Re: [PATCH for 5.4] Fix unsynchronized access to sev members through svm_register_enc_region

[Dov Murik]

Hi Peter,

On 08/02/2021 18:48, 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@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Joerg Roedel <joro@xxxxxxxxxx>
> Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
> Cc: Brijesh Singh <brijesh.singh@xxxxxxx>
> Cc: Sean Christopherson <seanjc@xxxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: kvm@xxxxxxxxxxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Fixes: 1e80fdc09d12 ("KVM: SVM: Pin guest memory when SEV is active")
> Signed-off-by: Peter Gonda <pgonda@xxxxxxxxxx>
> 
> 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@xxxxxxxxxx/
> 
> Message-Id: <20210127161524.2832400-1-pgonda@xxxxxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
>  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);

This extra mutex_lock call doesn't appear in the upstream patch (committed 
as 19a23da5393), but does appear in the 5.4 and 4.19 backports.  Is it
needed here?

-Dov


> +	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:
>