firmware: qcom: scm: Fix missing read barrier in qcom_scm_get_tzmem_pool()

JIRA: https://issues.redhat.com/browse/RHEL-73299

commit b628510
Author: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Date:   Mon Dec 9 15:27:55 2024 +0100

    firmware: qcom: scm: Fix missing read barrier in qcom_scm_get_tzmem_pool()

    Commit 2e49551 ("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: ca61d68 ("firmware: qcom: scm: fix a NULL-pointer dereference")
    Fixes: 449d0d8 ("firmware: qcom: scm: smc: switch to using the SCM allocator")
    Cc: stable@vger.kernel.org
    Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
    Link: https://lore.kernel.org/r/20241209-qcom-scm-missing-barriers-and-all-sort-of-srap-v2-2-9061013c8d92@linaro.org
    Signed-off-by: Bjorn Andersson <andersson@kernel.org>

Signed-off-by: Jennifer Berringer <jberring@redhat.com>

Author: Jennifer Berringer

drivers/firmware/qcom/qcom_scm.c

@@ -225,7 +225,10 @@ 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)