Two enclave threads may try to access the same non-present enclave page simultaneously (e.g., if the SGX runtime supports lazy allocation). The threads will end up in sgx_encl_eaug_page(), racing to acquire the enclave lock. The winning thread will perform EAUG, set up the page table entry, and insert the page into encl->page_array. The losing thread will then get -EBUSY on xa_insert(&encl->page_array) and proceed to error handling path.
This error handling path contains two bugs: (1) SIGBUS is sent to userspace even though the enclave page is correctly installed by another thread, and (2) sgx_encl_free_epc_page() is called that performs EREMOVE even though the enclave page was never intended to be removed. The first bug is less severe because it impacts only the user space; the second bug is more severe because it also impacts the OS state by ripping the page (added by the winning thread) from the enclave.
Fix these two bugs (1) by returning VM_FAULT_NOPAGE to the generic Linux fault handler so that no signal is sent to userspace, and (2) by replacing sgx_encl_free_epc_page() with sgx_free_epc_page() so that no EREMOVE is performed.
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 --- 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);