Re: [PATCH] x86/sgx: Free backing memory after faulting the enclave page

[Dave Hansen]

On 11/7/21 10:06 AM, Jarkko Sakkinen wrote:
> On Thu, Nov 04, 2021 at 03:38:55PM -0700, Dave Hansen wrote:
>> On 11/3/21 4:22 PM, Jarkko Sakkinen wrote:
>>> --- a/arch/x86/kernel/cpu/sgx/encl.c
>>> +++ b/arch/x86/kernel/cpu/sgx/encl.c
>>> @@ -22,6 +22,7 @@ static int __sgx_encl_eldu(struct sgx_encl_page *encl_page,
>>>  {
>>>  	unsigned long va_offset = encl_page->desc & SGX_ENCL_PAGE_VA_OFFSET_MASK;
>>>  	struct sgx_encl *encl = encl_page->encl;
>>> +	struct inode *inode = file_inode(encl->backing);
>>>  	struct sgx_pageinfo pginfo;
>>>  	struct sgx_backing b;
>>>  	pgoff_t page_index;
>>> @@ -60,6 +61,9 @@ static int __sgx_encl_eldu(struct sgx_encl_page *encl_page,
>>>  
>>>  	sgx_encl_put_backing(&b, false);
>>>  
>>> +	/* Free the backing memory. */
>>> +	shmem_truncate_range(inode, PFN_PHYS(page_index), PFN_PHYS(page_index) + PAGE_SIZE - 1);
>>> +
>>>  	return ret;
>>>  }
>>
>> This also misses tearing down the backing storage if it is in place at
>> sgx_encl_release().
> 
> Hmm... sgx_encl_release() does fput(). Isn't that enough to tear it down,
> or does it require explicit truncate, i.e. something like
> 
>         shmem_truncate_range(file_inode(encl->backing), encl->base, encl->size - 1);

That's true, the page cache should all be torn down along with the
fput().  *But*, it would be a very nice property if the backing storage
was empty by this point.  It essentially ensures that no enclave-runtime
cases missed truncating the backing storage away.

>> Does a entry->epc_page==NULL page in there guarantee that it has backing
>> storage?
> 
> Yes, it is an invariant. That what I was thinking to use for PCMD: iterate
> 32 pages and check if they have a faulted page.

I think the rule should be that entry->epc_page==NULL enclave pages have
backing storage.  All entry->epc_page!=NULL do *not* have backing storage.