CVE-2024-35791

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

KVM: SVM: Flush pages under kvm->lock to fix UAF in svm_register_enc_region()

Do the cache flush of converted pages in svm_register_enc_region() before dropping kvm->lock to fix use-after-free issues where region and/or its array of pages could be freed by a different task, e.g. if userspace has __unregister_enc_region_locked() already queued up for the region.

Note, the obvious alternative of using local variables doesnt fully resolve the bug, as region->pages is also dynamically allocated. I.e. the region structure itself would be fine, but region->pages could be freed.

Flushing multiple pages under kvm->lock is unfortunate, but the entire flow is a rare slow path, and the manual flush is only needed on CPUs that lack coherency for encrypted memory.

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/12f8e32a5a389a5d58afc67728c76e61beee1ad4
• https://git.kernel.org/stable/c/2d13b79640b147bd77c34a5998533b2021a4122d
• https://git.kernel.org/stable/c/4868c0ecdb6cfde7c70cf478c46e06bb9c7e5865
• https://git.kernel.org/stable/c/5ef1d8c1ddbf696e47b226e11888eaf8d9e8e807
• https://git.kernel.org/stable/c/e126b508ed2e616d679d85fca2fbe77bb48bbdd7
• https://git.kernel.org/stable/c/f6d53d8a2617dd58c89171a6b9610c470ebda38a
• https://git.kernel.org/stable/c/12f8e32a5a389a5d58afc67728c76e61beee1ad4
• https://git.kernel.org/stable/c/2d13b79640b147bd77c34a5998533b2021a4122d
• https://git.kernel.org/stable/c/4868c0ecdb6cfde7c70cf478c46e06bb9c7e5865
• https://git.kernel.org/stable/c/5ef1d8c1ddbf696e47b226e11888eaf8d9e8e807
• https://git.kernel.org/stable/c/e126b508ed2e616d679d85fca2fbe77bb48bbdd7
• https://git.kernel.org/stable/c/f6d53d8a2617dd58c89171a6b9610c470ebda38a
• https://lists.debian.org/debian-lts-announce/2024/06/msg00017.html