Re: [PATCH 1/2] x86: sgx_vepc: extract sgx_vepc_remove_page

Paolo Bonzini
2021-09-13T14:26:43Z

On 13/09/21 16:05, Dave Hansen wrote:
> On 9/13/21 6:11 AM, Paolo Bonzini wrote:
>> Windows expects all pages to be in uninitialized state on startup.
>> In order to implement this, we will need a ioctl that performs
>> EREMOVE on all pages mapped by a /dev/sgx_vepc file descriptor:
>> other possibilities, such as closing and reopening the device,
>> are racy.
>
> Hi Paolo,
>
> How does this end up happening in the first place?
>
> All enclave pages should start out on 'sgx_dirty_page_list' and
> ksgxd sanitizes them with EREMOVE before making them available. That
> should cover EREMOVE after reboots while SGX pages are initialized,
> including kexec().

By "Windows startup" I mean even after guest reboot. Because another
process could sneak in and steal your EPC pages between a close() and an
open(), I'd like to have a way to EREMOVE the pages while keeping them
assigned to the specific vEPC instance, i.e. *without* going through
sgx_vepc_free_page().

Thanks,

Paolo

> sgx_vepc_free_page() should do the same for pages that a guest not not
> clean up properly.
>
> sgx_encl_free_epc_page() does an EREMOVE after a normal enclave has used
> a page.
>
> Those are the only three cases that I can think of. So, it sounds like
> one of those is buggy, or there's another unexpected path out there.
> Ultimately, I think it would be really handy if we could do this EREMOVE
> implicitly and without any new ABI.
>