linux-kernel - Re: [PATCH v1 1/1] KVM: add missing kvm_put

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Date:   Tue, 27 Jun 2017 14:34:43 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Claudio Imbrenda <imbrenda@...ux.vnet.ibm.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        borntraeger@...ibm.com, frankja@...ux.vnet.ibm.com,
        pbonzini@...hat.com
Subject: Re: [PATCH v1 1/1] KVM: add missing kvm_put_kvm in case of failure

On Tue, Jun 27, 2017 at 01:57:16PM +0200, Claudio Imbrenda wrote:
> If I'm not missing anything, in case kvm_create_vm_debugfs fails, we
> will have a memory leak due to not freeing the kvm object.
> 
> A call to kvm_put_kvm was accidentally removed from an error handling in
> commit 506cfba9e726 ("KVM: don't use anon_inode_getfd() before possible failures")
> 
> This patch simply restores the call to kvm_put_kvm, so that the kvm
> object is destroyed before returning an error.
> 
> Signed-off-by: Claudio Imbrenda <imbrenda@...ux.vnet.ibm.com>
> Fixes: 506cfba9e726 ("KVM: don't use anon_inode_getfd() before possible failures")

What you are missing is that by that point we have file->f_op already
set, with ->release() being kvm_vm_release().  It will be called by
final fput(), and that's what will do your "missing" kvm_put_kvm().

IOW, NAK - removal was not accidental and you've just introduced
double-put there.