[PATCH v2 2/6] firmware: qcom: scm: Fix missing read barrier in qcom_scm_get_tzmem_pool()

[Krzysztof Kozlowski]

Commit 2e4955167ec5 ("firmware: qcom: scm: Fix __scm and waitq
completion variable initialization") introduced a write barrier in probe
function to store global '__scm' variable.  We all known barriers are
paired (see memory-barriers.txt: "Note that write barriers should
normally be paired with read or address-dependency barriers"), therefore
accessing it from concurrent contexts requires read barrier.  Previous
commit added such barrier in qcom_scm_is_available(), so let's use that
directly.

Lack of this read barrier can result in fetching stale '__scm' variable
value, NULL, and dereferencing it.

Note that barrier in qcom_scm_is_available() satisfies here the control
dependency.

Fixes: ca61d6836e6f ("firmware: qcom: scm: fix a NULL-pointer dereference")
Fixes: 449d0d84bcd8 ("firmware: qcom: scm: smc: switch to using the SCM allocator")
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@xxxxxxxxxx>
---
 drivers/firmware/qcom/qcom_scm.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/firmware/qcom/qcom_scm.c b/drivers/firmware/qcom/qcom_scm.c
index 246d672e8f7f0e2a326a03a5af40cd434a665e67..5d91b8e22844608f35432f1ba9c08d477d4ff762 100644
--- a/drivers/firmware/qcom/qcom_scm.c
+++ b/drivers/firmware/qcom/qcom_scm.c
@@ -217,7 +217,10 @@ static DEFINE_SPINLOCK(scm_query_lock);
 
 struct qcom_tzmem_pool *qcom_scm_get_tzmem_pool(void)
 {
-	return __scm ? __scm->mempool : NULL;
+	if (!qcom_scm_is_available())
+		return NULL;
+
+	return __scm->mempool;
 }
 
 static enum qcom_scm_convention __get_convention(void)

-- 
2.43.0