Re: [RFC PATCH 2/4] x86/sgx: Set dirty bit after modifying page contents

[Reinette Chatre <reinette.chatre@intel.com>]

Hi Dave,

On 4/28/2022 2:40 PM, Dave Hansen wrote:
> On 4/28/22 13:11, Reinette Chatre wrote:
>>
>> Since the PCMD page in the backing store is modified the page
>> should be set as dirty when releasing the reference to
>> ensure the modified data is retained.
>>
>> Fixes: 08999b2489b4 ("x86/sgx: Free backing memory after faulting the enclave page")
>> Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
>> ---
>>  arch/x86/kernel/cpu/sgx/encl.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
>> index e5d2661800ac..e03f124ce772 100644
>> --- a/arch/x86/kernel/cpu/sgx/encl.c
>> +++ b/arch/x86/kernel/cpu/sgx/encl.c
>> @@ -98,7 +98,7 @@ static int __sgx_encl_eldu(struct sgx_encl_page *encl_page,
>>  	kunmap_atomic(pcmd_page);
>>  	kunmap_atomic((void *)(unsigned long)pginfo.contents);
>>  
>> -	sgx_encl_put_backing(&b, false);
>> +	sgx_encl_put_backing(&b, true);
> 
> I think you're on the right track here.  The concept is right.  The
> memset() wrote fresh data into the b.pcmd page.  But, if it were clean
> swap cache, it can be discarded again and the memset() might be lost.
> 
> I *think* all that would do in the end is leave us with a PCMD page that
> will never be truncated because the page has non-zero PCMD data that
> will never be used, the result of the thrown-away memset().

Thank you very much for the analysis. This explains why this change
did not impact the issue I am chasing.

> 
> But, I'd rather this be done more directly and closer to the actual
> dirtying of the page.  Perhaps:
> 
> +	set_page_dirty(b.pcmd);
>         memset(pcmd_page + b.pcmd_offset, 0, sizeof(struct sgx_pcmd));
> 
> 

Will do (with comments to explain why this line was extracted from
the subsequent sgx_encl_put_backing() call). 


> I don't think the "b.contents" page needs the same treatment because its
> contents are always discarded in this path while some of the PCMD page's
> contents need to be preserved.

That's right. The consistent symmetrical API of get()/put() was appealing
to me.

Thank you very much.

Reinette