linux-kernel - re: KVM: Introduce a 'release' method for KVM devices

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Message-ID: <5a34a77e-d1bf-c630-ef9b-4f94c2c0c221@canonical.com>
Date:   Wed, 1 May 2019 15:42:53 +0100
From:   Colin Ian King <colin.king@...onical.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org,
        Cédric Le Goater <clg@...d.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: re: KVM: Introduce a 'release' method for KVM devices

Hi,

Static analysis with Coverity picked up an issue in the following commit:

commit 2bde9b3ec8bdf60788e9e2ce8c07a2f8d6003dbd
Author: Cédric Le Goater <clg@...d.org>
Date:   Thu Apr 18 12:39:41 2019 +0200

    KVM: Introduce a 'release' method for KVM devices

        struct kvm *kvm = dev->kvm;

+       if (!dev)
+               return -ENODEV;

If dev is null then the dereference of dev->kvm when assigning pointer
kvm will cause an null pointer dereference.  This is easily fixed by
assigning kvm after the dev null check.

+
+       if (dev->kvm != kvm)
+               return -EPERM;

I don't understand the logic of the above check. kvm is the same
dev->kvm on the earlier assignment, so dev->kvm != kvm seems to be
always false, so this check seems to be redundant. Am I missing
something more fundamental here?

Colin