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

Huang, Kai
2024-07-25T00:52:18Z

On 5/07/2024 7:45 pm, 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, ...);

Seems the reason of this issue is we allocate encl_page outside of the
encl->lock mutex, and the current way to detect "whether the fault has
been handled by another thread" is by checking whether xa_insert()
returns -EBUSY -- which ...

>
> 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;
> + }

... isn't done in the current code despite there's a comment for it

ret = xa_insert(&encl->page_array, PFN_DOWN(encl_page->desc),
encl_page, GFP_KERNEL);
/*
* If ret == -EBUSY then page was created in another flow while
* running without encl->lock
*/
if (ret)
goto err_out_shrink;

And this patch actually does that.

But instead of using xa_insert() to detect such case, where we have done
bunch of things and needs to revert of all them if xa_insert() fails,
could we just re-check the encl_page inside the encl->lock and quickly
mark it as done if another thread has already done the job?

Something like below (build tested only):

diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
index 279148e72459..7bf63d1b047b 100644
--- a/arch/x86/kernel/cpu/sgx/encl.c
+++ b/arch/x86/kernel/cpu/sgx/encl.c
@@ -339,6 +339,18 @@ static vm_fault_t sgx_encl_eaug_page(struct
vm_area_struct *vma,
if (!test_bit(SGX_ENCL_INITIALIZED, &encl->flags))
return VM_FAULT_SIGBUS;

+
+ mutex_lock(&encl->lock);
+
+ /*
+ * Multiple threads may try to fault in the same EPC page
+ * concurrently. Re-check if another thread has already
+ * done that.
+ */
+ encl_page = xa_load(&encl->page_array, PFN_DOWN(addr));
+ if(encl_page)
+ goto done;
+
/*
* Ignore internal permission checking for dynamically added pages.
* They matter only for data added during the pre-initialization
@@ -347,10 +359,10 @@ static vm_fault_t sgx_encl_eaug_page(struct
vm_area_struct *vma,
*/
secinfo_flags = SGX_SECINFO_R | SGX_SECINFO_W | SGX_SECINFO_X;
encl_page = sgx_encl_page_alloc(encl, addr - encl->base,
secinfo_flags);
- if (IS_ERR(encl_page))
- return VM_FAULT_OOM;
-
- mutex_lock(&encl->lock);
+ if (IS_ERR(encl_page)) {
+ vmret = VM_FAULT_OOM;
+ goto err_out;
+ }

epc_page = sgx_encl_load_secs(encl);
if (IS_ERR(epc_page)) {
@@ -378,10 +390,6 @@ static vm_fault_t sgx_encl_eaug_page(struct
vm_area_struct *vma,

ret = xa_insert(&encl->page_array, PFN_DOWN(encl_page->desc),
encl_page, GFP_KERNEL);
- /*
- * If ret == -EBUSY then page was created in another flow while
- * running without encl->lock
- */
if (ret)
goto err_out_shrink;

@@ -391,7 +399,7 @@ static vm_fault_t sgx_encl_eaug_page(struct
vm_area_struct *vma,

ret = __eaug(&pginfo, sgx_get_epc_virt_addr(epc_page));
if (ret)
- goto err_out;
+ goto err_out_eaug;

encl_page->encl = encl;
encl_page->epc_page = epc_page;
@@ -410,10 +418,11 @@ static vm_fault_t sgx_encl_eaug_page(struct
vm_area_struct *vma,
mutex_unlock(&encl->lock);
return VM_FAULT_SIGBUS;
}
+done:
mutex_unlock(&encl->lock);
return VM_FAULT_NOPAGE;

-err_out:
+err_out_eaug:
xa_erase(&encl->page_array, PFN_DOWN(encl_page->desc));

err_out_shrink:
@@ -421,9 +430,9 @@ static vm_fault_t sgx_encl_eaug_page(struct
vm_area_struct *vma,
err_out_epc:
sgx_encl_free_epc_page(epc_page);
err_out_unlock:
- mutex_unlock(&encl->lock);
kfree(encl_page);
-
+err_out:
+ mutex_unlock(&encl->lock);
return vmret;
}