linux-kernel - Re: [PATCH for 4.19] Fix unsynchronized access to sev members through svm_register_enc

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Message-ID: <2654c1ab-0178-e980-8573-3893e0bbcd60@redhat.com>
Date:   Mon, 8 Feb 2021 17:53:56 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Gonda <pgonda@...gle.com>, stable@...r.kernel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Joerg Roedel <joro@...tes.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Brijesh Singh <brijesh.singh@....com>,
        Sean Christopherson <seanjc@...gle.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH for 4.19] Fix unsynchronized access to sev members through
 svm_register_enc_region

On 08/02/21 17:48, Peter Gonda wrote:
> commit 19a23da53932bc8011220bd8c410cb76012de004 upstream.
> 
> Grab kvm->lock before pinning memory when registering an encrypted
> region; sev_pin_memory() relies on kvm->lock being held to ensure
> correctness when checking and updating the number of pinned pages.
> 
> Add a lockdep assertion to help prevent future regressions.
> 
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Joerg Roedel <joro@...tes.org>
> Cc: Tom Lendacky <thomas.lendacky@....com>
> Cc: Brijesh Singh <brijesh.singh@....com>
> Cc: Sean Christopherson <seanjc@...gle.com>
> Cc: x86@...nel.org
> Cc: kvm@...r.kernel.org
> Cc: stable@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Fixes: 1e80fdc09d12 ("KVM: SVM: Pin guest memory when SEV is active")
> Signed-off-by: Peter Gonda <pgonda@...gle.com>
> 
> V2
>   - Fix up patch description
>   - Correct file paths svm.c -> sev.c
>   - Add unlock of kvm->lock on sev_pin_memory error
> 
> V1
>   - https://lore.kernel.org/kvm/20210126185431.1824530-1-pgonda@google.com/
> 
> Message-Id: <20210127161524.2832400-1-pgonda@...gle.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>   arch/x86/kvm/svm.c | 18 +++++++++++-------
>   1 file changed, 11 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 2b506904be02..93c89f1ffc5d 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -1830,6 +1830,8 @@ static struct page **sev_pin_memory(struct kvm *kvm, unsigned long uaddr,
>   	struct page **pages;
>   	unsigned long first, last;
>   
> +	lockdep_assert_held(&kvm->lock);
> +
>   	if (ulen == 0 || uaddr + ulen < uaddr)
>   		return NULL;
>   
> @@ -7086,12 +7088,21 @@ static int svm_register_enc_region(struct kvm *kvm,
>   	if (!region)
>   		return -ENOMEM;
>   
> +	mutex_lock(&kvm->lock);
>   	region->pages = sev_pin_memory(kvm, range->addr, range->size, &region->npages, 1);
>   	if (!region->pages) {
>   		ret = -ENOMEM;
> +		mutex_unlock(&kvm->lock);
>   		goto e_free;
>   	}
>   
> +	region->uaddr = range->addr;
> +	region->size = range->size;
> +
> +	mutex_lock(&kvm->lock);
> +	list_add_tail(&region->list, &sev->regions_list);
> +	mutex_unlock(&kvm->lock);
> +
>   	/*
>   	 * The guest may change the memory encryption attribute from C=0 -> C=1
>   	 * or vice versa for this memory range. Lets make sure caches are
> @@ -7100,13 +7111,6 @@ static int svm_register_enc_region(struct kvm *kvm,
>   	 */
>   	sev_clflush_pages(region->pages, region->npages);
>   
> -	region->uaddr = range->addr;
> -	region->size = range->size;
> -
> -	mutex_lock(&kvm->lock);
> -	list_add_tail(&region->list, &sev->regions_list);
> -	mutex_unlock(&kvm->lock);
> -
>   	return ret;
>   
>   e_free:
> 

Acked-by: Paolo Bonzini <pbonzini@...hat.com>