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

[Paolo Bonzini <pbonzini@redhat.com>]

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@linutronix.de>
>> Cc: Ingo Molnar <mingo@redhat.com>
>> Cc: "H. Peter Anvin" <hpa@zytor.com>
>> Cc: Paolo Bonzini <pbonzini@redhat.com>
>> Cc: Joerg Roedel <joro@8bytes.org>
>> Cc: Tom Lendacky <thomas.lendacky@amd.com>
>> Cc: Brijesh Singh <brijesh.singh@amd.com>
>> Cc: Sean Christopherson <seanjc@google.com>
>> Cc: x86@kernel.org
>> Cc: kvm@vger.kernel.org
>> Cc: stable@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 <pgonda@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-pgonda@google.com/
>>
>> Message-Id: <20210127161524.2832400-1-pgonda@google.com>
>> Signed-off-by: Paolo Bonzini <pbonzini@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?

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:
>>
>