Re: [PATCH] x86/sgx: Add poison handling to reclaimer

[Reinette Chatre]

Hi Dave,

On 1/19/2022 11:51 AM, Dave Hansen wrote:
> On 1/18/22 3:05 PM, Reinette Chatre wrote:
>> The machine check recovery handling in SGX added the changes
>> listed below to the freeing of pages in sgx_free_epc_page().
>> The SGX reclaimer contains an open coded version of
>> sgx_free_epc_page() and thus did not obtain the changes in
>> support of poison handling.
> 
> I was trying to decide if this is an urgent fix or not.  A more crisp
> problem statement might have helped in the changelog.
> 
> But, from what I can tell, the most probable troublesome scenario here
> would be something like:
> 
>  1. Machine check (#MC) occurs (asynchronous, !MF_ACTION_REQUIRED)
>  2. arch_memory_failure() called is eventually
>  3. (SGX) page->poison set to 1
>  4. Page is reclaimed
>  5. Page added to normal free lists by sgx_reclaim_pages()
>     ^ This is the bug
>  6. Page is reallocated by some innocent enclave, a second (synchronous)
>     in-kernel #MC is induced, probably during EADD instruction.
>     ^ This is the fallout from the bug
> 
> #6 is unfortunate and can be avoided if this patch is applied.
> 
> Basically, this patch ensures that a bad enclave page is isolated
> quickly and causes a minimal amount of collateral damage.  Is this a
> valid summary?
> 
> 	The SGX reclaimer code lacks page poison handling in its free
> 	path.  This can lead to completely avoidable machine checks if a
> 	poisoned page is freed and reallocated instead of being
> 	isolated.

As I understand this code it does look like a valid summary to me. One
detail is that the poison page handling is currently done for SECS pages
when they are freed by the reclaimer (via sgx_reclaimer_write()).

Thank you very much for the detailed analysis. Should I resend with
an improved commit message that contains your scenario description
and summary?

Reinette