@@ -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);
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(-)