Re: [PATCH v2 1/3] KVM: Don't create VM debugfs files outside of the VM directory

From: Oliver Upton <oupton@google.com>
To: Marc Zyngier <maz@kernel.org>
Cc: kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org,
	James Morse <james.morse@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	linux-arm-kernel@lists.infradead.org,
	Peter Shier <pshier@google.com>,
	Ricardo Koller <ricarkol@google.com>,
	Reiji Watanabe <reijiw@google.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Sean Christopherson <seanjc@google.com>,
	stable@kernel.org
Subject: Re: [PATCH v2 1/3] KVM: Don't create VM debugfs files outside of the VM directory
Date: Wed, 6 Apr 2022 17:59:02 +0000	[thread overview]
Message-ID: <Yk3U5tfqBQBOeSs+@google.com> (raw)
In-Reply-To: <87fsmqvgaf.wl-maz@kernel.org>

On Wed, Apr 06, 2022 at 08:10:00AM +0100, Marc Zyngier wrote:
> Hi Oliver,
> 
> On Mon, 04 Apr 2022 19:21:17 +0100,
> Oliver Upton <oupton@google.com> wrote:
> > 
> > Unfortunately, there is no guarantee that KVM was able to instantiate a
> > debugfs directory for a particular VM. To that end, KVM shouldn't even
> > attempt to create new debugfs files in this case. If the specified
> > parent dentry is NULL, debugfs_create_file() will instantiate files at
> > the root of debugfs.
> > 
> > For arm64, it is possible to create the vgic-state file outside of a
> > VM directory, the file is not cleaned up when a VM is destroyed.
> > Nonetheless, the corresponding struct kvm is freed when the VM is
> > destroyed.
> > 
> > Nip the problem in the bud for all possible errant debugfs file
> > creations by initializing kvm->debugfs_dentry to -ENOENT. In so doing,
> > debugfs_create_file() will fail instead of creating the file in the root
> > directory.
> > 
> > Cc: stable@kernel.org
> > Fixes: 929f45e32499 ("kvm: no need to check return value of debugfs_create functions")
> > Signed-off-by: Oliver Upton <oupton@google.com>
> > ---
> >  virt/kvm/kvm_main.c | 10 ++++++++--
> >  1 file changed, 8 insertions(+), 2 deletions(-)
> > 
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 70e05af5ebea..04a426e65cb8 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -932,7 +932,7 @@ static void kvm_destroy_vm_debugfs(struct kvm *kvm)
> >  	int kvm_debugfs_num_entries = kvm_vm_stats_header.num_desc +
> >  				      kvm_vcpu_stats_header.num_desc;
> >  
> > -	if (!kvm->debugfs_dentry)
> > +	if (!IS_ERR(kvm->debugfs_dentry))
> >  		return;
> 
> Shouldn't this condition be inverted? It certainly looks odd.

Err... Yep, this is plain wrong. Let me fix this obvious mistake.

--
Thanks,
Oliver

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