Re: [PATCH v2 2/2] x86: sgx_vepc: implement SGX_IOC_VEPC_REMOVE ioctl

Paolo Bonzini
2021-10-16T07:15:03Z

On 16/10/21 00:29, Sean Christopherson wrote:
> On Tue, Oct 12, 2021, Paolo Bonzini wrote:
>> diff --git a/arch/x86/kernel/cpu/sgx/virt.c b/arch/x86/kernel/cpu/sgx/virt.c
>> index 59cdf3f742ac..81a0a0f22007 100644
>> --- a/arch/x86/kernel/cpu/sgx/virt.c
>> +++ b/arch/x86/kernel/cpu/sgx/virt.c
>> @@ -150,6 +150,46 @@ static int sgx_vepc_free_page(struct sgx_epc_page *epc_page)
>> return 0;
>> }
>>
>> +static long sgx_vepc_remove_all(struct sgx_vepc *vepc)
>> +{
>> + struct sgx_epc_page *entry;
>> + unsigned long index;
>> + long failures = 0;
>> +
>> + xa_for_each(&vepc->page_array, index, entry) {
>
> Might be worth a comment that xa_for_each() is safe to use concurrently with
> xa_load/xa_store, i.e. this doesn't need to take vepc->lock.

I considered that to be part of the xarray contract (xa_store uses
rcu_assign_pointer so it has release semantics, and vepc->page_array is
essentially "store once").

> It does raise the
> question of whether or not the kernel is responsible for providing deterministic
> results if userspace/guest is accessing previously-unallocated pages.

Garbage in, garbage out -- but you're right below that garbage in, WARN
out is not acceptable. I'm sending a v3 with documentation changes too.

Paolo

>> + int ret = sgx_vepc_remove_page(entry);
>
> I don't see anything that prevents userspace from doing SGX_IOC_VEPC_REMOVE_ALL
> on multiple threads with the same vEPC. That means userspace can induce a #GP
> due to concurrent access. Taking vepc->lock would solve that particular problem,
> but I think that's a moot point because the EREMOVE locking rules are relative to
> the SECS, not the individual page (because of refcounting). SGX_IOC_VEPC_REMOVE_ALL
> on any two arbitrary vEPCs could induce a fault if they have children belonging to
> the same enclave, i.e. share an SECS.
>
> Sadly, I think this needs to be:
>
> if (ret == SGX_CHILD_PRESENT)
> failures++;
> else if (ret)
> return -EBUSY;
>
>> + switch (ret) {
>> + case 0:
>> + break;
>> +
>> + case SGX_CHILD_PRESENT:
>> + failures++;
>> + break;
>> +
>> + case SGX_ENCLAVE_ACT:
>> + /*
>> + * Unlike in sgx_vepc_free_page, userspace could be calling
>> + * the ioctl while logical processors are running in the
>> + * enclave; do not warn.
>> + */
>> + return -EBUSY;
>> +
>> + default:
>> + WARN_ONCE(1, EREMOVE_ERROR_MESSAGE, ret, ret);
>> + failures++;
>> + break;
>> + }
>> + cond_resched();
>> + }
>> +
>> + /*
>> + * Return the number of pages that failed to be removed, so
>> + * userspace knows that there are still SECS pages lying
>> + * around.
>> + */
>> + return failures;
>> +}
>