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Message-ID: <CALCETrWwTiz3kZTkEgOW24-DvhQq6LftwEXh77FD2G5o71yD7g@mail.gmail.com>
Date: Fri, 26 Aug 2016 07:57:18 -0700
From: Andy Lutomirski <luto@...capital.net>
To: Mickaël Salaün <mic@...ikod.net>
Cc: "Eric W. Biederman" <ebiederm@...ssion.com>, LKML <linux-kernel@...r.kernel.org>,
Alexei Starovoitov <ast@...nel.org>, Arnd Bergmann <arnd@...db.de>, Casey Schaufler <casey@...aufler-ca.com>,
Daniel Borkmann <daniel@...earbox.net>, Daniel Mack <daniel@...que.org>,
David Drysdale <drysdale@...gle.com>, "David S . Miller" <davem@...emloft.net>,
Elena Reshetova <elena.reshetova@...el.com>, James Morris <james.l.morris@...cle.com>,
Kees Cook <keescook@...omium.org>, Paul Moore <pmoore@...hat.com>,
Sargun Dhillon <sargun@...gun.me>, "Serge E . Hallyn" <serge@...lyn.com>, Will Drewry <wad@...omium.org>,
kernel-hardening <kernel-hardening@...ts.openwall.com>, Linux API <linux-api@...r.kernel.org>,
LSM List <linux-security-module@...r.kernel.org>,
Network Development <netdev@...r.kernel.org>
Subject: Re: [RFC v2 08/10] landlock: Handle file system comparisons
On Thu, Aug 25, 2016 at 7:10 AM, Mickaël Salaün <mic@...ikod.net> wrote:
>
> On 25/08/2016 13:12, Andy Lutomirski wrote:
>> On Thu, Aug 25, 2016 at 3:32 AM, Mickaël Salaün <mic@...ikod.net> wrote:
>>> Add eBPF functions to compare file system access with a Landlock file
>>> system handle:
>>> * bpf_landlock_cmp_fs_prop_with_struct_file(prop, map, map_op, file)
>>> This function allows to compare the dentry, inode, device or mount
>>> point of the currently accessed file, with a reference handle.
>>> * bpf_landlock_cmp_fs_beneath_with_struct_file(opt, map, map_op, file)
>>> This function allows an eBPF program to check if the current accessed
>>> file is the same or in the hierarchy of a reference handle.
>>>
>>> The goal of file system handle is to abstract kernel objects such as a
>>> struct file or a struct inode. Userland can create this kind of handle
>>> thanks to the BPF_MAP_UPDATE_ELEM command. The element is a struct
>>> landlock_handle containing the handle type (e.g.
>>> BPF_MAP_HANDLE_TYPE_LANDLOCK_FS_FD) and a file descriptor. This could
>>> also be any descriptions able to match a struct file or a struct inode
>>> (e.g. path or glob string).
>>
>> This needs Eric's opinion.
>>
>> Also, where do all the struct file *'s get stashed? Are they
>> preserved in the arraymap? What prevents reference cycles or absurdly
>> large numbers of struct files getting pinned?
>
> Yes, the struct file are kept in the arraymap and dropped when there is
> no more reference on them. Currently, the limitations are the maximum
> number of open file descriptors referring to an arraymap and the maximum
> number of eBPF Landlock programs loaded in a process
> (LANDLOCK_PROG_LIST_MAX_PAGES in kernel/seccomp.c).
>
> What kind of reference cycles have you in mind?
Shoving evil things into the arraymaps, e.g. unix sockets with
SCM_RIGHTS messages pending, eBPF program references, the arraymap fd
itself, another arraymap fd, etc.
>
> It probably needs another limit for kernel object references as well.
> What is the best option here? Add another static limitation or use an
> existing one?
Dunno. If RLIMIT_FILE could be made to work, that would be nice.
--Andy
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