CVE-2024-43892: memcg: protect concurrent access to mem_cgroup_idr

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

memcg: protect concurrent access to memcgroupidr

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 memcgroupidr, the idralloc() and idrreplace() happen within css callback and thus are protected through cgroupmutex from concurrent modifications. However idrremove() for memcgroupidr 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 listlru based kernel crashes at a low frequency in our fleet for a long time. These crashes were in different part of listlru code including listlruadd(), listlrudel() and reparenting code. Upon further inspection, it looked like for a given object (dentry and inode), the superblock's listlru didn't have listlruone for the memcg of that object. The initial suspicions were either the object is not allocated through kmemcachealloclru() or somehow memcglistlrualloc() failed to allocate listlruone() 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 memcgroupidr and in some cases multiple valid memcgs have same id and memcgroupidr is pointing to one of them. So, the most reasonable explanation is that these situations can happen due to race between multiple idrremove() calls or race between idralloc()/idrreplace() and idrremove(). These races are causing multiple memcgs to acquire the same ID and then offlining of one of them would cleanup listlrus on the system for all of them. Later access from other memcgs to the listlru cause crashes due to missing listlruone.