Re: [RFC PATCH 3/4] x86/sgx: Obtain backing storage page with enclave mutex held

[Jarkko Sakkinen <jarkko@kernel.org>]

On Thu, Apr 28, 2022 at 01:11:26PM -0700, Reinette Chatre wrote:
> The SGX backing storage is accessed on two paths: when there
> are insufficient enclave pages in the EPC the reclaimer works
> to move enclave pages to the backing storage and as enclaves
> access pages that have been moved to the backing storage
> they are retrieved from there as part of page fault handling.
> 
> An oversubscribed SGX system will often run the reclaimer and
> page fault handler concurrently and needs to ensure that the
> backing store is accessed safely between the reclaimer and
> the page fault handler. The scenarios to consider here are:
> (a) faulting a page right after it was reclaimed,
> (b) faulting a page and reclaiming another page that are
>     sharing a PCMD page.
> 
> The reclaimer obtains pages from the backing storage without
> holding the enclave mutex and runs the risk of concurrently
> accessing the backing storage with the page fault handler that
> does access the backing storage with the enclave mutex held.
> 
> In the scenario below a page is written to the backing store
> by the reclaimer and then immediately faulted back, before
> the reclaimer is able to set the dirty bit of the page:
> 
> sgx_reclaim_pages() {                    sgx_vma_fault() {
> ...                                      ...
> /* write data to backing store */
> sgx_reclaimer_write();
>                                          mutex_lock(&encl->lock);
>                                          __sgx_encl_eldu() {
>                                          ...
>                                          /* page not dirty -
>                                           * contents may not be
>                                           * up to date
>                                          */
>                                          sgx_encl_get_backing();
>                                          ...
>                                          }
>                                          ...
> /* set page dirty */
> sgx_encl_put_backing();
> ...
>                                          mutex_unlock(&encl->lock);
> }                                        }
> 
> While it is not possible to concurrently reclaim and fault the same
> enclave page the PCMD pages are shared between enclave pages
> in the enclave and enclave pages in the backing store.
> In the below scenario a PCMD page is truncated from the backing
> store after all its pages have been loaded in to the enclave
> at the same time the PCMD page is loaded from the backing store
> when one of its pages are reclaimed:
> 
> sgx_reclaim_pages() {              sgx_vma_fault() {
>                                      ...
>                                      mutex_lock(&encl->lock);
>                                      ...
>                                      __sgx_encl_eldu() {
>                                        ...
>                                        if (pcmd_page_empty) {
> /*
>  * EPC page being reclaimed              /*
>  * shares a PCMD page with an             * PCMD page truncated
>  * enclave page that is being             * while requested from
>  * faulted in.                            * reclaimer.
>  */                                       */
> sgx_encl_get_backing()  <---------->      sgx_encl_truncate_backing_page()
>                                         }
> }                                    }
> 
> Protect the reclaimer's backing store access with the enclave's mutex
> to ensure that it can safely run concurrently with the page fault handler.
> 
> Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
> ---
>  arch/x86/kernel/cpu/sgx/main.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
> index 0e8741a80cf3..ae79b8d6f645 100644
> --- a/arch/x86/kernel/cpu/sgx/main.c
> +++ b/arch/x86/kernel/cpu/sgx/main.c
> @@ -252,6 +252,7 @@ static void sgx_reclaimer_write(struct sgx_epc_page *epc_page,
>  	sgx_encl_ewb(epc_page, backing);
>  	encl_page->epc_page = NULL;
>  	encl->secs_child_cnt--;
> +	sgx_encl_put_backing(backing, true);
>  
>  	if (!encl->secs_child_cnt && test_bit(SGX_ENCL_INITIALIZED, &encl->flags)) {
>  		ret = sgx_encl_get_backing(encl, PFN_DOWN(encl->size),
> @@ -323,11 +324,14 @@ static void sgx_reclaim_pages(void)
>  			goto skip;
>  
>  		page_index = PFN_DOWN(encl_page->desc - encl_page->encl->base);
> +
> +		mutex_lock(&encl_page->encl->lock);
>  		ret = sgx_encl_get_backing(encl_page->encl, page_index, &backing[i]);
> -		if (ret)
> +		if (ret) {
> +			mutex_unlock(&encl_page->encl->lock);
>  			goto skip;
> +		}
>  
> -		mutex_lock(&encl_page->encl->lock);
>  		encl_page->desc |= SGX_ENCL_PAGE_BEING_RECLAIMED;
>  		mutex_unlock(&encl_page->encl->lock);
>  		continue;
> @@ -355,7 +359,6 @@ static void sgx_reclaim_pages(void)
>  
>  		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->flags &= ~SGX_EPC_PAGE_RECLAIMER_TRACKED;
> -- 
> 2.25.1
> 

I fully agree with your fix but also there I would open code the required
statements from sgx_encL_put_backing(). Let's render that out overtime.
It masks more than helps.

BR, Jarkko