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>,
"Andrew Morton" <akpm@linux-foundation.org>,
"Andy Lutomirski" <luto@amacapital.net>,
"Anton Ivanov" <anton.ivanov@cambridgegreys.com>,
"Arnd Bergmann" <arnd@arndb.de>,
"Casey Schaufler" <casey@schaufler-ca.com>,
"David Howells" <dhowells@redhat.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 v30 07/12] landlock: Support filesystem access-control
Date: Tue, 23 Mar 2021 01:13:36 +0100 [thread overview]
Message-ID: <CAG48ez1arKO3uYzwng8fst-UHkcH6J7YzyHFN+vfXUT2=1HT+w@mail.gmail.com> (raw)
In-Reply-To: <20210316204252.427806-8-mic@digikod.net>
On Tue, Mar 16, 2021 at 9:43 PM Mickaël Salaün <mic@digikod.net> wrote:
> Using Landlock objects and ruleset, it is possible to tag inodes
> according to a process's domain.
[...]
> +static void release_inode(struct landlock_object *const object)
> + __releases(object->lock)
> +{
> + struct inode *const inode = object->underobj;
> + struct super_block *sb;
> +
> + if (!inode) {
> + spin_unlock(&object->lock);
> + return;
> + }
> +
> + /*
> + * Protects against concurrent use by hook_sb_delete() of the reference
> + * to the underlying inode.
> + */
> + object->underobj = NULL;
> + /*
> + * Makes sure that if the filesystem is concurrently unmounted,
> + * hook_sb_delete() will wait for us to finish iput().
> + */
> + sb = inode->i_sb;
> + atomic_long_inc(&landlock_superblock(sb)->inode_refs);
> + spin_unlock(&object->lock);
> + /*
> + * Because object->underobj was not NULL, hook_sb_delete() and
> + * get_inode_object() guarantee that it is safe to reset
> + * landlock_inode(inode)->object while it is not NULL. It is therefore
> + * not necessary to lock inode->i_lock.
> + */
> + rcu_assign_pointer(landlock_inode(inode)->object, NULL);
> + /*
> + * Now, new rules can safely be tied to @inode with get_inode_object().
> + */
> +
> + iput(inode);
> + if (atomic_long_dec_and_test(&landlock_superblock(sb)->inode_refs))
> + wake_up_var(&landlock_superblock(sb)->inode_refs);
> +}
[...]
> +static struct landlock_object *get_inode_object(struct inode *const inode)
> +{
> + struct landlock_object *object, *new_object;
> + struct landlock_inode_security *inode_sec = landlock_inode(inode);
> +
> + rcu_read_lock();
> +retry:
> + object = rcu_dereference(inode_sec->object);
> + if (object) {
> + if (likely(refcount_inc_not_zero(&object->usage))) {
> + rcu_read_unlock();
> + return object;
> + }
> + /*
> + * We are racing with release_inode(), the object is going
> + * away. Wait for release_inode(), then retry.
> + */
> + spin_lock(&object->lock);
> + spin_unlock(&object->lock);
> + goto retry;
> + }
> + rcu_read_unlock();
> +
> + /*
> + * If there is no object tied to @inode, then create a new one (without
> + * holding any locks).
> + */
> + new_object = landlock_create_object(&landlock_fs_underops, inode);
> + if (IS_ERR(new_object))
> + return new_object;
> +
> + /* Protects against concurrent get_inode_object() calls. */
> + spin_lock(&inode->i_lock);
> + object = rcu_dereference_protected(inode_sec->object,
> + lockdep_is_held(&inode->i_lock));
rcu_dereference_protected() requires that inode_sec->object is not
concurrently changed, but I think another thread could call
get_inode_object() while we're in landlock_create_object(), and then
we could race with the NULL write in release_inode() here? (It
wouldn't actually be a UAF though because we're not actually accessing
`object` here.) Or am I missing a lock that prevents this?
In v28 this wasn't an issue because release_inode() was holding
inode->i_lock (and object->lock) during the NULL store; but in v29 and
this version the NULL store in release_inode() moved out of the locked
region. I think you could just move the NULL store in release_inode()
back up (and maybe add a comment explaining the locking rules for
landlock_inode(...)->object)?
(Or alternatively you could use rcu_dereference_raw() with a comment
explaining that the read pointer is only used to check for NULL-ness,
and that it is guaranteed that the pointer can't change if it is NULL
and we're holding the lock. But that'd be needlessly complicated, I
think.)
> + if (unlikely(object)) {
> + /* Someone else just created the object, bail out and retry. */
> + spin_unlock(&inode->i_lock);
> + kfree(new_object);
> +
> + rcu_read_lock();
> + goto retry;
> + }
> +
> + rcu_assign_pointer(inode_sec->object, new_object);
> + /*
> + * @inode will be released by hook_sb_delete() on its superblock
> + * shutdown.
> + */
> + ihold(inode);
> + spin_unlock(&inode->i_lock);
> + return new_object;
> +}
next prev parent reply other threads:[~2021-03-23 0:14 UTC|newest]
Thread overview: 49+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-16 20:42 [PATCH v30 00/12] Landlock LSM Mickaël Salaün
2021-03-16 20:42 ` [PATCH v30 01/12] landlock: Add object management Mickaël Salaün
2021-03-19 18:13 ` Kees Cook
2021-03-19 18:57 ` Mickaël Salaün
2021-03-16 20:42 ` [PATCH v30 02/12] landlock: Add ruleset and domain management Mickaël Salaün
2021-03-19 18:40 ` Kees Cook
2021-03-19 19:03 ` Mickaël Salaün
2021-03-19 19:15 ` Kees Cook
2021-03-24 20:31 ` James Morris
2021-03-25 9:29 ` Mickaël Salaün
2021-03-23 0:13 ` Jann Horn
2021-03-16 20:42 ` [PATCH v30 03/12] landlock: Set up the security framework and manage credentials Mickaël Salaün
2021-03-19 18:45 ` Kees Cook
2021-03-19 19:07 ` Mickaël Salaün
2021-03-16 20:42 ` [PATCH v30 04/12] landlock: Add ptrace restrictions Mickaël Salaün
2021-03-19 18:45 ` Kees Cook
2021-03-16 20:42 ` [PATCH v30 05/12] LSM: Infrastructure management of the superblock Mickaël Salaün
2021-03-19 17:24 ` Kees Cook
2021-03-16 20:42 ` [PATCH v30 06/12] fs,security: Add sb_delete hook Mickaël Salaün
2021-03-19 17:24 ` Kees Cook
2021-03-16 20:42 ` [PATCH v30 07/12] landlock: Support filesystem access-control Mickaël Salaün
2021-03-18 23:10 ` James Morris
2021-03-19 18:57 ` Kees Cook
2021-03-19 19:19 ` Mickaël Salaün
2021-03-23 19:30 ` Mickaël Salaün
2021-03-23 0:13 ` Jann Horn [this message]
2021-03-23 15:55 ` Mickaël Salaün
2021-03-23 17:49 ` Jann Horn
2021-03-23 19:22 ` Mickaël Salaün
2021-03-24 3:10 ` Jann Horn
2021-03-16 20:42 ` [PATCH v30 08/12] landlock: Add syscall implementations Mickaël Salaün
2021-03-19 19:06 ` Kees Cook
2021-03-19 21:53 ` Mickaël Salaün
2021-03-24 15:03 ` Mickaël Salaün
2021-03-16 20:42 ` [PATCH v30 09/12] arch: Wire up Landlock syscalls Mickaël Salaün
2021-03-16 20:42 ` [PATCH v30 10/12] selftests/landlock: Add user space tests Mickaël Salaün
2021-03-19 17:56 ` Kees Cook
2021-03-19 18:41 ` Mickaël Salaün
2021-03-19 19:11 ` Kees Cook
2021-03-19 21:57 ` Mickaël Salaün
2021-03-16 20:42 ` [PATCH v30 11/12] samples/landlock: Add a sandbox manager example Mickaël Salaün
2021-03-19 17:26 ` Kees Cook
2021-03-16 20:42 ` [PATCH v30 12/12] landlock: Add user and kernel documentation Mickaël Salaün
2021-03-19 18:03 ` Kees Cook
2021-03-19 18:54 ` Mickaël Salaün
2021-03-23 19:25 ` Mickaël Salaün
2021-03-24 16:21 ` Mickaël Salaün
2021-03-18 23:26 ` [PATCH v30 00/12] Landlock LSM James Morris
2021-03-19 15:52 ` Mickaël Salaün
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