linux-kernel - Re: [PATCH] KVM: Add separate helper for putting borrowed reference to kvm

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Message-ID: <a924cb9c-8354-23fe-1052-8ad564edad7f@redhat.com>
Date:   Thu, 28 Nov 2019 14:49:42 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Leonardo Bras <leonardo@...ux.ibm.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Paul Mackerras <paulus@...abs.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm-ppc@...r.kernel.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: Add separate helper for putting borrowed reference
 to kvm

On 27/11/19 22:57, Leonardo Bras wrote:
> But on the above case, kvm_put_kvm{,_no_destroy}() would be called
> with refcount == 1, and if reorder patch is applied, it would not cause
> any use-after-free error, even on kvm_put_kvm() case.

I think this is what you're missing: kvm_put_kvm_no_destroy() does not
protect against bugs in the code that uses it.  It protect against bugs
_elsewhere_.

Therefore, kvm_put_kvm_no_destroy() is always a better choice when
applicable, because it turns bugs in _other parts of the code_ from
use-after-free to WARN+leak.

Paolo



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