Re: [PATCH v38 16/24] x86/sgx: Add a page reclaimer

Borislav Petkov
2020-09-22T10:45:49Z

On Tue, Sep 15, 2020 at 02:28:34PM +0300, Jarkko Sakkinen wrote:
> There is a limited amount of EPC available. Therefore, some of it must be
> copied to the regular memory, and only subset kept in the SGX reserved
> memory. While kernel cannot directly access enclave memory, SGX provides a
> set of ENCLS leaf functions to perform reclaiming.
>
> This commits implements a page reclaimer by using these leaf functions. It

s/This commits implements/Implement/

> picks the victim pages in LRU fashion from all the enclaves running in the
> system. The thread ksgxswapd reclaims pages on the event when the number of
> free EPC pages goes below SGX_NR_LOW_PAGES up until it reaches
> SGX_NR_HIGH_PAGES.

...

Had to grep the SDM for that "eldu" magic. It could use a comment:

/*
* ELDU: Load an EPC page as unblocked. For more info, see "OS
* Management of EPC Pages" in the SDM.
*/

or so, to denote what's going on here.

> +static struct sgx_epc_page *sgx_encl_eldu(struct sgx_encl_page *encl_page,
> + struct sgx_epc_page *secs_page)
> +{
> + unsigned long va_offset = SGX_ENCL_PAGE_VA_OFFSET(encl_page);
> + struct sgx_encl *encl = encl_page->encl;
> + struct sgx_epc_page *epc_page;
> + int ret;
> +
> + epc_page = sgx_alloc_epc_page(encl_page, false);
> + if (IS_ERR(epc_page))
> + return epc_page;
> +
> + ret = __sgx_encl_eldu(encl_page, epc_page, secs_page);
> + if (ret) {
> + sgx_free_epc_page(epc_page);
> + return ERR_PTR(ret);
> + }
> +
> + sgx_free_va_slot(encl_page->va_page, va_offset);
> + list_move(&encl_page->va_page->list, &encl->va_pages);
> + encl_page->desc &= ~SGX_ENCL_PAGE_VA_OFFSET_MASK;
> + encl_page->epc_page = epc_page;
> +
> + return epc_page;
> +}

...

> diff --git a/arch/x86/kernel/cpu/sgx/encl.h b/arch/x86/kernel/cpu/sgx/encl.h
> index 0448d22d3010..11dcf4e7fb3e 100644
> --- a/arch/x86/kernel/cpu/sgx/encl.h
> +++ b/arch/x86/kernel/cpu/sgx/encl.h
> @@ -19,6 +19,10 @@
>
> /**
> * enum sgx_encl_page_desc - defines bits for an enclave page's descriptor
> + * %SGX_ENCL_PAGE_RECLAIMED: The page is in the process of being
> + * reclaimed.

SGX_ENCL_PAGE_RECLAIMED means, tho, that the page has been reclaimed
already.

I guess that reads better:

+ if (entry->desc & SGX_ENCL_PAGE_BEING_RECLAIMED)
+ return ERR_PTR(-EBUSY);

> + * %SGX_ENCL_PAGE_VA_OFFSET_MASK: Holds the offset in the Version Array
> + * (VA) page for a swapped page.
> * %SGX_ENCL_PAGE_ADDR_MASK: Holds the virtual address of the page.
> *
> * The page address for SECS is zero and is used by the subsystem to recognize

...

> @@ -86,24 +123,34 @@ static int sgx_encl_create(struct sgx_encl *encl, struct sgx_secs *secs)
> {
> unsigned long encl_size = secs->size + PAGE_SIZE;
> struct sgx_epc_page *secs_epc;
> + struct sgx_va_page *va_page;
> struct sgx_pageinfo pginfo;
> struct sgx_secinfo secinfo;
> struct file *backing;
> long ret;
>
> + va_page = sgx_encl_grow(encl);
> + if (IS_ERR(va_page))
> + return PTR_ERR(va_page);
> + else if (va_page)

Not "else" simply?

AFAICT, sgx_encl_grow() would either return an ERR_PTR or the actual
page...

Also, should the growing happen *after* the SECS validation?

> + list_add(&va_page->list, &encl->va_pages);
> +
> if (sgx_validate_secs(secs)) {
> pr_debug("invalid SECS\n");
> - return -EINVAL;
> + ret = -EINVAL;
> + goto err_out_shrink;
> }
>
> backing = shmem_file_setup("SGX backing", encl_size + (encl_size >> 5),
> VM_NORESERVE);
> - if (IS_ERR(backing))
> - return PTR_ERR(backing);
> + if (IS_ERR(backing)) {
> + ret = PTR_ERR(backing);
> + goto err_out_shrink;
> + }
>
> encl->backing = backing;
>
> - secs_epc = __sgx_alloc_epc_page();
> + secs_epc = sgx_alloc_epc_page(&encl->secs, true);
> if (IS_ERR(secs_epc)) {
> ret = PTR_ERR(secs_epc);
> goto err_out_backing;
> @@ -151,6 +198,9 @@ static int sgx_encl_create(struct sgx_encl *encl, struct sgx_secs *secs)
> fput(encl->backing);
> encl->backing = NULL;
>
> +err_out_shrink:
> + sgx_encl_shrink(encl, va_page);
> +
> return ret;
> }

...

Let's add some comment blurb about "Write back/invalidate an EPC page"
to at least start to explain what that "ewb" is.

> +static void sgx_encl_ewb(struct sgx_epc_page *epc_page,
> + struct sgx_backing *backing)
> +{
> + struct sgx_encl_page *encl_page = epc_page->owner;
> + struct sgx_encl *encl = encl_page->encl;
> + struct sgx_va_page *va_page;
> + unsigned int va_offset;
> + void *va_slot;
> + int ret;
> +
> + encl_page->desc &= ~SGX_ENCL_PAGE_RECLAIMED;
> +
> + va_page = list_first_entry(&encl->va_pages, struct sgx_va_page,
> + list);
> + va_offset = sgx_alloc_va_slot(va_page);
> + va_slot = sgx_get_epc_addr(va_page->epc_page) + va_offset;
> + if (sgx_va_page_full(va_page))
> + list_move_tail(&va_page->list, &encl->va_pages);
> +
> + ret = __sgx_encl_ewb(epc_page, va_slot, backing);
> + if (ret == SGX_NOT_TRACKED) {
> + ret = __etrack(sgx_get_epc_addr(encl->secs.epc_page));
> + if (ret) {
> + if (encls_failed(ret))
> + ENCLS_WARN(ret, "ETRACK");
> + }
> +
> + ret = __sgx_encl_ewb(epc_page, va_slot, backing);
> + if (ret == SGX_NOT_TRACKED) {
> + /*
> + * Slow path, send IPIs to kick cpus out of the
> + * enclave. Note, it's imperative that the cpu
> + * mask is generated *after* ETRACK, else we'll
> + * miss cpus that entered the enclave between
> + * generating the mask and incrementing epoch.
> + */
> + on_each_cpu_mask(sgx_encl_ewb_cpumask(encl),
> + sgx_ipi_cb, NULL, 1);
> + ret = __sgx_encl_ewb(epc_page, va_slot, backing);
> + }
> + }
> +
> + if (ret) {
> + if (encls_failed(ret))
> + ENCLS_WARN(ret, "EWB");
> +
> + sgx_free_va_slot(va_page, va_offset);
> + } else {
> + encl_page->desc |= va_offset;
> + encl_page->va_page = va_page;
> + }

...

> +static void sgx_reclaim_pages(void)
> +{
> + struct sgx_epc_page *chunk[SGX_NR_TO_SCAN];
> + struct sgx_backing backing[SGX_NR_TO_SCAN];
> + struct sgx_epc_section *section;
> + struct sgx_encl_page *encl_page;
> + struct sgx_epc_page *epc_page;
> + int cnt = 0;
> + int ret;
> + int i;
> +
> + spin_lock(&sgx_active_page_list_lock);
> + for (i = 0; i < SGX_NR_TO_SCAN; i++) {
> + if (list_empty(&sgx_active_page_list))

Isn't it enough to do this once, i.e., not in the loop? You're holding
sgx_active_page_list_lock...

> + break;
> +
> + epc_page = list_first_entry(&sgx_active_page_list,
> + struct sgx_epc_page, list);
> + list_del_init(&epc_page->list);
> + encl_page = epc_page->owner;
> +
> + if (kref_get_unless_zero(&encl_page->encl->refcount) != 0)
> + chunk[cnt++] = epc_page;
> + else
> + /* The owner is freeing the page. No need to add the
> + * page back to the list of reclaimable pages.
> + */
> + epc_page->desc &= ~SGX_EPC_PAGE_RECLAIMABLE;
> + }
> + spin_unlock(&sgx_active_page_list_lock);
> +
> + for (i = 0; i < cnt; i++) {
> + epc_page = chunk[i];
> + encl_page = epc_page->owner;
> +
> + if (!sgx_reclaimer_age(epc_page))
> + goto skip;
> +
> + ret = sgx_encl_get_backing(encl_page->encl,
> + SGX_ENCL_PAGE_INDEX(encl_page),
> + &backing[i]);
> + if (ret)
> + goto skip;
> +
> + mutex_lock(&encl_page->encl->lock);
> + encl_page->desc |= SGX_ENCL_PAGE_RECLAIMED;
> + mutex_unlock(&encl_page->encl->lock);
> + continue;
> +
> +skip:
> + spin_lock(&sgx_active_page_list_lock);
> + list_add_tail(&epc_page->list, &sgx_active_page_list);
> + spin_unlock(&sgx_active_page_list_lock);
> +
> + kref_put(&encl_page->encl->refcount, sgx_encl_release);
> +
> + chunk[i] = NULL;
> + }
> +
> + for (i = 0; i < cnt; i++) {
> + epc_page = chunk[i];
> + if (epc_page)
> + sgx_reclaimer_block(epc_page);
> + }
> +
> + for (i = 0; i < cnt; i++) {
> + epc_page = chunk[i];
> + if (!epc_page)
> + continue;
> +
> + encl_page = epc_page->owner;
> + sgx_reclaimer_write(epc_page, &backing[i]);
> + sgx_encl_put_backing(&backing[i], true);
> +
> + kref_put(&encl_page->encl->refcount, sgx_encl_release);
> + epc_page->desc &= ~SGX_EPC_PAGE_RECLAIMABLE;
> +
> + section = sgx_get_epc_section(epc_page);
> + spin_lock(&section->lock);
> + list_add_tail(&epc_page->list, &section->page_list);
> + section->free_cnt++;
> + spin_unlock(&section->lock);
> + }
> +}
> +
>
> static void sgx_sanitize_section(struct sgx_epc_section *section)

...

--
Regards/Gruss,
Boris.