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 <t...@linutronix.de>
> Cc: Ingo Molnar <mi...@redhat.com>
> Cc: "H. Peter Anvin" <h...@zytor.com>
> Cc: Paolo Bonzini <pbonz...@redhat.com>
> Cc: Joerg Roedel <j...@8bytes.org>
> Cc: Tom Lendacky <thomas.lenda...@amd.com>
> Cc: Brijesh Singh <brijesh.si...@amd.com>
> Cc: Sean Christopherson <sea...@google.com>
> Cc: x...@kernel.org
> Cc: k...@vger.kernel.org
> Cc: sta...@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Fixes: 1e80fdc09d12 ("KVM: SVM: Pin guest memory when SEV is active")
> Signed-off-by: Peter Gonda <pgo...@google.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-pgo...@google.com/
> 
> Message-Id: <20210127161524.2832400-1-pgo...@google.com>
> Signed-off-by: Paolo Bonzini <pbonz...@redhat.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);

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:
>