KVM: SEV: Do cache maintenance on the source VM *before* clearing SEV state

Overview

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

KVM: SEV: Do cache maintenance on the source VM before clearing SEV state

Explicitly flush caches after intra-host migration before clearing "SEV active" on the source VM, as doing cache maintenance afterwards creates a tiny window where memory reclaim could return memory to the host without performing a cache flush, e.g. as pointed out by Sashiko:

CPU1 in sev_migrate_from(): src->active = false;

CPU2 running concurrent unmap: Since active is false, the automatic cache flush in sev_guest_memory_reclaimed is skipped. The host frees and reallocates the page.

CPU1 in sev_migrate_from(): sev_writeback_caches(src_kvm); Executes a hardware cache flush (wbnoinvd), which writes the guest's old dirty ciphertext over the new page owner's data.

Affected

• Linux >= 37dfb676fafd1eef36fe0bb649607afbb0d5521b < 8b7d4f68fef96cebe0b6dbb6e5607772337f7369
• Linux >= d4cc2dff12116a700add601c0262132cc26e1b2c < de83ff6a4d8f3618473bedb3c4400020c22e8a8a
• Linux >= 93de2a6a4b91b72607136dd656edf03fb399d27f < 973ea70393e885e540f714904e51bc6cac80e3d7
• Linux >= 6.18.55 < 6.18.56
• Linux >= 7.2.9 < 7.2.10
• Linux 7.3-rc5

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.