's the wrong approach.

not sure what you mean by 'policy driven'.
Proposed CAP_BPF is a policy?

My desire to do kernel.unprivileged_bpf_disabled=1 is driven by
text in Documentation/x86/mds.rst which says:
"There is one exception, which is untrusted BPF. The functionality of
untrusted BPF is limited, but it needs to be thoroughly investigated
whether it can be used to create such a construct."

commit 6a9e52927251 ("x86/speculation/mds: Add mds_clear_cpu_buffers()")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Jon Masters <jcm@redhat.com>
Tested-by: Jon Masters <jcm@redhat.com>

The way I read this text:
- there is a concern that mds is exploitable via bpf
- there is a desire to investigate to address this concern

I'm committed to help with the investigation.

In the mean time I propose a path to do
kernel.unprivileged_bpf_disabled=1 which is CAP_BPF.

Can kernel.unprivileged_bpf_disabled=1 be used now?
Yes, but it will weaken overall system security because things that
use unpriv to load bpf and CAP_NET_ADMIN to attach bpf would need
to move to stronger CAP_SYS_ADMIN.

With CAP_BPF both load and attach would happen under CAP_BPF
instead of CAP_SYS_ADMIN.

> So let's look at the mechanisms which we have at hand:
> 
>  1) Capabilities
>  
>  2) SUID and dropping priviledges
> 
>  3) Seccomp and LSM
> 
> Now the real interesting questions are:
> 
>  A) What kind of restrictions does BPF allow? Is it a binary on/off or is
>     there a more finegrained control of BPF functionality?
> 
>     TBH, I can't tell.
> 
>  B) Depending on the answer to #A what is the control possibility for
>     #1/#2/#3 ?

Can any of the mechanisms 1/2/3 address the concern in mds.rst?

I believe Andy wants to expand the attack surface when
kernel.unprivileged_bpf_disabled=0
Before that happens I'd like the community to work on addressing the text above.


  reply index

Thread overview: 61+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20190627201923.2589391-1-songliubraving@fb.com>
     [not found] ` <20190627201923.2589391-2-songliubraving@fb.com>
     [not found]   ` <21894f45-70d8-dfca-8c02-044f776c5e05@kernel.org>
     [not found]     ` <3C595328-3ABE-4421-9772-8D41094A4F57@fb.com>
     [not found]       ` <CALCETrWBnH4Q43POU8cQ7YMjb9LioK28FDEQf7aHZbdf1eBZWg@mail.gmail.com>
     [not found]         ` <0DE7F23E-9CD2-4F03-82B5-835506B59056@fb.com>
     [not found]           ` <CALCETrWBWbNFJvsTCeUchu3BZJ3SH3dvtXLUB2EhnPrzFfsLNA@mail.gmail.com>
     [not found]             ` <201907021115.DCD56BBABB@keescook>
     [not found]               ` <CALCETrXTta26CTtEDnzvtd03-WOGdXcnsAogP8JjLkcj4-mHvg@mail.gmail.com>
     [not found]                 ` <4A7A225A-6C23-4C0F-9A95-7C6C56B281ED@fb.com>
     [not found]                   ` <CALCETrX2bMnwC6_t4b_G-hzJSfMPrkK4YKs5ebcecv2LJ0rt3w@mail.gmail.com>
     [not found]                     ` <514D5453-0AEE-420F-AEB6-3F4F58C62E7E@fb.com>
     [not found]                       ` <1DE886F3-3982-45DE-B545-67AD6A4871AB@amacapital.net>
     [not found]                         ` <7F51F8B8-CF4C-4D82-AAE1-F0F28951DB7F@fb.com>
     [not found]                           ` <77354A95-4107-41A7-8936-D144F01C3CA4@fb.com>
     [not found]                             ` <369476A8-4CE1-43DA-9239-06437C0384C7@fb.com>
2019-07-30 20:24                               ` Andy Lutomirski
2019-07-31  8:10                                 ` Song Liu
2019-07-31 19:09                                   ` Andy Lutomirski
2019-08-02  7:21                                     ` Song Liu
2019-08-04 22:16                                       ` Andy Lutomirski
2019-08-05  0:08                                         ` Andy Lutomirski
2019-08-05  5:47                                           ` Andy Lutomirski
2019-08-05  7:36                                             ` Song Liu
2019-08-05 17:23                                               ` Andy Lutomirski
2019-08-05 19:21                                                 ` Alexei Starovoitov
2019-08-05 21:25                                                   ` Andy Lutomirski
2019-08-05 22:21                                                     ` Andy Lutomirski
2019-08-06  1:11                                                     ` Alexei Starovoitov
2019-08-07  5:24                                                       ` Andy Lutomirski
2019-08-07  9:03                                                         ` Lorenz Bauer
2019-08-07 13:52                                                           ` Andy Lutomirski
2019-08-13 21:58                                                         ` Alexei Starovoitov
2019-08-13 22:26                                                           ` Daniel Colascione
2019-08-13 23:24                                                             ` Andy Lutomirski
2019-08-13 23:06                                                           ` Andy Lutomirski
2019-08-14  0:57                                                             ` Alexei Starovoitov
2019-08-14 17:51                                                               ` Andy Lutomirski
2019-08-14 22:05                                                                 ` Alexei Starovoitov
2019-08-14 22:30                                                                   ` Andy Lutomirski
2019-08-14 23:33                                                                     ` Alexei Starovoitov
2019-08-14 23:59                                                                       ` Andy Lutomirski
2019-08-15  0:36                                                                         ` Alexei Starovoitov
2019-08-15 11:24                                                                   ` Jordan Glover
2019-08-15 17:28                                                                     ` Alexei Starovoitov
2019-08-15 18:36                                                                       ` Andy Lutomirski
2019-08-15 23:08                                                                         ` Alexei Starovoitov
2019-08-16  9:34                                                                           ` Jordan Glover
2019-08-16  9:59                                                                             ` Thomas Gleixner
2019-08-16 11:33                                                                               ` Jordan Glover
2019-08-16 19:52                                                                                 ` Alexei Starovoitov
2019-08-16 20:28                                                                                   ` Thomas Gleixner
2019-08-17 15:02                                                                                     ` Alexei Starovoitov [this message]
2019-08-17 15:44                                                                                       ` Andy Lutomirski
2019-08-19  9:15                                                                                       ` Thomas Gleixner
2019-08-19 17:27                                                                                         ` Alexei Starovoitov
2019-08-19 17:38                                                                                           ` Andy Lutomirski
2019-08-15 18:43                                                                       ` Jordan Glover
2019-08-15 19:46                                                           ` Kees Cook
2019-08-15 23:46                                                             ` Alexei Starovoitov
2019-08-16  0:54                                                               ` Andy Lutomirski
2019-08-16  5:56                                                                 ` Song Liu
2019-08-16 21:45                                                                 ` Alexei Starovoitov
2019-08-16 22:22                                                                   ` Christian Brauner
2019-08-17 15:08                                                                     ` Alexei Starovoitov
2019-08-17 15:16                                                                       ` Christian Brauner
2019-08-17 15:36                                                                         ` Alexei Starovoitov
2019-08-17 15:42                                                                           ` Christian Brauner
2019-08-22 14:17                                                         ` Daniel Borkmann
2019-08-22 15:16                                                           ` Andy Lutomirski
2019-08-22 15:17                                                             ` RFC: very rough draft of a bpf permission model Andy Lutomirski
2019-08-22 23:26                                                               ` Alexei Starovoitov
2019-08-23 23:09                                                                 ` Andy Lutomirski
2019-08-26 22:36                                                                   ` Alexei Starovoitov
2019-08-27  0:05                                                                     ` Andy Lutomirski
2019-08-27  0:34                                                                       ` Alexei Starovoitov
2019-08-22 22:48                                                           ` [PATCH v2 bpf-next 1/4] bpf: unprivileged BPF access via /dev/bpf Alexei Starovoitov

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