Re: [PATCH 2/2] mm/kmemleak: Fix UAF bug in kmemleak_scan()

From: Waiman Long
Date: Fri Dec 16 2022 - 11:41:47 EST


On 12/16/22 05:32, Catalin Marinas wrote:
On Wed, Dec 14, 2022 at 10:54:28AM -0500, Waiman Long wrote:
On 12/14/22 06:16, Catalin Marinas wrote:
On Sat, Dec 10, 2022 at 06:00:48PM -0500, Waiman Long wrote:
Commit 6edda04ccc7c ("mm/kmemleak: prevent soft lockup in first
object iteration loop of kmemleak_scan()") fixes soft lockup problem
in kmemleak_scan() by periodically doing a cond_resched(). It does
take a reference of the current object before doing it. Unfortunately,
if the object has been deleted from the object_list, the next object
pointed to by its next pointer may no longer be valid after coming
back from cond_resched(). This can result in use-after-free and other
nasty problem.
Ah, kmemleak_cond_resched() releases the rcu lock, so using
list_for_each_entry_rcu() doesn't help.
[...]
Another potential issue with re-scanning is that the loop may never
complete if it always goes from the beginning. Yet another problem with
restarting is that we may count references to an object multiple times
and get more false negatives.

I'd keep the OBJECT_ALLOCATED logic in the main kmemleak_scan() loop and
retake the object->lock if cond_resched() was called
(kmemleak_need_resched() returning true), check if it was freed and
restart the loop. We could add a new OBJECT_SCANNED flag so that we
skip such objects if we restarted the loop. The flag is reset during
list preparation.
[...]
Thanks for the review. Another alternative way to handle that is to add an
OBJECT_ANCHORED flag to indicate that this object shouldn't be deleted from
the object list yet. Maybe also an OBJECT_DELETE_PENDING flag so that
kmemleak_cond_resched() will delete it after returning from cond_resched()
when set by another function that want to delete this object. All these
checks and flag setting will be done with object lock held. How do you
think?
I think we are over-complicating this. The problems I see with deleting
an object are that (1) only the object being deleted is locked (so that
the corresponding memory block is not freed while scanning) and (2)
kmemleak_cond_resched() releases the RCU lock briefly. A list_del_rcu()
on the object next to the one being scanned (and locked) will leave the
current object->next pointer dangling.

Yes, I believe that is the cause of the UAF error that I saw from KASAN.


If we get rid of object->lock and just use kmemleak_lock instead, we can
have a big lock around the scanning, released briefly in
kmemleak_cond_resched(). A standard list_del() (not _rcu) could be run
during the resched but it also updates the current object->next. Once
the lock is re-acquired, the list traversal can continue safely. The
current object cannot be freed due to get_object(). No need for
restarting the loop.

The problem with a big lock (kmemleak_lock) is that we will be disabing interrupt for an extended period of time which is not ideal.

I have posted a v2 patch that drop the idea of restarting the loop. Instead, I just block the current object from being removed from the object_list to make sure its next pointer will point to a valid object.


I don't think we'd miss much in terms of scalability for a debug
feature. Object freeing already takes the kmemleak_lock, it's just that
during scanning it will have to wait for the scanning loop to release
it. We might as well release it within the loop on each iteration.

So my proposal is to replace the rcu list traversal with the classic
one and kmemleak_lock held (some functions like __find_and_get_object()
will have to skip the lock). With this in place, we can subsequently
remove all object->lock instances, just rely on the big lock (we do need
to run lockdep if we do the latter separately, some nesting is a bit
weird; my preference would be to remove the object->lock at the same
time). We still need the rcu freeing in put_object() but for a
completely different reason: the sl*b allocators don't like being called
recursively, so we just use the RCU mechanism to free the kmemleak
structures in a separate thread.

That was what I thought about when you said you wanted to use a big lock instead of object->lock in the last email. As I said above, we can't hold the kmemleak_lock with interrupt disabled for an extended period of time especially if RT tasks are running. So we may need to release the lock frequently like per dozen objects or so. I believe we still need rcu_read_lock() just to be safe.

Cheers,
Longman