[PATCH v3] x86/sgx: Synchronize encl->srcu in sgx_encl_release().

[Jarkko Sakkinen <jarkko@kernel.org>]

Add synchronize_srcu_expedited() to sgx_encl_release() to catch a grace
period initiated by sgx_mmu_notifier_release().

A trivial example of a failing sequence with tasks A and B:

1. A: -> sgx_release()
2. B: -> sgx_mmu_notifier_release()
3. B: -> list_del_rcu()
3. A: -> sgx_encl_release()
4. A: -> cleanup_srcu_struct()

The loop in sgx_release() observes an empty list because B has removed its
entry in the middle, and calls cleanup_srcu_struct() before B has a chance
to calls synchronize_srcu().

Fixes: 1728ab54b4be ("x86/sgx: Add a page reclaimer")
Suggested-by: Sean Christopherson <seanjc@google.com>
Suggested-by: Haitao Huang <haitao.huang@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
---
v3: Fine-tuned tags, and added missing change log for v2.
v2: Switch to synchronize_srcu_expedited().
 arch/x86/kernel/cpu/sgx/encl.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
index ee50a5010277..fe7256db6e73 100644
--- a/arch/x86/kernel/cpu/sgx/encl.c
+++ b/arch/x86/kernel/cpu/sgx/encl.c
@@ -438,6 +438,12 @@ void sgx_encl_release(struct kref *ref)
 	if (encl->backing)
 		fput(encl->backing);
 
+	/*
+	 * Each sgx_mmu_notifier_release() starts a grace period. Therefore, an
+	 * additional sync is required here.
+	 */
+	synchronize_srcu_expedited(&encl->srcu);
+
 	cleanup_srcu_struct(&encl->srcu);
 
 	WARN_ON_ONCE(!list_empty(&encl->mm_list));
-- 
2.27.0