CVE-2022-49647
Published: 26 February 2025
Description
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.
Security Summary
CVE-2022-49647 is a use-after-free vulnerability in the Linux kernel's cgroup subsystem, specifically during task migrations across css_sets (csets). The issue arises when preloading source and destination csets for migration reuses the same cset->mg_preload_node for both mgctx->preloaded_src_csets and mgctx->preloaded_dst_csets lists. This overloading fails in scenarios involving multiple tasks, such as mixed identity noop migrations and actual cross-cgroup moves, leading to incorrect list management. For instance, in cgroup v1 with the misc controller, sequences like creating subdirectories, moving process leader and threads between /sys/fs/cgroup/misc/a and /sys/fs/cgroup/misc/a/b, then back, can inadvertently remove a cset from the preload list, allowing its destruction and resulting in a use-after-free if tasks unpin it mid-migration.
A local attacker with low privileges (PR:L) can exploit this by writing to cgroup.procs and tasks files in accessible cgroup hierarchies, such as the misc controller. By executing a crafted sequence—e.g., creating cgroup paths, forking threads, and migrating the process group leader and threads in a specific order—the attacker triggers the flawed preload logic. This leads to a use-after-free of a css_set structure, enabling kernel memory corruption with high impact on confidentiality, integrity, and availability (CVSS 7.8: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H, CWE-416).
The provided kernel stable commit references detail the fix: separating the preload nodes into cset->mg_src_preload_node and cset->mg_dst_preload_node to prevent interference between source and destination lists during migrations. Security practitioners should apply these patches (e.g., commits 05f7658210d1, 07fd5b6cdf3c) to vulnerable Linux kernel versions supporting cgroup migrations.
Details
- CWE(s)