CVE-2024-43892

In the Linux kernel, the following vulnerability has been resolved:

memcg: protect concurrent access to mem_cgroup_idr

Commit 73f576c04b94 ("mm: memcontrol: fix cgroup creation failure after

many small jobs") decoupled the memcg IDs from the CSS ID space to fix the

cgroup creation failures. It introduced IDR to maintain the memcg ID

space. The IDR depends on external synchronization mechanisms for

modifications. For the mem_cgroup_idr, the idr_alloc() and idr_replace()

happen within css callback and thus are protected through cgroup_mutex

from concurrent modifications. However idr_remove() for mem_cgroup_idr

was not protected against concurrency and can be run concurrently for

different memcgs when they hit their refcnt to zero. Fix that.

We have been seeing list_lru based kernel crashes at a low frequency in

our fleet for a long time. These crashes were in different part of

list_lru code including list_lru_add(), list_lru_del() and reparenting

code. Upon further inspection, it looked like for a given object (dentry

and inode), the super_block's list_lru didn't have list_lru_one for the

memcg of that object. The initial suspicions were either the object is

not allocated through kmem_cache_alloc_lru() or somehow

memcg_list_lru_alloc() failed to allocate list_lru_one() for a memcg but

returned success. No evidence were found for these cases.

Looking more deeply, we started seeing situations where valid memcg's id

is not present in mem_cgroup_idr and in some cases multiple valid memcgs

have same id and mem_cgroup_idr is pointing to one of them. So, the most

reasonable explanation is that these situations can happen due to race

between multiple idr_remove() calls or race between

idr_alloc()/idr_replace() and idr_remove(). These races are causing

multiple memcgs to acquire the same ID and then offlining of one of them

would cleanup list_lrus on the system for all of them. Later access from

other memcgs to the list_lru cause crashes due to missing list_lru_one.