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From: Andy Lutomirski <luto@kernel.org>
To: Song Liu <songliubraving@fb.com>
Cc: Andy Lutomirski <luto@kernel.org>,
	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: Sun, 4 Aug 2019 15:16:46 -0700
Message-ID: <CALCETrU7NbBnXXsw1B+DvTkfTVRBFWXuJ8cZERCCNvdFG6KqRw@mail.gmail.com> (raw)
In-Reply-To: <5A2FCD7E-7F54-41E5-BFAE-BB9494E74F2D@fb.com>

On Fri, Aug 2, 2019 at 12:22 AM Song Liu <songliubraving@fb.com> wrote:
>
> Hi Andy,
>
 >> I actually agree CAP_BPF_ADMIN makes sense. The hard part is to make
> >> existing tools (setcap, getcap, etc.) and libraries aware of the new CAP.
> >
> > It's been done before -- it's not that hard.  IMO the main tricky bit
> > would be try be somewhat careful about defining exactly what
> > CAP_BPF_ADMIN does.
>
> Agreed. I think defining CAP_BPF_ADMIN could be a good topic for the
> Plumbers conference.
>
> OTOH, I don't think we have to wait for CAP_BPF_ADMIN to allow daemons
> like systemd to do sys_bpf() without root.

I don't understand the use case here.  Are you talking about systemd
--user?  As far as I know, a user is expected to be able to fully
control their systemd --user process, so giving it unrestricted bpf
access is very close to giving it superuser access, and this doesn't
sound like a good idea.  I think that, if systemd --user needs bpf(),
it either needs real unprivileged bpf() or it needs a privileged
helper (SUID or a daemon) to intermediate this access.

>
> >
> >>> I don't see why you need to invent a whole new mechanism for this.
> >>> The entire cgroup ecosystem outside bpf() does just fine using the
> >>> write permission on files in cgroupfs to control access.  Why can't
> >>> bpf() do the same thing?
> >>
> >> It is easier to use write permission for BPF_PROG_ATTACH. But it is
> >> not easy to do the same for other bpf commands: BPF_PROG_LOAD and
> >> BPF_MAP_*. A lot of these commands don't have target concept. Maybe
> >> we should have target concept for all these commands. But that is a
> >> much bigger project. OTOH, "all or nothing" model allows all these
> >> commands at once.
> >
> > For BPF_PROG_LOAD, I admit I've never understood why permission is
> > required at all.  I think that CAP_SYS_ADMIN or similar should be
> > needed to get is_priv in the verifier, but I think that should mainly
> > be useful for tracing, and that requires lots of privilege anyway.
> > BPF_MAP_* is probably the trickiest part.  One solution would be some
> > kind of bpffs, but I'm sure other solutions are possible.
>
> Improving permission management of cgroup_bpf is another good topic to
> discuss. However, it is also an overkill for current use case.
>

I looked at the code some more, and I don't think this is so hard
after all.  As I understand it, all of the map..by_id stuff is, to
some extent, deprecated in favor of persistent maps.  As I see it, the
map..by_id calls should require privilege forever, although I can
imagine ways to scope that privilege to a namespace if the maps
themselves were to be scoped to a namespace.

Instead, unprivileged tools would use the persistent map interface
roughly like this:

$ bpftool map create /sys/fs/bpf/my_dir/filename type hash key 8 value
8 entries 64 name mapname

This would require that the caller have either CAP_DAC_OVERRIDE or
that the caller have permission to create files in /sys/fs/bpf/my_dir
(using the same rules as for any filesystem), and the resulting map
would end up owned by the creating user and have mode 0600 (or maybe
0666, or maybe a new bpf_attr parameter) modified by umask.  Then all
the various capable() checks that are currently involved in accessing
a persistent map would instead check FMODE_READ or FMODE_WRITE on the
map file as appropriate.

Half of this stuff already works.  I just set my system up like this:

$ ls -l /sys/fs/bpf
total 0
drwxr-xr-x. 3 luto luto 0 Aug  4 15:10 luto

$ mkdir /sys/fs/bpf/luto/test

$ ls -l /sys/fs/bpf/luto
total 0
drwxrwxr-x. 2 luto luto 0 Aug  4 15:10 test

I bet that making the bpf() syscalls work appropriately in this
context without privilege would only be a couple of hours of work.
The hard work, creating bpffs and making it function, is already done
:)

P.S. The docs for bpftool create are less than fantastic.  The
complete lack of any error message at all when the syscall returns
-EACCES is also not fantastic.

  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 [this message]
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
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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