Re: [PATCH] x86/sgx: Return VM_FAULT_SIGBUS for EPC exhaustion

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

On 10/25/2023 4:58 PM, Huang, Kai wrote:
> On Wed, 2023-10-25 at 07:31 -0700, Hansen, Dave wrote:
>> On 10/19/23 19:53, Haitao Huang wrote:
>>> In the EAUG on page fault path, VM_FAULT_OOM is returned when the
>>> Enclave Page Cache (EPC) runs out. This may trigger unneeded OOM kill
>>> that will not free any EPCs. Return VM_FAULT_SIGBUS instead.

This commit message does not seem accurate to me. From what I can tell
VM_FAULT_SIGBUS is indeed returned when EPC runs out. What is addressed
with this patch is the error returned when kernel (not EPC) memory runs
out.

>> So, when picking an error code and we look the documentation for the
>> bits, we see:
>>
>>>  * @VM_FAULT_OOM:               Out Of Memory
>>>  * @VM_FAULT_SIGBUS:            Bad access
>>
>> So if anything we'll need a bit more changelog where you explain how
>> running out of enclave memory is more "Bad access" than "Out Of Memory".
>>  Because on the surface this patch looks wrong.
>>
>> But that's just a naming thing.  What *behavior* is bad here?  With the
>> old code, what happens?  With the new code, what happens?  Why is the
>> old better than the new?
> 
> I think Haitao meant if we return OOM, the core-MM fault handler will believe
> the fault couldn't be handled because of running out of memory, and then it
> could invoke the OOM killer which might select an unrelated victim who might
> have no EPC at all.

Since the issue is that system is out of kernel memory the resolution may need to
look further than owners with EPC memory.

...

> 
> (Also, currently the non-EAUG code path (ELDU) in sgx_vma_fault() also returns
> SIGBUS if it fails to allocate EPC, so making EAUG code path return SIGBUS also
> matches the ELDU path.)
> 

These errors all seem related to EPC memory to me, not kernel memory.

Reinette