Re: [PATCH v26 07/12] landlock: Support filesystem access-control

From: Jann Horn <jannh@google.com>
To: "Mickaël Salaün" <mic@digikod.net>
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: Thu, 14 Jan 2021 23:43:02 +0100	[thread overview]
Message-ID: <CAG48ez2HJCFvmFALDYDYnufE755Dqh3JquAMf-1mnzmRrdKaoQ@mail.gmail.com> (raw)
In-Reply-To: <aeb3e152-8108-89d2-0577-4b130368f14f@digikod.net>

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:

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 /.