linux-kernel - [PATCH v3] KVM: SEV: Mark nested locking of vcpu->lock

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Message-Id: <20220407195908.633003-1-pgonda@google.com>
Date:   Thu,  7 Apr 2022 12:59:08 -0700
From:   Peter Gonda <pgonda@...gle.com>
To:     kvm@...r.kernel.org
Cc:     Peter Gonda <pgonda@...gle.com>,
        John Sperbeck <jsperbeck@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH v3] KVM: SEV: Mark nested locking of vcpu->lock

svm_vm_migrate_from() uses sev_lock_vcpus_for_migration() to lock all
source and target vcpu->locks. Mark the nested subclasses to avoid false
positives from lockdep.

Warning example:
============================================
WARNING: possible recursive locking detected
5.17.0-dbg-DEV #15 Tainted: G           O
--------------------------------------------
sev_migrate_tes/18859 is trying to acquire lock:
ffff8d672d484238 (&vcpu->mutex){+.+.}-{3:3}, at: sev_lock_vcpus_for_migration+0x7e/0x150
but task is already holding lock:
ffff8d67703f81f8 (&vcpu->mutex){+.+.}-{3:3}, at: sev_lock_vcpus_for_migration+0x7e/0x150
other info that might help us debug this:
 Possible unsafe locking scenario:
       CPU0
       ----
  lock(&vcpu->mutex);
  lock(&vcpu->mutex);
 *** DEADLOCK ***
 May be due to missing lock nesting notation
3 locks held by sev_migrate_tes/18859:
 #0: ffff9302f91323b8 (&kvm->lock){+.+.}-{3:3}, at: sev_vm_move_enc_context_from+0x96/0x740
 #1: ffff9302f906a3b8 (&kvm->lock/1){+.+.}-{3:3}, at: sev_vm_move_enc_context_from+0xae/0x740
 #2: ffff8d67703f81f8 (&vcpu->mutex){+.+.}-{3:3}, at: sev_lock_vcpus_for_migration+0x7e/0x150

Fixes: b56639318bb2b ("KVM: SEV: Add support for SEV intra host migration")
Reported-by: John Sperbeck<jsperbeck@...gle.com>
Suggested-by: David Rientjes <rientjes@...gle.com>
Suggested-by: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org
Cc: linux-kernel@...r.kernel.org
Signed-off-by: Peter Gonda <pgonda@...gle.com>

---

V3
 * Updated signature to enum to self-document argument.
 * Updated comment as Seanjc@ suggested.

Tested by running sev_migrate_tests with lockdep enabled. Before we see
a warning from sev_lock_vcpus_for_migration(). After we get no warnings.

---
 arch/x86/kvm/svm/sev.c | 22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 75fa6dd268f0..f66550ec8eaf 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -1591,14 +1591,26 @@ static void sev_unlock_two_vms(struct kvm *dst_kvm, struct kvm *src_kvm)
 	atomic_set_release(&src_sev->migration_in_progress, 0);
 }
 
+/*
+ * To suppress lockdep false positives, subclass all vCPU mutex locks by
+ * assigning even numbers to the source vCPUs and odd numbers to destination
+ * vCPUs based on the vCPU's index.
+ */
+enum sev_migration_role {
+	SEV_MIGRATION_SOURCE = 0,
+	SEV_MIGRATION_TARGET,
+	SEV_NR_MIGRATION_ROLES,
+};
 
-static int sev_lock_vcpus_for_migration(struct kvm *kvm)
+static int sev_lock_vcpus_for_migration(struct kvm *kvm,
+					enum sev_migration_role role)
 {
 	struct kvm_vcpu *vcpu;
 	unsigned long i, j;
 
-	kvm_for_each_vcpu(i, vcpu, kvm) {
-		if (mutex_lock_killable(&vcpu->mutex))
+	kvm_for_each_vcpu(i, vcpu, kvm) {
+		if (mutex_lock_killable_nested(
+			    &vcpu->mutex, i * SEV_NR_MIGRATION_ROLES + role))
 			goto out_unlock;
 	}
 
@@ -1745,10 +1757,10 @@ int sev_vm_move_enc_context_from(struct kvm *kvm, unsigned int source_fd)
 		charged = true;
 	}
 
-	ret = sev_lock_vcpus_for_migration(kvm);
+	ret = sev_lock_vcpus_for_migration(kvm, SEV_MIGRATION_SOURCE);
 	if (ret)
 		goto out_dst_cgroup;
-	ret = sev_lock_vcpus_for_migration(source_kvm);
+	ret = sev_lock_vcpus_for_migration(source_kvm, SEV_MIGRATION_TARGET);
 	if (ret)
 		goto out_dst_vcpu;
 
-- 
2.35.1.1178.g4f1659d476-goog