From: Sean Christopherson <seanjc@google.com>
Date: Mon, 12 Apr 2021 15:20:48 -0700
Subject: KVM: Destroy I/O bus devices on unregister failure _after_ sync'ing
 SRCU
Git-commit: 2ee3757424be7c1cd1d0bbfa6db29a7edd82a250
Patch-mainline: v5.13-rc1
References: CVE-2020-36312 bsc#1184509

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>
Message-Id: <20210412222050.876100-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Joerg Roedel <jroedel@suse.de>
---
 virt/kvm/kvm_main.c |   10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3731,7 +3731,13 @@ void kvm_io_bus_unregister_dev(struct kv
 		new_bus->dev_count--;
 		memcpy(new_bus->range + i, bus->range + i + 1,
 		       (new_bus->dev_count - i) * sizeof(struct kvm_io_range));
-	} 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)
@@ -3740,8 +3746,6 @@ void kvm_io_bus_unregister_dev(struct kv
 		}
 	}
 
-	rcu_assign_pointer(kvm->buses[bus_idx], new_bus);
-	synchronize_srcu_expedited(&kvm->srcu);
 	kfree(bus);
 	return;
 }