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From: Andy Lutomirski <luto@kernel.org>
To: Andy Lutomirski <luto@kernel.org>
Cc: 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: Sun, 4 Aug 2019 22:47:22 -0700	[thread overview]
Message-ID: <CALCETrWtE2U4EvZVYeq8pSmQjBzF2PHH+KxYW8FSeF+W=1FYjw@mail.gmail.com> (raw)
In-Reply-To: <CALCETrUjh6DdgW1qSuSRd1_=0F9CqB8+sNj__e_6AHEvh_BaxQ@mail.gmail.com>

On Sun, Aug 4, 2019 at 5:08 PM Andy Lutomirski <luto@kernel.org> wrote:
>
> On Sun, Aug 4, 2019 at 3:16 PM Andy Lutomirski <luto@kernel.org> wrote:
> >
> > 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.
>
> This isn't remotely finished, but I spent a bit of time fiddling with this:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=bpf/perms
>
> What do you think?  (It's obviously not done.  It doesn't compile, and
> I haven't gotten to the permissions needed to do map operations.  I
> also haven't touched the capable() checks.)

I updated the branch.  It compiles, and basic map functionality works!

# mount -t bpf bpf /sys/fs/bpf
# cd /sys/fs/bpf
# mkdir luto
# chown luto: luto
# setpriv --euid=1000 --ruid=1000 bash
$ pwd
/sys/fs/bpf
bash-5.0$ ls -l
total 0
drwxr-xr-x 2 luto luto 0 Aug  4 22:41 luto
bash-5.0$ bpftool map create /sys/fs/bpf/luto/filename type hash key 8
value 8 entries 64 name mapname
bash-5.0$ bpftool map dump pinned /sys/fs/bpf/luto/filename
Found 0 elements

# chown root: /sys/fs/bpf/luto/filename

$ bpftool map dump pinned /sys/fs/bpf/luto/filename
Error: bpf obj get (/sys/fs/bpf/luto): Permission denied

So I think it's possible to get a respectable subset of bpf()
functionality working without privilege in short order :)

(FWIW, a decent fraction of this probably works even without my
patches, but it's going to have nonsensical semantics and may fail for
silly reasons.)

  reply	other threads:[~2019-08-05  5:47 UTC|newest]

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                               ` [PATCH v2 bpf-next 1/4] bpf: unprivileged BPF access via /dev/bpf 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 [this message]
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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