[PATCH v3 0/2] KVM: x86: Fix VM-Entry fail due to stale CR8 intercept

[PATCH v3 0/2] KVM: x86: Fix VM-Entry fail due to stale CR8 intercept

[Sean Christopherson]

Carlos' fix for a bug where KVM neglects to update TPR_THRESHOLD and runs
afoul of VMX's consistency check that the current TPR won't immediately
generate a VM-Exit.

v3: Call get_vmcs12() iff is_guest_mode() is true to workaround a lockdep
    false positive. [syzbot]

v2:
 - https://lore.kernel.org/all/20260610214523.2905255-2-clopez@suse.de
 - Call kvm_lapic_update_cr8_intercept() from apic_update_ppr() instead
   of issuing KVM_REQ_EVENTS from handle_tpr_below_threshold().

Carlos López (1):
  KVM: x86: Unconditionally recompute CR8 intercept on PPR update

Sean Christopherson (1):
  KVM: VMX: Grab vmcs12 on CR8 interception update iff vCPU is in guest
    mode

 arch/x86/kvm/lapic.c   | 2 ++
 arch/x86/kvm/vmx/vmx.c | 3 +--
 arch/x86/kvm/x86.c     | 5 +----
 3 files changed, 4 insertions(+), 6 deletions(-)


base-commit: 9d4853b044beefa21c4ee3e18c40653601a64ced
-- 
2.55.0.rc0.738.g0c8ab3ebcc-goog

[PATCH v3 1/2] KVM: VMX: Grab vmcs12 on CR8 interception update iff vCPU is in guest mode

[Sean Christopherson]

When updating CR8 intercepts, get vmcs12 if and only if the vCPU is in
guest mode so that a future change can have update CR8 intercepts during
vCPU creation, without running afoul of get_vmcs12()'s lockdep assertion.

  ------------[ cut here ]------------
  debug_locks && !(lock_is_held(&(&vcpu->mutex)->dep_map) || !refcount_read(&vcpu->kvm->users_count))
  WARNING: arch/x86/kvm/vmx/nested.h:61 at get_vmcs12 arch/x86/kvm/vmx/nested.h:60 [inline], CPU#0: syz.2.19/5879
  WARNING: arch/x86/kvm/vmx/nested.h:61 at vmx_update_cr8_intercept+0x3de/0x4e0 arch/x86/kvm/vmx/vmx.c:6879, CPU#0: syz.2.19/5879
  Modules linked in:
  CPU: 0 UID: 0 PID: 5879 Comm: syz.2.19 Not tainted syzkaller #0 PREEMPT(full)
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
  RIP: 0010:get_vmcs12 arch/x86/kvm/vmx/nested.h:60 [inline]
  RIP: 0010:vmx_update_cr8_intercept+0x3de/0x4e0 arch/x86/kvm/vmx/vmx.c:6879
  Call Trace:
   <TASK>
   apic_update_ppr arch/x86/kvm/lapic.c:984 [inline]
   kvm_lapic_reset+0x1c24/0x2980 arch/x86/kvm/lapic.c:3023
   kvm_vcpu_reset+0x44c/0x1bf0 arch/x86/kvm/x86.c:12986
   kvm_arch_vcpu_create+0x746/0x8b0 arch/x86/kvm/x86.c:12847
   kvm_vm_ioctl_create_vcpu+0x428/0x930 virt/kvm/kvm_main.c:4201
   kvm_vm_ioctl+0x893/0xd50 virt/kvm/kvm_main.c:5159
   vfs_ioctl fs/ioctl.c:51 [inline]
   __do_sys_ioctl fs/ioctl.c:597 [inline]
   __se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
   do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
   do_syscall_64+0x174/0x580 arch/x86/entry/syscall_64.c:94
   entry_SYSCALL_64_after_hwframe+0x77/0x7f
   </TASK>

No functional change intended.

Reported-by: syzbot ci <syzbot+ci493c6d734b63e050@syzkaller.appspotmail.com>
Closes: https://lore.kernel.org/all/6a2adf3b.3b0a2d4e.8c8d1.0012.GAE@google.com
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/vmx/vmx.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index a1a5edb39a7e..125994ed3db5 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6872,11 +6872,10 @@ int vmx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t exit_fastpath)
 
 void vmx_update_cr8_intercept(struct kvm_vcpu *vcpu, int tpr, int irr)
 {
-	struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
 	int tpr_threshold;
 
 	if (is_guest_mode(vcpu) &&
-		nested_cpu_has(vmcs12, CPU_BASED_TPR_SHADOW))
+	    nested_cpu_has(get_vmcs12(vcpu), CPU_BASED_TPR_SHADOW))
 		return;
 
 	guard(vmx_vmcs01)(vcpu);
-- 
2.55.0.rc0.738.g0c8ab3ebcc-goog

[PATCH v3 2/2] KVM: x86: Unconditionally recompute CR8 intercept on PPR update

[Sean Christopherson]

From: Carlos López <clopez@suse.de>

The TPR_THRESHOLD field in the VMCS is used by VMX to induce VM exits
when the guest's virtual TPR falls under the specified threshold,
allowing KVM to inject previously masked interrupts.

KVM handles these VM exits in handle_tpr_below_threshold().
Commit eb90f3417a0c ("KVM: vmx: speed up TPR below threshold vmexits")
optimized this function by calling apic_update_ppr() instead of raising
KVM_REQ_EVENT. apic_update_ppr() then raises KVM_REQ_EVENT if there is
a pending, deliverable interrupt.

However, if there are no new interrupts pending, apic_update_ppr() does
not issue the request. Thus, kvm_lapic_update_cr8_intercept() and
vmx_update_cr8_intercept() are not called before VM entry, which results
in a high, stale TPR_THRESHOLD. This is problematic due to the following
sentence in 28.2.1.1 "VM-Execution Control Fields" in the SDM:

  The following check is performed if the “use TPR shadow” VM-execution
  control is 1 and the “virtualize APIC accesses” and “virtual-interrupt
  delivery” VM-execution controls are both 0: the value of bits 3:0 of
  the TPR threshold VM-execution control field should not be greater
  than the value of bits 7:4 of VTPR.

This error condition is typically not observed when KVM runs on a bare
metal system because modern processors support APICv, which enables
virtual-interrupt delivery, and which KVM uses when possible. This
causes the processor to no longer generate TPR-below-threshold exits
and to no longer check TPR_THRESHOLD on entry. However, when running
on older platforms, or under nested virtualization on a hypervisor that
does not support virtual-interrupt delivery and enforces this check
(like Hyper-V) this can cause a VM entry failure with hardware error
0x7, as seen in [1].

Call kvm_lapic_update_cr8_intercept() if apic_update_ppr() does not
find a deliverable interrupt (and thus does not raise KVM_REQ_EVENT).
Remove calls to kvm_lapic_update_cr8_intercept() on paths that end up in
apic_update_ppr(), as they now become redundant. This ensures that any
path that updates the guest's PPR also figures out if KVM needs to wait
for a TPR change (using TPR_THRESHOLD on VMX or CR8 intercepts on SVM).

Link: https://github.com/coconut-svsm/svsm/issues/1081 [1]
Tested-by: Stefano Garzarella <sgarzare@redhat.com>
Cc: stable@vger.kernel.org
Fixes: eb90f3417a0c ("KVM: vmx: speed up TPR below threshold vmexits")
Signed-off-by: Carlos López <clopez@suse.de>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/lapic.c | 2 ++
 arch/x86/kvm/x86.c   | 5 +----
 2 files changed, 3 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 9d2df8623f6d..f6a289d01a26 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -980,6 +980,8 @@ static void apic_update_ppr(struct kvm_lapic *apic)
 	if (__apic_update_ppr(apic, &ppr) &&
 	    apic_has_interrupt_for_ppr(apic, ppr) != -1)
 		kvm_make_request(KVM_REQ_EVENT, apic->vcpu);
+	else
+		kvm_lapic_update_cr8_intercept(apic->vcpu);
 }
 
 void kvm_apic_update_ppr(struct kvm_vcpu *vcpu)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index d9d51803b7b2..96c465040756 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5317,7 +5317,6 @@ static int kvm_vcpu_ioctl_set_lapic(struct kvm_vcpu *vcpu,
 	r = kvm_apic_set_state(vcpu, s);
 	if (r)
 		return r;
-	kvm_lapic_update_cr8_intercept(vcpu);
 
 	return 0;
 }
@@ -12418,8 +12417,6 @@ static int __set_sregs_common(struct kvm_vcpu *vcpu, struct kvm_sregs *sregs,
 	kvm_register_mark_dirty(vcpu, VCPU_REG_CR3);
 	kvm_x86_call(post_set_cr3)(vcpu, sregs->cr3);
 
-	kvm_set_cr8(vcpu, sregs->cr8);
-
 	*mmu_reset_needed |= vcpu->arch.efer != sregs->efer;
 	kvm_x86_call(set_efer)(vcpu, sregs->efer);
 
@@ -12448,7 +12445,7 @@ static int __set_sregs_common(struct kvm_vcpu *vcpu, struct kvm_sregs *sregs,
 	kvm_set_segment(vcpu, &sregs->tr, VCPU_SREG_TR);
 	kvm_set_segment(vcpu, &sregs->ldt, VCPU_SREG_LDTR);
 
-	kvm_lapic_update_cr8_intercept(vcpu);
+	kvm_set_cr8(vcpu, sregs->cr8);
 
 	/* Older userspace won't unhalt the vcpu on reset. */
 	if (kvm_vcpu_is_bsp(vcpu) && kvm_rip_read(vcpu) == 0xfff0 &&
-- 
2.55.0.rc0.738.g0c8ab3ebcc-goog

Re: [PATCH v3 0/2] KVM: x86: Fix VM-Entry fail due to stale CR8 intercept

[Paolo Bonzini]

Queued, thanks.

Paolo