CVE-2021-47318

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

arch_topology: Avoid use-after-free for scale_freq_data

Currently topology_scale_freq_tick() (which gets called from scheduler_tick()) may end up using a pointer to struct scale_freq_data, which was previously cleared by topology_clear_scale_freq_source(), as there is no protection in place here. The users of topology_clear_scale_freq_source() though needs a guarantee that the previously cleared scale_freq_data isnt used anymore, so they can free the related resources.

Since topology_scale_freq_tick() is called from scheduler tick, we dont want to add locking in there. Use the RCU update mechanism instead (which is already used by the schedulers utilization update path) to guarantee race free updates here.

synchronize_rcu() makes sure that all RCU critical sections that started before it is called, will finish before it returns. And so the callers of topology_clear_scale_freq_source() dont need to worry about their callback getting called anymore.

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/83150f5d05f065fb5c12c612f119015cabdcc124
• https://git.kernel.org/stable/c/ccdf7e073170886bc370c613e269de610a794c4a
• https://git.kernel.org/stable/c/83150f5d05f065fb5c12c612f119015cabdcc124
• https://git.kernel.org/stable/c/ccdf7e073170886bc370c613e269de610a794c4a