ACK: [SRU][F][PATCH 0/1] CVE-2024-43892

[Magali Lemes]

Acked-by: Magali Lemes <magali.lemes at canonical.com>

On 20/11/2024 04:25, Koichiro Den wrote:
> [Impact]
> 
> 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.
> 
> [Fix]
> 
> Oracular: not affected
> Noble:    fixed via stable upstream update (LP: #2083656)
> Jammy:    fixed via stable upstream update (LP: #2081279)
> Focal:    Clean cherry-pick
> Bionic:   fix sent to esm ML
> Xenial:   fix sent to esm ML
> Trusty:   not affected
> 
> [Test Case]
> 
> Compile and boot tested
> 
> [Where problems could occur]
> 
> This fix affects those who do not disable memory cgroup controller, an issue
> with this fix would be visible to the user via unpredicted system behavior or a
> system crash.
> 
> 
> Shakeel Butt (1):
>    memcg: protect concurrent access to mem_cgroup_idr
> 
>   mm/memcontrol.c | 23 ++++++++++++++++++++---
>   1 file changed, 20 insertions(+), 3 deletions(-)
>