CVE-2022-49647

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 were moving tasks around across csets for a migration, we need to hold the source and destination csets to ensure that they dont go away while were 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 cant 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, lets 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 its 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 isnt 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 dont interfere with each other.

Weakness

Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code.

Affected Software

Extended Description

The use of previously-freed memory can have any number of adverse consequences, ranging from the corruption of valid data to the execution of arbitrary code, depending on the instantiation and timing of the flaw. The simplest way data corruption may occur involves the system’s reuse of the freed memory. Use-after-free errors have two common and sometimes overlapping causes:

In this scenario, the memory in question is allocated to another pointer validly at some point after it has been freed. The original pointer to the freed memory is used again and points to somewhere within the new allocation. As the data is changed, it corrupts the validly used memory; this induces undefined behavior in the process. If the newly allocated data happens to hold a class, in C++ for example, various function pointers may be scattered within the heap data. If one of these function pointers is overwritten with an address to valid shellcode, execution of arbitrary code can be achieved.

Potential Mitigations

References

• https://git.kernel.org/stable/c/05f7658210d1d331e8dd4cb6e7bbbe3df5f5ac27
• https://git.kernel.org/stable/c/07fd5b6cdf3cc30bfde8fe0f644771688be04447
• https://git.kernel.org/stable/c/0e41774b564befa6d271e8d5086bf870d617a4e6
• https://git.kernel.org/stable/c/54aee4e5ce8c21555286a6333e46c1713880cf93
• https://git.kernel.org/stable/c/7657e3958535d101a24ab4400f9b8062b9107cc4
• https://git.kernel.org/stable/c/ad44e05f3e016bdcb1ad25af35ade5b5f41ccd68
• https://git.kernel.org/stable/c/cec2bbdcc14fbaa6b95ee15a7c423b05d97038be