From: "Mickaël Salaün" <mic@digikod.net>
To: Jann Horn <jannh@google.com>
Cc: "James Morris" <jmorris@namei.org>,
"Serge E . Hallyn" <serge@hallyn.com>,
"Al Viro" <viro@zeniv.linux.org.uk>,
"Andy Lutomirski" <luto@amacapital.net>,
"Anton Ivanov" <anton.ivanov@cambridgegreys.com>,
"Arnd Bergmann" <arnd@arndb.de>,
"Casey Schaufler" <casey@schaufler-ca.com>,
"Jeff Dike" <jdike@addtoit.com>,
"Jonathan Corbet" <corbet@lwn.net>,
"Kees Cook" <keescook@chromium.org>,
"Michael Kerrisk" <mtk.manpages@gmail.com>,
"Richard Weinberger" <richard@nod.at>,
"Shuah Khan" <shuah@kernel.org>,
"Vincent Dagonneau" <vincent.dagonneau@ssi.gouv.fr>,
"Kernel Hardening" <kernel-hardening@lists.openwall.com>,
"Linux API" <linux-api@vger.kernel.org>,
linux-arch <linux-arch@vger.kernel.org>,
"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
"kernel list" <linux-kernel@vger.kernel.org>,
"open list:KERNEL SELFTEST FRAMEWORK"
<linux-kselftest@vger.kernel.org>,
linux-security-module <linux-security-module@vger.kernel.org>,
"the arch/x86 maintainers" <x86@kernel.org>,
"Mickaël Salaün" <mic@linux.microsoft.com>
Subject: Re: [PATCH v26 07/12] landlock: Support filesystem access-control
Date: Fri, 15 Jan 2021 10:10:36 +0100 [thread overview]
Message-ID: <9be6481f-9c03-dd32-378f-20bc7c52315c@digikod.net> (raw)
In-Reply-To: <CAG48ez2HJCFvmFALDYDYnufE755Dqh3JquAMf-1mnzmRrdKaoQ@mail.gmail.com>
On 14/01/2021 23:43, Jann Horn wrote:
> On Thu, Jan 14, 2021 at 7:54 PM Mickaël Salaün <mic@digikod.net> wrote:
>> On 14/01/2021 04:22, Jann Horn wrote:
>>> On Wed, Dec 9, 2020 at 8:28 PM Mickaël Salaün <mic@digikod.net> wrote:
>>>> Thanks to the Landlock objects and ruleset, it is possible to identify
>>>> inodes according to a process's domain. To enable an unprivileged
>>>> process to express a file hierarchy, it first needs to open a directory
>>>> (or a file) and pass this file descriptor to the kernel through
>>>> landlock_add_rule(2). When checking if a file access request is
>>>> allowed, we walk from the requested dentry to the real root, following
>>>> the different mount layers. The access to each "tagged" inodes are
>>>> collected according to their rule layer level, and ANDed to create
>>>> access to the requested file hierarchy. This makes possible to identify
>>>> a lot of files without tagging every inodes nor modifying the
>>>> filesystem, while still following the view and understanding the user
>>>> has from the filesystem.
>>>>
>>>> Add a new ARCH_EPHEMERAL_INODES for UML because it currently does not
>>>> keep the same struct inodes for the same inodes whereas these inodes are
>>>> in use.
>>>>
>>>> This commit adds a minimal set of supported filesystem access-control
>>>> which doesn't enable to restrict all file-related actions. This is the
>>>> result of multiple discussions to minimize the code of Landlock to ease
>>>> review. Thanks to the Landlock design, extending this access-control
>>>> without breaking user space will not be a problem. Moreover, seccomp
>>>> filters can be used to restrict the use of syscall families which may
>>>> not be currently handled by Landlock.
>>> [...]
>>>> +static bool check_access_path_continue(
>>>> + const struct landlock_ruleset *const domain,
>>>> + const struct path *const path, const u32 access_request,
>>>> + u64 *const layer_mask)
>>>> +{
>>> [...]
>>>> + /*
>>>> + * An access is granted if, for each policy layer, at least one rule
>>>> + * encountered on the pathwalk grants the access, regardless of their
>>>> + * position in the layer stack. We must then check not-yet-seen layers
>>>> + * for each inode, from the last one added to the first one.
>>>> + */
>>>> + for (i = 0; i < rule->num_layers; i++) {
>>>> + const struct landlock_layer *const layer = &rule->layers[i];
>>>> + const u64 layer_level = BIT_ULL(layer->level - 1);
>>>> +
>>>> + if (!(layer_level & *layer_mask))
>>>> + continue;
>>>> + if ((layer->access & access_request) != access_request)
>>>> + return false;
>>>> + *layer_mask &= ~layer_level;
>>>
>>> Hmm... shouldn't the last 5 lines be replaced by the following?
>>>
>>> if ((layer->access & access_request) == access_request)
>>> *layer_mask &= ~layer_level;
>>>
>>> And then, since this function would always return true, you could
>>> change its return type to "void".
>>>
>>>
>>> As far as I can tell, the current version will still, if a ruleset
>>> looks like this:
>>>
>>> /usr read+write
>>> /usr/lib/ read
>>>
>>> reject write access to /usr/lib, right?
>>
>> If these two rules are from different layers, then yes it would work as
>> intended. However, if these rules are from the same layer the path walk
>> will not stop at /usr/lib but go down to /usr, which grants write
>> access.
>
> I don't see why the code would do what you're saying it does. And an
> experiment seems to confirm what I said; I checked out landlock-v26,
> and the behavior I get is:
There is a misunderstanding, I was responding to your proposition to
modify check_access_path_continue(), not about the behavior of landlock-v26.
>
> user@vm:~/landlock$ dd if=/dev/null of=/tmp/aaa
> 0+0 records in
> 0+0 records out
> 0 bytes copied, 0.00106365 s, 0.0 kB/s
> user@vm:~/landlock$ LL_FS_RO='/lib' LL_FS_RW='/' ./sandboxer dd
> if=/dev/null of=/tmp/aaa
> 0+0 records in
> 0+0 records out
> 0 bytes copied, 0.000491814 s, 0.0 kB/s
> user@vm:~/landlock$ LL_FS_RO='/tmp' LL_FS_RW='/' ./sandboxer dd
> if=/dev/null of=/tmp/aaa
> dd: failed to open '/tmp/aaa': Permission denied
> user@vm:~/landlock$
>
> Granting read access to /tmp prevents writing to it, even though write
> access was granted to /.
>
It indeed works like this with landlock-v26. However, with your above
proposition, it would work like this:
$ LL_FS_RO='/tmp' LL_FS_RW='/' ./sandboxer dd if=/dev/null of=/tmp/aaa
0+0 records in
0+0 records out
0 bytes copied, 0.000187265 s, 0.0 kB/s
…which is not what users would expect I guess. :)
next prev parent reply other threads:[~2021-01-15 9:11 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-09 19:28 [PATCH v26 00/12] Landlock LSM Mickaël Salaün
2020-12-09 19:28 ` [PATCH v26 01/12] landlock: Add object management Mickaël Salaün
2020-12-09 19:28 ` [PATCH v26 02/12] landlock: Add ruleset and domain management Mickaël Salaün
2021-01-14 3:21 ` Jann Horn
2020-12-09 19:28 ` [PATCH v26 03/12] landlock: Set up the security framework and manage credentials Mickaël Salaün
2020-12-09 19:28 ` [PATCH v26 04/12] landlock: Add ptrace restrictions Mickaël Salaün
2020-12-09 19:28 ` [PATCH v26 05/12] LSM: Infrastructure management of the superblock Mickaël Salaün
2020-12-09 19:28 ` [PATCH v26 06/12] fs,security: Add sb_delete hook Mickaël Salaün
2020-12-09 19:28 ` [PATCH v26 07/12] landlock: Support filesystem access-control Mickaël Salaün
2021-01-14 3:22 ` Jann Horn
2021-01-14 18:54 ` Mickaël Salaün
2021-01-14 22:43 ` Jann Horn
2021-01-15 9:10 ` Mickaël Salaün [this message]
2021-01-15 18:31 ` Jann Horn
2021-01-16 17:16 ` Mickaël Salaün
2020-12-09 19:28 ` [PATCH v26 08/12] landlock: Add syscall implementations Mickaël Salaün
2020-12-10 10:38 ` Mickaël Salaün
2020-12-09 19:28 ` [PATCH v26 09/12] arch: Wire up Landlock syscalls Mickaël Salaün
2020-12-09 19:28 ` [PATCH v26 10/12] selftests/landlock: Add user space tests Mickaël Salaün
2020-12-09 19:28 ` [PATCH v26 11/12] samples/landlock: Add a sandbox manager example Mickaël Salaün
2021-01-14 3:21 ` Jann Horn
2021-01-14 18:59 ` Mickaël Salaün
2020-12-09 19:28 ` [PATCH v26 12/12] landlock: Add user and kernel documentation Mickaël Salaün
2021-01-14 3:22 ` [PATCH v26 00/12] Landlock LSM Jann Horn
2021-01-14 19:03 ` Mickaël Salaün
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