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SubjectRe: [PATCH] cgroup: handle cset multiidentity issue when migration
Hello.

On Fri, Jun 03, 2022 at 12:34:48AM +0800, shisiyuan <shisiyuan19870131@gmail.com> wrote:
> Bug code flow:
> cset X's initial refcount is 1.
> 1. cgroup_attach_task()
> 2. [For thread1]
> cgroup_migrate_add_src()
> [For thread2]
> cgroup_migrate_add_src()
> cset X is thread2's src_cset , ref->2,
> and its mg_preload_node is added to
> mgctx->preloaded_src_csets.
> 3. cgroup_migrate_prepare_dst()
> [For thread1]
> find_css_set()
> cset X is thread1's dst_cset, ref->3
> put_css_set()
> ref->2 because cset X's mg_preload_node is not
> empty(already in mgctx->preloaded_src_csets).
> [For thread2]
> find_css_cset()
> cset X is also thread2's dst_cset, ref->3
> then drop src_cset, ref->1
> [cgroup_free] ref->0
> 4. cgroup_migrate_execute
> [For thread1]
> ref -> 0xc0000000(UAF)

I'm trying to understand when this happens.
You migrate a process with two threads while one of them exits?

This should be properly synchronized with cgroup_threadgroup_rwsem, so I
don't understand where does the [cgroup_free] between 3. and 4. come
from.

Do you have a reproducer for this?

Thanks,
Michal

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