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PUBLISHED

CVE-2021-47011

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

Reserved:2024-02-27
Published:2024-02-28
Updated:2024-04-04

Description

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().

Product status

Default status
unaffected

26f54dac1564 before 31df8bc4d3fe
affected

3de7d4f25a74 before 89b1ed358e01
affected

3de7d4f25a74 before c3ae6a3f3ca4
affected

3de7d4f25a74 before 9f38f03ae8d5
affected

Default status
affected

5.11
affected

Any version before 5.11
unaffected

5.10.37
unaffected

5.11.21
unaffected

5.12.4
unaffected

5.13
unaffected

References

https://git.kernel.org/stable/c/31df8bc4d3feca9f9c6b2cd06fd64a111ae1a0e6

https://git.kernel.org/stable/c/89b1ed358e01e1b0417f5d3b0082359a23355552

https://git.kernel.org/stable/c/c3ae6a3f3ca4f02f6ccddf213c027302586580d0

https://git.kernel.org/stable/c/9f38f03ae8d5f57371b71aa6b4275765b65454fd

cve.org CVE-2021-47011

nvd.nist.gov CVE-2021-47011

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