linux-kernel - Re: [PATCH 1/2] x86: sgx_vepc: extract sgx_vepc_remove

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Message-ID: <8105a379-195e-8c9b-5e06-f981f254707f@redhat.com>
Date:   Mon, 13 Sep 2021 16:24:39 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Dave Hansen <dave.hansen@...el.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     x86@...nel.org, linux-sgx@...r.kernel.org, jarkko@...nel.org,
        dave.hansen@...ux.intel.com, yang.zhong@...el.com
Subject: Re: [PATCH 1/2] x86: sgx_vepc: extract sgx_vepc_remove_page

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.
>