[PATCH v5] x86/sgx: Fix use-after-free in sgx_mmu_notifier_release()

[Jarkko Sakkinen <jarkko@kernel.org>]

The most trivial example of a race condition can be demonstrated by this
sequence where mm_list contains just one entry:

CPU A                           CPU B
-> sgx_release()
                                -> sgx_mmu_notifier_release()
                                -> list_del_rcu()
                                <- list_del_rcu()
-> kref_put()
-> sgx_encl_release()
                                -> synchronize_srcu()
-> cleanup_srcu_struct()

A sequence similar to this has also been spotted in tests under high
stress:

[  +0.000008] WARNING: CPU: 3 PID: 7620 at kernel/rcu/srcutree.c:374 cleanup_srcu_struct+0xed/0x100

Albeit not spotted in the tests, it's also entirely possible that the
following scenario could happen:

CPU A                           CPU B
-> sgx_release()
                                -> sgx_mmu_notifier_release()
                                -> list_del_rcu()
-> kref_put()
-> sgx_encl_release()
-> cleanup_srcu_struct()
<- cleanup_srcu_struct()
                                -> synchronize_srcu()

This scenario would lead into use-after free in cleaup_srcu_struct().

Fix this by taking a reference to the enclave in
sgx_mmu_notifier_release().

Cc: stable@vger.kernel.org
Fixes: 1728ab54b4be ("x86/sgx: Add a page reclaimer")
Suggested-by: Sean Christopherson <seanjc@google.com>
Reported-by: Haitao Huang <haitao.huang@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
---
v5:
- To make sure that the instance does not get deleted use kref_get()
  kref_put(). This also removes the need for additional
  synchronize_srcu().
v4:
- Rewrite the commit message.
- Just change the call order. *_expedited() is out of scope for this
  bug fix.
v3: Fine-tuned tags, and added missing change log for v2.
v2: Switch to synchronize_srcu_expedited().
 arch/x86/kernel/cpu/sgx/encl.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
index ee50a5010277..5ecbcf94ec2a 100644
--- a/arch/x86/kernel/cpu/sgx/encl.c
+++ b/arch/x86/kernel/cpu/sgx/encl.c
@@ -465,6 +465,7 @@ static void sgx_mmu_notifier_release(struct mmu_notifier *mn,
 	spin_lock(&encl_mm->encl->mm_lock);
 	list_for_each_entry(tmp, &encl_mm->encl->mm_list, list) {
 		if (tmp == encl_mm) {
+			kref_get(&encl_mm->encl->refcount);
 			list_del_rcu(&encl_mm->list);
 			break;
 		}
@@ -474,6 +475,7 @@ static void sgx_mmu_notifier_release(struct mmu_notifier *mn,
 	if (tmp == encl_mm) {
 		synchronize_srcu(&encl_mm->encl->srcu);
 		mmu_notifier_put(mn);
+		kref_put(&encl_mm->encl->refcount, sgx_encl_release);
 	}
 }
 
-- 
2.30.0