Re: [PATCH v4 2/3] x86/sgx: Resolve EAUG race where losing thread returns SIGBUS

Jarkko Sakkinen
2024-07-17T10:38:41Z

On Fri Jul 5, 2024 at 10:45 AM EEST, Dmitrii Kuvaiskii wrote:
> 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 c0a3c00284c8..9f7f9e57cdeb 100644
> --- a/arch/x86/kernel/cpu/sgx/encl.c
> +++ b/arch/x86/kernel/cpu/sgx/encl.c
> @@ -380,8 +380,11 @@ 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;
> @@ -417,7 +420,7 @@ 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);

Fixes should be in the head of the series so please reorder.

BR, Jarkko