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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