On 23/03/2021 18:49, Jann Horn wrote:
On Tue, Mar 23, 2021 at 4:54 PM Mickaël Salaün [off-list ref] wrote:
quoted
On 23/03/2021 01:13, Jann Horn wrote:
quoted
On Tue, Mar 16, 2021 at 9:43 PM Mickaël Salaün [off-list ref] wrote:
quoted
Using Landlock objects and ruleset, it is possible to tag inodes
according to a process's domain.
[...]
quoted
+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);
+}
[...]
quoted
+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.)
To reach rcu_assign_pointer(landlock_inode(inode)->object, NULL) in
release_inode() or in hook_sb_delete(), the
landlock_inode(inode)->object need to be non-NULL,
Yes.
quoted
which implies that a
call to get_inode_object(inode) either "retry" (because release_inode is
only called by landlock_put_object, which set object->usage to 0) until
it creates a new object, or reuses the existing referenced object (and
increments object->usage).
But it can be that landlock_inode(inode)->object only becomes non-NULL
after get_inode_object() has checked
rcu_dereference(inode_sec->object).
quoted
The worse case would be if
get_inode_object(inode) is called just before the
rcu_assign_pointer(landlock_inode(inode)->object, NULL) from
hook_sb_delete(), which would result in an object with a NULL underobj,
which is the expected behavior (and checked by release_inode).
The scenario I'm talking about doesn't involve hook_sb_delete().
quoted
The line rcu_assign_pointer(inode_sec->object, new_object) from
get_inode_object() can only be reached if the underlying inode doesn't
reference an object,
Yes.
quoted
in which case hook_sb_delete() will not reach the
rcu_assign_pointer(landlock_inode(inode)->object, NULL) line for this
same inode.
This works because get_inode_object(inode) is mutually exclusive to
itself with the same inode (i.e. an inode can only point to an object
that references this same inode).
To clarify: You can concurrently call get_inode_object() multiple
times on the same inode, right? There are no locks held on entry to
that function.
quoted
I tried to explain this with the comment "Protects against concurrent
get_inode_object() calls" in get_inode_object(), and the comments just
before both rcu_assign_pointer(landlock_inode(inode)->object, NULL).
The scenario I'm talking about is:
Initially the inode does not have an associated landlock_object. There
are two threads A and B. Thread A is going to execute
get_inode_object(). Thread B is going to execute get_inode_object()
followed immediately by landlock_put_object().
thread A: enters get_inode_object()
thread A: rcu_dereference(inode_sec->object) returns NULL
thread A: enters landlock_create_object()
thread B: enters get_inode_object()
thread B: rcu_dereference(inode_sec->object) returns NULL
thread B: calls landlock_create_object()
thread B: sets inode_sec->object while holding inode->i_lock
thread B: leaves get_inode_object()
thread B: enters landlock_put_object()
thread B: object->usage drops to 0, object->lock is taken
thread B: calls release_inode()
thread B: drops object->lock
thread A: returns from landlock_create_object()
thread A: takes inode->i_lock
At this point, thread B will run:
rcu_assign_pointer(landlock_inode(inode)->object, NULL);
while thread A runs:
rcu_dereference_protected(inode_sec->object,
lockdep_is_held(&inode->i_lock));
meaning there is a (theoretical) data race, since
rcu_dereference_protected() doesn't use READ_ONCE().
Hum, I see, that is what I was missing. And that explain why there is
(in practice) no impact on winning the race.
I would prefer to use rcu_access_pointer() instead of
rcu_dereference_protected() to avoid pitfall, and it reflects what I was
expecting:
--- a/security/landlock/fs.c
+++ b/security/landlock/fs.c
@@ -117,9 +117,7 @@ static struct landlock_object
*get_inode_object(struct inode *const inode)
/* 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));
- if (unlikely(object)) {
+ if (unlikely(rcu_access_pointer(inode_sec->object))) {
/* Someone else just created the object, bail out and
retry. */
spin_unlock(&inode->i_lock);
kfree(new_object);
But I'm not sure about your proposition to move the NULL store in
release_inode() back up. Do you mean to add back the inode lock in
release_inode() like this?
--- a/security/landlock/fs.c
+++ b/security/landlock/fs.c
@@ -59,16 +59,12 @@ static void release_inode(struct landlock_object
*const object)
* Makes sure that if the filesystem is concurrently unmounted,
* hook_sb_delete() will wait for us to finish iput().
*/
+ spin_lock(&inode->i_lock);
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);
+ spin_unlock(&inode->i_lock);
/*
* Now, new rules can safely be tied to @inode with get_inode_object().
*/
I would prefer to avoid nested locks if it is not necessary though.
quoted
quoted
quoted
+ 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;
+}