linux-kernel - Re: [PATCH] KVM: SEV: Clear the pages pointer in sev_unpin

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Date:   Mon, 27 Jun 2022 18:10:37 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Peter Gonda <pgonda@...gle.com>
Cc:     kvm@...r.kernel.org, Greg Thelen <gthelen@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: SEV: Clear the pages pointer in sev_unpin_memory

On Mon, Jun 27, 2022, Peter Gonda wrote:
> Clear to the @pages array pointer in sev_unpin_memory to avoid leaving a
> dangling pointer to invalid memory.
> 
> Signed-off-by: Peter Gonda <pgonda@...gle.com>
> Cc: Greg Thelen <gthelen@...gle.com>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Sean Christopherson <seanjc@...gle.com>
> Cc: Tom Lendacky <thomas.lendacky@....com>
> Cc: kvm@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> ---
>  arch/x86/kvm/svm/sev.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 309bcdb2f929..485ad86c01c6 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -452,6 +452,7 @@ static void sev_unpin_memory(struct kvm *kvm, struct page **pages,
>  	unpin_user_pages(pages, npages);
>  	kvfree(pages);
>  	sev->pages_locked -= npages;
> +	*pages = NULL;

Would this have helped detect a real bug?  I generally like cleaning up, but this
leaves things in a somewhat inconsistent state, e.g. when unpinning a kvm_enc_region,
pages will be NULL but npages will be non-zero.  It's somewhat moot because the
region is immediately freed in that case, but that begs the question of what real
benefit this provides.  sev_dbg_crypt() is the only flow where there's much danger
of a use-after-free.

>  }
>  
>  static void sev_clflush_pages(struct page *pages[], unsigned long npages)
> -- 
> 2.37.0.rc0.161.g10f37bed90-goog
>