Re: CVE-2024-36000: mm/hugetlb: fix missing hugetlb_lock for resv uncharge

[Michal Hocko <mhocko@suse.com>]

On Thu 23-05-24 12:33:37, Oscar Salvador wrote:
> On Thu, May 23, 2024 at 09:30:59AM +0200, Michal Hocko wrote:
> > Let me add Oscar,
> 
> Thanks
> 
> > 
> > On Tue 21-05-24 15:38:45, Peter Xu wrote:
> > > That commit mentioned that we rely on the lock to make sure all hugetlb
> > > folios on the active list will have a valid memcg.  However I'm not sure
> > > whether it's still required now (after all that's 2012..), e.g., I'm
> > > looking at hugetlb_cgroup_css_offline(), and hugetlb_cgroup_move_parent()
> > > looks all safe to even take empty memcg folios with the latest code at
> > > least:
> > > 
> > > 	/*
> > > 	 * We can have pages in active list without any cgroup
> > > 	 * ie, hugepage with less than 3 pages. We can safely
> > > 	 * ignore those pages.
> > > 	 */
> > > 	if (!page_hcg || page_hcg != h_cg)
> > > 		goto out;
> 
> Ok, I had a look at hugetlb_cgroup implementation.
> First time looking at that code, so bear with me.
> 
> I looked back at commit
> 
>  commit 94ae8ba7176666d1e7d8bbb9f93670a27540b6a8 (HEAD)
>  Author: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
>  Date:   Tue Jul 31 16:42:35 2012 -0700
>  
>      hugetlb/cgroup: assign the page hugetlb cgroup when we move the page to active list.
> 
> to understand why the lock was needed.
> 
> On the theoretical part:
> 
> And we could have
> 
>      CPU0                                   CPU1
>  dequeue_huge_page_vma
>   dequeue_huge_page_node
>    move_page_to_active_list
>  release_lock
>                                            hugetlb_cgroup_pre_destroy
>                                             for_each_page_in_active_list
>                                             <-- pages empty cgroups are skipped -->
>                                              hugetlb_cgroup_move_parent
>                                              move_page_to_parent
>  hugetlb_cgroup_commit_charge <-- too late
>   page[2].lru.next = (void *)h_cg;
> 
> So, the above page should have been moved to the parent, but since by the time
> we were checking the activelist this page did not have any cgroup attach ot it,
> it was skipped.
> 
> Notice I said theoretical because I noticed that
> cgroup_call_pre_destroy()->hugetlb_cgroup_pre_destroy() is called from
> cgroup_rmdir(). I am not sure under which circumstances cgroup_rmdir()
> can succeed (does the cgroup refcount have dropped to 0?)

Now, it just cannot have any tasks attached nor any subgroups. So is the
race actually possible?
-- 
Michal Hocko
SUSE Labs