Patch "KVM: PPC: Book3S HV: Drop locks before reading guest memory" has been added to the 4.15-stable tree

This is a note to let you know that I've just added the patch titled

KVM: PPC: Book3S HV: Drop locks before reading guest memory

to the 4.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
kvm-ppc-book3s-hv-drop-locks-before-reading-guest-memory.patch
and it can be found in the queue-4.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Paul Mackerras <[email protected]>
Date: Tue, 30 Jan 2018 10:51:32 +1100
Subject: KVM: PPC: Book3S HV: Drop locks before reading guest memory

From: Paul Mackerras <[email protected]>

commit 36ee41d161c67a6fcf696d4817a0da31f778938c upstream.

Running with CONFIG_DEBUG_ATOMIC_SLEEP reveals that HV KVM tries to read guest memory, in order to emulate guest instructions, while preempt is disabled and a vcore lock is held. This occurs in kvmppc_handle_exit_hv(), called from post_guest_process(), when emulating guest doorbell instructions on POWER9 systems, and also when checking whether we have hit a hypervisor breakpoint. Reading guest memory can cause a page fault and thus cause the task to sleep, so we need to avoid reading guest memory while holding a spinlock or when preempt is disabled.

To fix this, we move the preempt_enable() in kvmppc_run_core() to before the loop that calls post_guest_process() for each vcore that has just run, and we drop and re-take the vcore lock around the calls to kvmppc_emulate_debug_inst() and kvmppc_emulate_doorbell_instr().

Dropping the lock is safe with respect to the iteration over the runnable vcpus in post_guest_process(); for_each_runnable_thread is actually safe to use locklessly. It is possible for a vcpu to become runnable and add itself to the runnable_threads array (code near the beginning of kvmppc_run_vcpu()) and then get included in the iteration in post_guest_process despite the fact that it has not just run. This is benign because vcpu->arch.trap and vcpu->arch.ceded will be zero.

---
 arch/powerpc/kvm/book3s_hv.c |   16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

@@ -1005,8 +1005,6 @@ static int kvmppc_emulate_doorbell_instr
 	struct kvm *kvm = vcpu->kvm;
 	struct kvm_vcpu *tvcpu;
 
-	if (!cpu_has_feature(CPU_FTR_ARCH_300))
-		return EMULATE_FAIL;
 	if (kvmppc_get_last_inst(vcpu, INST_GENERIC, &inst) != EMULATE_DONE)
 		return RESUME_GUEST;
 	if (get_op(inst) != 31)

@@ -1056,6 +1054,7 @@ static int kvmppc_emulate_doorbell_instr
 	return RESUME_GUEST;
 }
 
+/* Called with vcpu->arch.vcore->lock held */
 static int kvmppc_handle_exit_hv(struct kvm_run *run, struct kvm_vcpu *vcpu,
 				 struct task_struct *tsk)
 {

@@ -1176,7 +1175,10 @@ static int kvmppc_handle_exit_hv(struct
 				swab32(vcpu->arch.emul_inst) :
 				vcpu->arch.emul_inst;
 		if (vcpu->guest_debug & KVM_GUESTDBG_USE_SW_BP) {
+			/* Need vcore unlocked to call kvmppc_get_last_inst */
+			spin_unlock(&vcpu->arch.vcore->lock);
 			r = kvmppc_emulate_debug_inst(run, vcpu);
+			spin_lock(&vcpu->arch.vcore->lock);
 		} else {
 			kvmppc_core_queue_program(vcpu, SRR1_PROGILL);
 			r = RESUME_GUEST;

@@ -1191,8 +1193,13 @@ static int kvmppc_handle_exit_hv(struct
 	 */
 	case BOOK3S_INTERRUPT_H_FAC_UNAVAIL:
 		r = EMULATE_FAIL;
-		if ((vcpu->arch.hfscr >> 56) == FSCR_MSGP_LG)
+		if (((vcpu->arch.hfscr >> 56) == FSCR_MSGP_LG) &&
+		    cpu_has_feature(CPU_FTR_ARCH_300)) {
+			/* Need vcore unlocked to call kvmppc_get_last_inst */
+			spin_unlock(&vcpu->arch.vcore->lock);
 			r = kvmppc_emulate_doorbell_instr(vcpu);
+			spin_lock(&vcpu->arch.vcore->lock);
+		}
 		if (r == EMULATE_FAIL) {
 			kvmppc_core_queue_program(vcpu, SRR1_PROGILL);
 			r = RESUME_GUEST;

@@ -2934,13 +2941,14 @@ static noinline void kvmppc_run_core(str
 	/* make sure updates to secondary vcpu structs are visible now */
 	smp_mb();
 
+	preempt_enable();
+
 	for (sub = 0; sub < core_info.n_subcores; ++sub) {
 		pvc = core_info.vc[sub];
 		post_guest_process(pvc, pvc == vc);
 	}
 
 	spin_lock(&vc->lock);
-	preempt_enable();
 
  out:
 	vc->vcore_state = VCORE_INACTIVE;

Patches currently in stable-queue which might be from [email protected] are

queue-4.15/kvm-ppc-book3s-hv-make-sure-we-don-t-re-enter-guest-without-xive-loaded.patch queue-4.15/kvm-ppc-book3s-hv-drop-locks-before-reading-guest-memory.patch queue-4.15/kvm-ppc-book3s-pr-fix-broken-select-due-to-misspelling.patch