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

From: Huang, Kai
Date: Wed Jul 24 2024 - 20:52:28 EST



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@xxxxxxxxxxxxxxx
Reported-by: Marcelina Kościelnicka <mwk@xxxxxxxxxxxxxxxxxxxxxx>
Suggested-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
Signed-off-by: Dmitrii Kuvaiskii <dmitrii.kuvaiskii@xxxxxxxxx>
Reviewed-by: Haitao Huang <haitao.huang@xxxxxxxxxxxxxxx>
Reviewed-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
Reviewed-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
---
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;
}