On 2022/10/24 04:39, jarkko@xxxxxxxxxx wrote:
>> As you can see if the EPC page has already been populated at a given index of
>> one virtual EPC instance, the current fault handler just assumes the mapping is
>> already there and returns success immediately. This causes a bug when one
>> virtual EPC instance is shared by multi processes via fork(): if the EPC page at
>> one index is already populated by the parent process, when the child accesses
>> the same page using different virtual address, the fault handler just returns
>> success w/o actually setting up the mapping for the child, resulting in endless
>> page fault.
>>
>> This needs to be fixed in no matter what way.
> I think you mean that vm_insert_pfn() does not happen for child because
> of early return? I did not understand the part about "different virtual
> addresses", as it is the same mapping.
>
If userspace do something like this, the child will get "different
virtual address":
... parent run enclave within VM ...
if (fork() == 0) {
int *caddr = mmap(NULL, 4096, PROT_READ, MAP_SHARED, vepc_fd, 0);
printf("child: %d\n", caddr[0]);
}
- "vepc_fd" is inherited from parent which had opened /dev/sgx_vepc.
- mmap() will create a VMA in child with "different virtual addresses".
- "caddr[0]" will cause a page fault as it's a new mapping.
1. Then kernel will run into the code snippet referenced by Kai.
2. The early return 0 will result in sgx_vepc_fault() return
"VM_FAULT_NOPAGE".
3. This return value will make the condition as true at
function do_user_addr_fault()
if (likely(!(fault & VM_FAULT_ERROR)))
return;
4. Since this page fault has not been handled and "$RIP" is still the
original value, it will result in the same page fault again. Namely,
it's an endless page fault.
But the problem is neither the early return in __sgx_vepc_fault() nor
the return of VM_FAULT_NOPAGE at sgx_vepc_fault(). The root cause has
been highlighted by Kai, one virtual EPC instance
can only be mmap()-ed by the process which opens /dev/sgx_vepc.
In fact, to share a virtual EPC instance in userspace doesn't make any
sense. Even though it can be shared by child, the virtual EPC page
cannot be used by child correctly.
Best Regards,
Zhiquan
