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

cgroup/cpuset: rebind mm mempolicy to effective_mems, not mems_allowed

Creating a child cpuset where cpuset.mems is never set leads to a div/0
when a VMA mempolicy with MPOL_F_RELATIVE_NODES rebinds in response to a
CPU hotplug event.

Reproduction steps:
 1) Create a cgroup w/ cpuset controls (do not set cpuset.mems)
 2) Move the task into the child cpuset
 3) Create a VMA mempolicy for that task with MPOL_F_RELATIVE_NODES
 4) unplug and hotplug a cpu
      echo 0 > /sys/devices/system/cpu/cpu1/online
      echo 1 > /sys/devices/system/cpu/cpu1/online
 5) mempolicy rebind does a div/0 in mpol_relative_nodemask on the
    call to __nodes_fold()

The cpuset code passes (cs->mems_allowed) which is not guaranteed to have
nodes to the rebind routine.  Use cs->effective_mems instead, which is
guaranteed to have a non-empty nodemask once we reach that code path.

[ david: add a comment, slightly rephrase description ]

https://git.kernel.org/stable/c/02f67c4f88be8e3dbd91951d13cdcc5bcc82e9c7

https://git.kernel.org/stable/c/4b06449384b9ee5b3372bab60601ba5cd4162086

https://git.kernel.org/stable/c/b7adeba2a21c98c7b20f18e27e3ead86bdb5e08d

https://git.kernel.org/stable/c/b983c56426383e4a06fa5970c4e33cee879b1482

https://git.kernel.org/stable/c/c17f06d8a085d6be58b544a440ce243f5e441a60

https://git.kernel.org/stable/c/c844b7d9a9586de15dd28c06da5cc7f6ab28787d

https://git.kernel.org/stable/c/fc680afc510157f6b137956011c09abc23eb0842

https://git.kernel.org/stable/c/fcc8c310539c3cc523419113b227161a96b50550

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