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

[Paolo Bonzini <pbonzini@xxxxxxxxxx>]

On 17/02/21 10:18, Dov Murik wrote:
> 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?

Ouch.  No it isn't and it's an insta-deadlock.  Let me send a fix.

Paolo

> -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: