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

[Jarkko Sakkinen]

Each sgx_mmun_notifier_release() starts a grace period, which means that
one extra synchronize_rcu() in sgx_encl_release(). Add it there.

sgx_release() has the loop that drains the list but with bad luck the
entry is already gone from the list before that loop processes it.

Fixes: 1728ab54b4be ("x86/sgx: Add a page reclaimer")
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Reported-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
---
 arch/x86/kernel/cpu/sgx/encl.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
index ee50a5010277..48539a6ee315 100644
--- a/arch/x86/kernel/cpu/sgx/encl.c
+++ b/arch/x86/kernel/cpu/sgx/encl.c
@@ -438,6 +438,13 @@ void sgx_encl_release(struct kref *ref)
 	if (encl->backing)
 		fput(encl->backing);
 
+	/*
+	 * Each sgx_mmun_notifier_release() starts a grace period. Thus one
+	 * "extra" synchronize_rcu() is required here. This can go undetected by
+	 * sgx_release() when it drains the mm list.
+	 */
+	synchronize_srcu(&encl->srcu);
+
 	cleanup_srcu_struct(&encl->srcu);
 
 	WARN_ON_ONCE(!list_empty(&encl->mm_list));
-- 
2.27.0