KVM: SEV: Nullify "have run CPUs" mask pointer when freeing it

Overview

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

KVM: SEV: Nullify "have run CPUs" mask pointer when freeing it

Nullify have_run_cpus when freeing the mask, particularly in the error path of __sev_guest_init(), so that KVM doesn't have to subtly use sev->active to track whether or not the mask has been freed. As pointed out by Sashiko, blindly freeing the mask in sev_vm_destroy() results in a double-free if the mask is freed if __sev_guest_init() fails.

Throw the logic in a helper as nullifying the pointer is frustratingly difficult and weird due to have_run_cpus being a single-entry array when CPUMASK_OFFSTACK=n. Deliberately don't use CPUMASK_VAR_NULL, as it's not directly assignable when the cpumask is on-stack, e.g. requires using a local variable and a memcpy(), which is beyond ridiculous. Furthermore, while clearing the on-stack bitmask is an unnecessary and arguably unwanted side effect, KVM absolutely relies on '0' being the "null" value given that the struct is zero-allocated.

Opportunistically add an alloc() helper to pair with free(); there are just enough call sites to make doing so worthwhile.

Affected

• Linux >= 6aef7cf800098749ff5d660222efaf6e01e98c09 < b1de3ebf59507e0edacaa5cfa2225bc1869e1b63
• Linux >= 9784e5ca91b37b387f3f414af4340c54f7a63566 < 0fbc7fec2242de6c3dcea6de55573c428e820173
• Linux >= 12c1f6e03f944e399bd2c88441dca5dc702b95a5 < 8abbc76120a74bfba1851bd97528099d715e45c6
• 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.