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Reply-To: Sean Christopherson <seanjc@google.com>
Date:   Mon, 12 Apr 2021 15:20:48 -0700
In-Reply-To: <20210412222050.876100-1-seanjc@google.com>
Message-Id: <20210412222050.876100-2-seanjc@google.com>
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References: <20210412222050.876100-1-seanjc@google.com>
Subject: [PATCH 1/3] KVM: Destroy I/O bus devices on unregister failure
 _after_ sync'ing SRCU
From:   Sean Christopherson <seanjc@google.com>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        Sean Christopherson <seanjc@google.com>,
        Hao Sun <sunhao.th@gmail.com>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

If allocating a new instance of an I/O bus fails when unregistering a
device, wait to destroy the device until after all readers are guaranteed
to see the new null bus.  Destroying devices before the bus is nullified
could lead to use-after-free since readers expect the devices on their
reference of the bus to remain valid.

Fixes: f65886606c2d ("KVM: fix memory leak in kvm_io_bus_unregister_dev()")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 virt/kvm/kvm_main.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 383df23514b9..d6e2b570e430 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4511,7 +4511,13 @@ void kvm_io_bus_unregister_dev(struct kvm *kvm, enum kvm_bus bus_idx,
 		new_bus->dev_count--;
 		memcpy(new_bus->range + i, bus->range + i + 1,
 				flex_array_size(new_bus, range, new_bus->dev_count - i));
-	} else {
+	}
+
+	rcu_assign_pointer(kvm->buses[bus_idx], new_bus);
+	synchronize_srcu_expedited(&kvm->srcu);
+
+	/* Destroy the old bus _after_ installing the (null) bus. */
+	if (!new_bus) {
 		pr_err("kvm: failed to shrink bus, removing it completely\n");
 		for (j = 0; j < bus->dev_count; j++) {
 			if (j == i)
@@ -4520,8 +4526,6 @@ void kvm_io_bus_unregister_dev(struct kvm *kvm, enum kvm_bus bus_idx,
 		}
 	}
 
-	rcu_assign_pointer(kvm->buses[bus_idx], new_bus);
-	synchronize_srcu_expedited(&kvm->srcu);
 	kfree(bus);
 	return;
 }
-- 
2.31.1.295.g9ea45b61b8-goog