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From:   Waiman Long <longman@redhat.com>
To:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>
Cc:     Phil Auld <pauld@redhat.com>, linux-kernel@vger.kernel.org,
        Waiman Long <longman@redhat.com>,
        kernel test robot <yujie.liu@intel.com>
Subject: [PATCH] sched/core: Fix NULL pointer access fault in sched_setaffinity() with non-SMP configs
Date:   Sun, 15 Jan 2023 14:31:22 -0500
Message-Id: <20230115193122.563036-1-longman@redhat.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

The kernel commit 9a5418bc48ba ("sched/core: Use kfree_rcu() in
do_set_cpus_allowed()") introduces a bug for kernels built with non-SMP
configs. Calling sched_setaffinity() on such a uniprocessor kernel will
cause cpumask_copy() to be called with a NULL pointer leading to general
protection fault. This is not really a problem in real use cases as
there aren't that many uniprocessor kernel configs in use and calling
sched_setaffinity() on such a uniprocessor system doesn't make sense.

Fix this problem by making sure cpumask_copy() will not be called in
such a case.

Fixes: 9a5418bc48ba ("sched/core: Use kfree_rcu() in do_set_cpus_allowed()")
Reported-by: kernel test robot <yujie.liu@intel.com>
Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/sched/core.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index bb1ee6d7bdde..e838feb6adc5 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8290,12 +8290,18 @@ long sched_setaffinity(pid_t pid, const struct cpumask *in_mask)
 	if (retval)
 		goto out_put_task;
 
+	/*
+	 * With non-SMP configs, user_cpus_ptr/user_mask isn't used and
+	 * alloc_user_cpus_ptr() returns NULL.
+	 */
 	user_mask = alloc_user_cpus_ptr(NUMA_NO_NODE);
-	if (IS_ENABLED(CONFIG_SMP) && !user_mask) {
+	if (user_mask) {
+		cpumask_copy(user_mask, in_mask);
+	} else if (IS_ENABLED(CONFIG_SMP)) {
 		retval = -ENOMEM;
 		goto out_put_task;
 	}
-	cpumask_copy(user_mask, in_mask);
+
 	ac = (struct affinity_context){
 		.new_mask  = in_mask,
 		.user_mask = user_mask,
-- 
2.31.1