CVE-2021-47011

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

mm: memcontrol: slab: fix obtain a reference to a freeing memcg

Patch series "Use obj_cgroup APIs to charge kmem pages", v5.

Since Roman's series "The new cgroup slab memory controller" applied.

All slab objects are charged with the new APIs of obj_cgroup. The new

APIs introduce a struct obj_cgroup to charge slab objects. It prevents

long-living objects from pinning the original memory cgroup in the

memory. But there are still some corner objects (e.g. allocations

larger than order-1 page on SLUB) which are not charged with the new

APIs. Those objects (include the pages which are allocated from buddy

allocator directly) are charged as kmem pages which still hold a

reference to the memory cgroup.

E.g. We know that the kernel stack is charged as kmem pages because the

size of the kernel stack can be greater than 2 pages (e.g. 16KB on

x86_64 or arm64). If we create a thread (suppose the thread stack is

charged to memory cgroup A) and then move it from memory cgroup A to

memory cgroup B. Because the kernel stack of the thread hold a

reference to the memory cgroup A. The thread can pin the memory cgroup

A in the memory even if we remove the cgroup A. If we want to see this

scenario by using the following script. We can see that the system has

added 500 dying cgroups (This is not a real world issue, just a script

to show that the large kmallocs are charged as kmem pages which can pin

the memory cgroup in the memory).

#!/bin/bash

cat /proc/cgroups | grep memory

cd /sys/fs/cgroup/memory

echo 1 > memory.move_charge_at_immigrate

for i in range{1..500}

do

mkdir kmem_test

echo $$ > kmem_test/cgroup.procs

sleep 3600 &

echo $$ > cgroup.procs

echo `cat kmem_test/cgroup.procs` > cgroup.procs

rmdir kmem_test

done

cat /proc/cgroups | grep memory

This patchset aims to make those kmem pages to drop the reference to

memory cgroup by using the APIs of obj_cgroup. Finally, we can see that

the number of the dying cgroups will not increase if we run the above test

script.

This patch (of 7):

The rcu_read_lock/unlock only can guarantee that the memcg will not be

freed, but it cannot guarantee the success of css_get (which is in the

refill_stock when cached memcg changed) to memcg.

rcu_read_lock()

memcg = obj_cgroup_memcg(old)

__memcg_kmem_uncharge(memcg)

refill_stock(memcg)

if (stock->cached != memcg)

// css_get can change the ref counter from 0 back to 1.

css_get(&memcg->css)

rcu_read_unlock()

This fix is very like the commit:

eefbfa7fd678 ("mm: memcg/slab: fix use after free in obj_cgroup_charge")

Fix this by holding a reference to the memcg which is passed to the

__memcg_kmem_uncharge() before calling __memcg_kmem_uncharge().