cgroup: Use separate src/dst nodes when preloading css_sets for migration

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

cgroup: Use separate src/dst nodes when preloading css_sets for migration

Each cset (css_set) is pinned by its tasks. When we're moving tasks around

across csets for a migration, we need to hold the source and destination

csets to ensure that they don't go away while we're moving tasks about. This

is done by linking cset->mg_preload_node on either the

mgctx->preloaded_src_csets or mgctx->preloaded_dst_csets list. Using the

same cset->mg_preload_node for both the src and dst lists was deemed okay as

a cset can't be both the source and destination at the same time.

Unfortunately, this overloading becomes problematic when multiple tasks are

involved in a migration and some of them are identity noop migrations while

others are actually moving across cgroups. For example, this can happen with

the following sequence on cgroup1:

#1> mkdir -p /sys/fs/cgroup/misc/a/b

#2> echo $$ > /sys/fs/cgroup/misc/a/cgroup.procs

#3> RUN_A_COMMAND_WHICH_CREATES_MULTIPLE_THREADS &

#4> PID=$!

#5> echo $PID > /sys/fs/cgroup/misc/a/b/tasks

#6> echo $PID > /sys/fs/cgroup/misc/a/cgroup.procs

the process including the group leader back into a. In this final migration,

non-leader threads would be doing identity migration while the group leader

is doing an actual one.

After #3, let's say the whole process was in cset A, and that after #4, the

leader moves to cset B. Then, during #6, the following happens:

1. cgroup_migrate_add_src() is called on B for the leader.

2. cgroup_migrate_add_src() is called on A for the other threads.

3. cgroup_migrate_prepare_dst() is called. It scans the src list.

4. It notices that B wants to migrate to A, so it tries to A to the dst

list but realizes that its ->mg_preload_node is already busy.

5. and then it notices A wants to migrate to A as it's an identity

migration, it culls it by list_del_init()'ing its ->mg_preload_node and

putting references accordingly.

6. The rest of migration takes place with B on the src list but nothing on

the dst list.

This means that A isn't held while migration is in progress. If all tasks

leave A before the migration finishes and the incoming task pins it, the

cset will be destroyed leading to use-after-free.

This is caused by overloading cset->mg_preload_node for both src and dst

preload lists. We wanted to exclude the cset from the src list but ended up

inadvertently excluding it from the dst list too.

This patch fixes the issue by separating out cset->mg_preload_node into

->mg_src_preload_node and ->mg_dst_preload_node, so that the src and dst

preloadings don't interfere with each other.