linux-kernel - Re: [PATCH] x86/sgx: Free backing memory after faulting the enclave page

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Date:   Sun, 7 Nov 2021 11:58:21 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Jarkko Sakkinen <jarkko@...nel.org>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Jethro Beekman <jethro@...tanix.com>,
        Sean Christopherson <seanjc@...gle.com>,
        reinette.chatre@...el.com, tony.luck@...el.com,
        nathaniel@...fian.com, stable@...r.kernel.org,
        Borislav Petkov <bp@...e.de>, linux-sgx@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/sgx: Free backing memory after faulting the enclave
 page

On 11/7/21 11:45 AM, Jarkko Sakkinen wrote:
> On Sun, Nov 07, 2021 at 11:06:01AM -0800, Dave Hansen wrote:
>> That's true, the page cache should all be torn down along with the
>> fput().  *But*, it would be a very nice property if the backing storage
>> was empty by this point.  It essentially ensures that no enclave-runtime
>> cases missed truncating the backing storage away.
> 
> What if an enclave is released a point when all of its pages
> are swapped out? Or even simpler case would an enclave that is
> larger than all of EPC.

In this loop:

void sgx_encl_release(struct kref *ref)
{
	...
        xa_for_each(&encl->page_array, index, entry) {
		if (entry->epc_page == NULL)
			// truncate backing storage

> What can be made sure is that for all pages, which are in EPC,
> the backing page is truncated.

Right, and that should be utterly trivial to do.