[PATCH v5] x86/sgx: Resolves SECS reclaim vs. page fault for EAUG race

[Haitao Huang]

Under heavy load, the SGX EPC reclaimer (ksgxd) may reclaim the SECS EPC
page for an enclave and set encl->secs.epc_page to NULL. But the SECS
EPC page is used for EAUG in the SGX page fault handler without checking
for NULL and reloading.

Fix this by checking if SECS is loaded before EAUG and loading it if it
was reclaimed.

The SECS page holds global enclave metadata. It can only be reclaimed
when there are no other enclave pages remaining. At that point,
virtually nothing can be done with the enclave until the SECS page is
paged back in.

An enclave can not run nor generate page faults without a resident SECS
page. But it is still possible for a #PF for a non-SECS page to race
with paging out the SECS page: when the last resident non-SECS page A
triggers a #PF in a non-resident page B, and then page A and the SECS
both are paged out before the #PF on B is handled.

Hitting this bug requires that race triggered with a #PF for EAUG.
Following is a trace when it happens.

BUG: kernel NULL pointer dereference, address: 0000000000000000
RIP: 0010:sgx_encl_eaug_page+0xc7/0x210
Call Trace:
 ? __kmem_cache_alloc_node+0x16a/0x440
 ? xa_load+0x6e/0xa0
 sgx_vma_fault+0x119/0x230
 __do_fault+0x36/0x140
 do_fault+0x12f/0x400
 __handle_mm_fault+0x728/0x1110
 handle_mm_fault+0x105/0x310
 do_user_addr_fault+0x1ee/0x750
 ? __this_cpu_preempt_check+0x13/0x20
 exc_page_fault+0x76/0x180
 asm_exc_page_fault+0x27/0x30

Fixes: 5a90d2c3f5ef ("x86/sgx: Support adding of pages to an initialized enclave")
Cc: stable@xxxxxxxxxxxxxxx # v6.0+
Signed-off-by: Haitao Huang <haitao.huang@xxxxxxxxxxxxxxx>
Reviewed-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
Acked-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
---
v5:
- Trimmed trace and added Acked-by (Reinette)

v4:
- Refined the title (Kai, Dave)
- Added a trace to commit meesage (Kai)
- Added a few details for the race.

v3:
- Added comments on sgx_encl_load_secs(). (Dave)
- Added theory of the race condition to hit the bug. (Dave)
- Added Reviewed-by, and applicable stable release. (Jarkko)

v2:
- Fixes for style, commit message (Jarkko, Kai)
- Removed unneeded WARN_ON (Kai)
---
 arch/x86/kernel/cpu/sgx/encl.c | 30 +++++++++++++++++++++++++-----
 1 file changed, 25 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
index 91fa70e51004..279148e72459 100644
--- a/arch/x86/kernel/cpu/sgx/encl.c
+++ b/arch/x86/kernel/cpu/sgx/encl.c
@@ -235,6 +235,21 @@ static struct sgx_epc_page *sgx_encl_eldu(struct sgx_encl_page *encl_page,
 	return epc_page;
 }
 
+/*
+ * Ensure the SECS page is not swapped out.  Must be called with encl->lock
+ * to protect the enclave states including SECS and ensure the SECS page is
+ * not swapped out again while being used.
+ */
+static struct sgx_epc_page *sgx_encl_load_secs(struct sgx_encl *encl)
+{
+	struct sgx_epc_page *epc_page = encl->secs.epc_page;
+
+	if (!epc_page)
+		epc_page = sgx_encl_eldu(&encl->secs, NULL);
+
+	return epc_page;
+}
+
 static struct sgx_encl_page *__sgx_encl_load_page(struct sgx_encl *encl,
 						  struct sgx_encl_page *entry)
 {
@@ -248,11 +263,9 @@ static struct sgx_encl_page *__sgx_encl_load_page(struct sgx_encl *encl,
 		return entry;
 	}
 
-	if (!(encl->secs.epc_page)) {
-		epc_page = sgx_encl_eldu(&encl->secs, NULL);
-		if (IS_ERR(epc_page))
-			return ERR_CAST(epc_page);
-	}
+	epc_page = sgx_encl_load_secs(encl);
+	if (IS_ERR(epc_page))
+		return ERR_CAST(epc_page);
 
 	epc_page = sgx_encl_eldu(entry, encl->secs.epc_page);
 	if (IS_ERR(epc_page))
@@ -339,6 +352,13 @@ static vm_fault_t sgx_encl_eaug_page(struct vm_area_struct *vma,
 
 	mutex_lock(&encl->lock);
 
+	epc_page = sgx_encl_load_secs(encl);
+	if (IS_ERR(epc_page)) {
+		if (PTR_ERR(epc_page) == -EBUSY)
+			vmret = VM_FAULT_NOPAGE;
+		goto err_out_unlock;
+	}
+
 	epc_page = sgx_alloc_epc_page(encl_page, false);
 	if (IS_ERR(epc_page)) {
 		if (PTR_ERR(epc_page) == -EBUSY)

base-commit: 6eaae198076080886b9e7d57f4ae06fa782f90ef
-- 
2.25.1