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

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Date:   Sun, 7 Nov 2021 20:06:42 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
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 Thu, Nov 04, 2021 at 03:38:55PM -0700, Dave Hansen wrote:
> On 11/3/21 4:22 PM, Jarkko Sakkinen wrote:
> > --- a/arch/x86/kernel/cpu/sgx/encl.c
> > +++ b/arch/x86/kernel/cpu/sgx/encl.c
> > @@ -22,6 +22,7 @@ static int __sgx_encl_eldu(struct sgx_encl_page *encl_page,
> >  {
> >  	unsigned long va_offset = encl_page->desc & SGX_ENCL_PAGE_VA_OFFSET_MASK;
> >  	struct sgx_encl *encl = encl_page->encl;
> > +	struct inode *inode = file_inode(encl->backing);
> >  	struct sgx_pageinfo pginfo;
> >  	struct sgx_backing b;
> >  	pgoff_t page_index;
> > @@ -60,6 +61,9 @@ static int __sgx_encl_eldu(struct sgx_encl_page *encl_page,
> >  
> >  	sgx_encl_put_backing(&b, false);
> >  
> > +	/* Free the backing memory. */
> > +	shmem_truncate_range(inode, PFN_PHYS(page_index), PFN_PHYS(page_index) + PAGE_SIZE - 1);
> > +
> >  	return ret;
> >  }
> 
> This also misses tearing down the backing storage if it is in place at
> sgx_encl_release().

Hmm... sgx_encl_release() does fput(). Isn't that enough to tear it down,
or does it require explicit truncate, i.e. something like

        shmem_truncate_range(file_inode(encl->backing), encl->base, encl->size - 1);


> Does a entry->epc_page==NULL page in there guarantee that it has backing
> storage?

Yes, it is an invariant. That what I was thinking to use for PCMD: iterate
32 pages and check if they have a faulted page.

/Jarkko