CVE-2022-48991

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

mm/khugepaged: invoke MMU notifiers in shmem/file collapse paths

Any codepath that zaps page table entries must invoke MMU notifiers to ensure that secondary MMUs (like KVM) dont keep accessing pages which arent mapped anymore. Secondary MMUs dont hold their own references to pages that are mirrored over, so failing to notify them can lead to page use-after-free.

Im marking this as addressing an issue introduced in commit f3f0e1d2150b (khugepaged: add support of collapse for tmpfs/shmem pages), but most of the security impact of this only came in commit 27e1f8273113 (khugepaged: enable collapse pmd for pte-mapped THP), which actually omitted flushes for the removal of present PTEs, not just for the removal of empty page tables.

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/1a3f8c6cd29d9078cc81b29d39d0e9ae1d6a03c3
• https://git.kernel.org/stable/c/275c626c131cfe141beeb6c575e31fa53d32da19
• https://git.kernel.org/stable/c/5450535901d89a5dcca5fbbc59a24fe89caeb465
• https://git.kernel.org/stable/c/5ffc2a75534d9d74d49760f983f8eb675fa63d69
• https://git.kernel.org/stable/c/7f445ca2e0e59c7971d0b7b853465e50844ab596
• https://git.kernel.org/stable/c/c23105673228c349739e958fa33955ed8faddcaf
• https://git.kernel.org/stable/c/f268f6cf875f3220afc77bdd0bf1bb136eb54db9
• https://git.kernel.org/stable/c/ff2a1a6f869650aec99e9d070b5ab625bfbc5bc3