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From: "Mickaël Salaün" <mic@digikod.net>
To: Jann Horn <jann@thejh.net>
Cc: linux-kernel@vger.kernel.org, Alexei Starovoitov <ast@kernel.org>,
	Andy Lutomirski <luto@amacapital.net>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Daniel Mack <daniel@zonque.org>,
	David Drysdale <drysdale@google.com>,
	"David S . Miller" <davem@davemloft.net>,
	"Eric W . Biederman" <ebiederm@xmission.com>,
	James Morris <james.l.morris@oracle.com>,
	Kees Cook <keescook@chromium.org>, Paul Moore <pmoore@redhat.com>,
	Sargun Dhillon <sargun@sargun.me>,
	"Serge E . Hallyn" <serge@hallyn.com>, Tejun Heo <tj@kernel.org>,
	Thomas Graf <tgraf@suug.ch>, Will Drewry <wad@chromium.org>,
	kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org,
	linux-security-module@vger.kernel.org, netdev@vger.kernel.org,
	cgroups@vger.kernel.org
Subject: Re: [kernel-hardening] [RFC v4 03/18] bpf,landlock: Add a new arraymap type to deal with (Landlock) handles
Date: Wed, 26 Oct 2016 22:03:09 +0200
Message-ID: <58110BFD.5090005@digikod.net> (raw)
In-Reply-To: <20161026190147.GN3334@pc.thejh.net>

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On 26/10/2016 21:01, Jann Horn wrote:
> On Wed, Oct 26, 2016 at 08:56:39AM +0200, Mickaël Salaün wrote:
>> This new arraymap looks like a set and brings new properties:
>> * strong typing of entries: the eBPF functions get the array type of
>>   elements instead of CONST_PTR_TO_MAP (e.g.
>>   CONST_PTR_TO_LANDLOCK_HANDLE_FS);
>> * force sequential filling (i.e. replace or append-only update), which
>>   allow quick browsing of all entries.
>>
>> This strong typing is useful to statically check if the content of a map
>> can be passed to an eBPF function. For example, Landlock use it to store
>> and manage kernel objects (e.g. struct file) instead of dealing with
>> userland raw data. This improve efficiency and ensure that an eBPF
>> program can only call functions with the right high-level arguments.
>>
>> The enum bpf_map_handle_type list low-level types (e.g.
>> BPF_MAP_HANDLE_TYPE_LANDLOCK_FS_FD) which are identified when
>> updating a map entry (handle). This handle types are used to infer a
>> high-level arraymap type which are listed in enum bpf_map_array_type
>> (e.g. BPF_MAP_ARRAY_TYPE_LANDLOCK_FS).
>>
>> For now, this new arraymap is only used by Landlock LSM (cf. next
>> commits) but it could be useful for other needs.
>>
>> Changes since v3:
>> * make handle arraymap safe (RCU) and remove buggy synchronize_rcu()
>> * factor out the arraymay walk
>>
>> Changes since v2:
>> * add a RLIMIT_NOFILE-based limit to the maximum number of arraymap
>>   handle entries (suggested by Andy Lutomirski)
>> * remove useless checks
>>
>> Changes since v1:
>> * arraymap of handles replace custom checker groups
>> * simpler userland API
> [...]
>> +	case BPF_MAP_HANDLE_TYPE_LANDLOCK_FS_FD:
>> +		handle_file = fget(handle->fd);
>> +		if (IS_ERR(handle_file))
>> +			return ERR_CAST(handle_file);
>> +		/* check if the FD is tied to a user mount point */
>> +		if (unlikely(handle_file->f_path.mnt->mnt_flags & MNT_INTERNAL)) {
>> +			fput(handle_file);
>> +			return ERR_PTR(-EINVAL);
>> +		}
>> +		path_get(&handle_file->f_path);
>> +		ret = kmalloc(sizeof(*ret), GFP_KERNEL);
>> +		ret->path = handle_file->f_path;
>> +		fput(handle_file);
> 
> You can use fdget() and fdput() here because the reference to
> handle_file is dropped before the end of the syscall.

The reference to handle_file is dropped but not the reference to the
(inner) path thanks to path_get().

> 
> 
>> +		break;
>> +	case BPF_MAP_HANDLE_TYPE_UNSPEC:
>> +	default:
>> +		return ERR_PTR(-EINVAL);
>> +	}
>> +	ret->type = handle_type;
>> +	return ret;
>> +}
>> +
>> +static void *nop_map_lookup_elem(struct bpf_map *map, void *key)
>> +{
>> +	return ERR_PTR(-EINVAL);
>> +}
>> +
>> +/* called from syscall or from eBPF program */
>> +static int landlock_array_map_update_elem(struct bpf_map *map, void *key,
>> +		void *value, u64 map_flags)
>> +{
> 
> This being callable from eBPF context is IMO pretty surprising and should
> at least be well-documented. Also: What is the usecase here?
> 

This may be callable but is restricted to CAP_SYS_ADMIN. Any update with
an FD should indeed be denied, but updates with raw values (e.g. GLOB,
IP or port numbers, not yet implemented) may be allowed. Because an eBPF
program is trusted by the process which loaded it (and have the same
rights), this program should be able to update a map like its creator
process. One usecase may be to adjust a map of handles by removing
entries or tightening them (i.e. drop privileges) when a specific
behavior of a monitored process is detected by the eBPF program.

I'm going to fix this update-with-FD case which make no sense anyway.

Thanks,
 Mickaël


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  reply index

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-10-26  6:56 [RFC v4 00/18] Landlock LSM: Unprivileged sandboxing Mickaël Salaün
2016-10-26  6:56 ` [RFC v4 01/18] landlock: Add Kconfig Mickaël Salaün
2016-10-26  6:56 ` [RFC v4 02/18] bpf: Move u64_to_ptr() to BPF headers and inline it Mickaël Salaün
2016-10-26  7:19   ` Arnd Bergmann
2016-10-26 13:52     ` [kernel-hardening] " David Sterba
2016-10-26  6:56 ` [RFC v4 03/18] bpf,landlock: Add a new arraymap type to deal with (Landlock) handles Mickaël Salaün
2016-10-26 19:01   ` [kernel-hardening] " Jann Horn
2016-10-26 20:03     ` Mickaël Salaün [this message]
2016-10-26 20:16       ` Jann Horn
2016-10-26  6:56 ` [RFC v4 04/18] bpf,landlock: Add eBPF program subtype and is_valid_subtype() verifier Mickaël Salaün
2016-10-26  6:56 ` [RFC v4 05/18] bpf,landlock: Define an eBPF program type for Landlock Mickaël Salaün
2016-10-26  6:56 ` [RFC v4 06/18] fs: Constify path_is_under()'s arguments Mickaël Salaün
2016-10-26  6:56 ` [RFC v4 07/18] landlock: Add LSM hooks Mickaël Salaün
2016-10-26  6:56 ` [RFC v4 08/18] landlock: Handle file comparisons Mickaël Salaün
2016-10-26  6:56 ` [RFC v4 09/18] landlock: Add manager functions Mickaël Salaün
2016-10-26  6:56 ` [RFC v4 10/18] seccomp: Split put_seccomp_filter() with put_seccomp() Mickaël Salaün
2016-10-26  6:56 ` [RFC v4 11/18] seccomp,landlock: Handle Landlock hooks per process hierarchy Mickaël Salaün
2016-10-26  6:56 ` [RFC v4 12/18] bpf: Cosmetic change for bpf_prog_attach() Mickaël Salaün
2016-10-26  6:56 ` [RFC v4 13/18] bpf/cgroup: Replace struct bpf_prog with struct bpf_object Mickaël Salaün
2016-10-26  6:56 ` [RFC v4 14/18] bpf/cgroup: Make cgroup_bpf_update() return an error code Mickaël Salaün
2016-10-26  6:56 ` [RFC v4 15/18] bpf/cgroup: Move capability check Mickaël Salaün
2016-10-26  6:56 ` [RFC v4 16/18] bpf/cgroup,landlock: Handle Landlock hooks per cgroup Mickaël Salaün
2016-10-26  6:56 ` [RFC v4 17/18] landlock: Add update and debug access flags Mickaël Salaün
2016-10-26  6:56 ` [RFC v4 18/18] samples/landlock: Add sandbox example Mickaël Salaün
2016-10-26 14:52 ` [RFC v4 00/18] Landlock LSM: Unprivileged sandboxing Jann Horn
2016-10-26 16:56   ` Mickaël Salaün
2016-10-26 17:24     ` Mickaël Salaün
2016-11-13 14:23 ` Mickaël Salaün
2016-11-14 10:35   ` Sargun Dhillon
2016-11-14 20:51     ` Mickaël Salaün

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