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KVM: nVMX: Explicitly invalidate posted_intr_nv if PI is disabled at VM-Enter

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Explicitly invalidate posted_intr_nv when emulating nested VM-Enter and
posted interrupts are disabled to make it clear that posted_intr_nv is
valid if and only if nested posted interrupts are enabled, and as a cheap
way to harden against KVM bugs.

KVM initializes posted_intr_nv to -1 at vCPU creation and resets it to -1
when unloading vmcs12 and/or leaving nested mode, i.e. this is not a bug
fix (or at least, it's not intended to be a bug fix).

Note, tracking nested.posted_intr_nv as a u16 subtly adds a measure of
safety, as it prevents unintentionally matching KVM's informal "no IRQ"
vector of -1, stored as a signed int.  Because a u16 can be always be
represented as a signed int, the effective "invalid" value of
posted_intr_nv, 65535, will be preserved as-is when comparing against an
int, i.e. will be zero-extended, not sign-extended, and thus won't get a
false positive if KVM is buggy and compares posted_intr_nv against -1.

Opportunistically add a comment in vmx_deliver_nested_posted_interrupt()
to call out that it must check vmx->nested.posted_intr_nv, not the vector
in vmcs12, which is presumably the _entire_ reason nested.posted_intr_nv
exists.  E.g. vmcs12 is a KVM-controlled snapshot, so there are no TOCTOU
races to worry about, the only potential badness is if the vCPU leaves
nested and frees vmcs12 between the sender checking is_guest_mode() and
dereferencing the vmcs12 pointer.

Link: https://lore.kernel.org/r/20240906043413.1049633-7-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Sean Christopherson 2024-09-05 21:34:12 -07:00
parent aa9477966a
commit 1ed0f119c5
2 changed files with 11 additions and 2 deletions
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6
arch/x86/kvm/vmx/nested.c
View File

@ -2317,10 +2317,12 @@ static void prepare_vmcs02_early(struct vcpu_vmx *vmx, struct loaded_vmcs *vmcs0
/* Posted interrupts setting is only taken from vmcs12. */
vmx->nested.pi_pending = false;
if (nested_cpu_has_posted_intr(vmcs12))
if (nested_cpu_has_posted_intr(vmcs12)) {
vmx->nested.posted_intr_nv = vmcs12->posted_intr_nv;
else
} else {
vmx->nested.posted_intr_nv = -1;
exec_control &= ~PIN_BASED_POSTED_INTR;
}
pin_controls_set(vmx, exec_control);
/*

7
arch/x86/kvm/vmx/vmx.c
View File

@ -4215,6 +4215,13 @@ static int vmx_deliver_nested_posted_interrupt(struct kvm_vcpu *vcpu,
{
struct vcpu_vmx *vmx = to_vmx(vcpu);
/*
* DO NOT query the vCPU's vmcs12, as vmcs12 is dynamically allocated
* and freed, and must not be accessed outside of vcpu->mutex. The
* vCPU's cached PI NV is valid if and only if posted interrupts
* enabled in its vmcs12, i.e. checking the vector also checks that
* L1 has enabled posted interrupts for L2.
*/
if (is_guest_mode(vcpu) &&
vector == vmx->nested.posted_intr_nv) {
/*