[PATCH v3 1/2] x86/sgx: Resolve EAUG race where losing thread returns SIGBUS

[Dmitrii Kuvaiskii <dmitrii.kuvaiskii@intel.com>]

Imagine an mmap()'d file. Two threads touch the same address at the same
time and fault. Both allocate a physical page and race to install a PTE
for that page. Only one will win the race. The loser frees its page, but
still continues handling the fault as a success and returns
VM_FAULT_NOPAGE from the fault handler.

The same race can happen with SGX. But there's a bug: the loser in the
SGX steers into a failure path. The loser EREMOVE's the winner's EPC
page, then returns SIGBUS, likely killing the app.

Fix the SGX loser's behavior. Change the return code to VM_FAULT_NOPAGE
to avoid SIGBUS and call sgx_free_epc_page() which avoids EREMOVE'ing
the winner's page and only frees the page that the loser allocated.

The race can be illustrated as follows:

/*                             /*
 * Fault on CPU1                * Fault on CPU2
 * on enclave page X            * on enclave page X
 */                             */
sgx_vma_fault() {              sgx_vma_fault() {

  xa_load(&encl->page_array)     xa_load(&encl->page_array)
      == NULL -->                    == NULL -->

  sgx_encl_eaug_page() {         sgx_encl_eaug_page() {

    ...                            ...

    /*                             /*
     * alloc encl_page              * alloc encl_page
     */                             */
                                   mutex_lock(&encl->lock);
                                   /*
                                    * alloc EPC page
                                    */
                                   epc_page = sgx_alloc_epc_page(...);
                                   /*
                                    * add page to enclave's xarray
                                    */
                                   xa_insert(&encl->page_array, ...);
                                   /*
                                    * add page to enclave via EAUG
                                    * (page is in pending state)
                                    */
                                   /*
                                    * add PTE entry
                                    */
                                   vmf_insert_pfn(...);

                                   mutex_unlock(&encl->lock);
                                   return VM_FAULT_NOPAGE;
                                 }
                               }
                               /*
                                * All good up to here: enclave page
                                * successfully added to enclave,
                                * ready for EACCEPT from user space
                                */
    mutex_lock(&encl->lock);
    /*
     * alloc EPC page
     */
    epc_page = sgx_alloc_epc_page(...);
    /*
     * add page to enclave's xarray,
     * this fails with -EBUSY as this
     * page was already added by CPU2
     */
    xa_insert(&encl->page_array, ...);

  err_out_shrink:
    sgx_encl_free_epc_page(epc_page) {
      /*
       * remove page via EREMOVE
       *
       * *BUG*: page added by CPU2 is
       * yanked from enclave while it
       * remains accessible from OS
       * perspective (PTE installed)
       */
      /*
       * free EPC page
       */
      sgx_free_epc_page(epc_page);
    }

    mutex_unlock(&encl->lock);
    /*
     * *BUG*: SIGBUS is returned
     * for a valid enclave page
     */
    return VM_FAULT_SIGBUS;
  }
}

Fixes: 5a90d2c3f5ef ("x86/sgx: Support adding of pages to an initialized enclave")
Cc: stable@vger.kernel.org
Reported-by: Marcelina Kościelnicka <mwk@invisiblethingslab.com>
Suggested-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Dmitrii Kuvaiskii <dmitrii.kuvaiskii@intel.com>
Reviewed-by: Haitao Huang <haitao.huang@linux.intel.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
---
 arch/x86/kernel/cpu/sgx/encl.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
index 279148e72459..41f14b1a3025 100644
--- a/arch/x86/kernel/cpu/sgx/encl.c
+++ b/arch/x86/kernel/cpu/sgx/encl.c
@@ -382,8 +382,11 @@ static vm_fault_t sgx_encl_eaug_page(struct vm_area_struct *vma,
 	 * If ret == -EBUSY then page was created in another flow while
 	 * running without encl->lock
 	 */
-	if (ret)
+	if (ret) {
+		if (ret == -EBUSY)
+			vmret = VM_FAULT_NOPAGE;
 		goto err_out_shrink;
+	}
 
 	pginfo.secs = (unsigned long)sgx_get_epc_virt_addr(encl->secs.epc_page);
 	pginfo.addr = encl_page->desc & PAGE_MASK;
@@ -419,7 +422,7 @@ static vm_fault_t sgx_encl_eaug_page(struct vm_area_struct *vma,
 err_out_shrink:
 	sgx_encl_shrink(encl, va_page);
 err_out_epc:
-	sgx_encl_free_epc_page(epc_page);
+	sgx_free_epc_page(epc_page);
 err_out_unlock:
 	mutex_unlock(&encl->lock);
 	kfree(encl_page);
-- 
2.34.1