Re: [syzbot] [cgroups?] KASAN: slab-use-after-free Read in pressure_write

[Edward Adam Davis]

#syz test

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 4ca3cb993da2..c94a16352c33 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -3995,33 +3995,38 @@ static int cgroup_cpu_pressure_show(struct seq_file *seq, void *v)
 static ssize_t pressure_write(struct kernfs_open_file *of, char *buf,
 			      size_t nbytes, enum psi_res res)
 {
-	struct cgroup_file_ctx *ctx = of->priv;
+	struct cgroup_file_ctx *ctx;
 	struct psi_trigger *new;
 	struct cgroup *cgrp;
 	struct psi_group *psi;
+	ssize_t ret = 0;
 
 	cgrp = cgroup_kn_lock_live(of->kn, false);
 	if (!cgrp)
 		return -ENODEV;
 
-	cgroup_get(cgrp);
-	cgroup_kn_unlock(of->kn);
+	/* of->priv can not be NULL, because cgroup is CSS_ONLINE */
+	ctx = of->priv;
 
 	/* Allow only one trigger per file descriptor */
 	if (ctx->psi.trigger) {
-		cgroup_put(cgrp);
-		return -EBUSY;
+		ret = -EBUSY;
+		goto out_unlock;
 	}
 
 	psi = cgroup_psi(cgrp);
 	new = psi_trigger_create(psi, buf, res, of->file, of);
 	if (IS_ERR(new)) {
-		cgroup_put(cgrp);
-		return PTR_ERR(new);
+		ret = PTR_ERR(new);
+		goto out_unlock;
 	}
 
 	smp_store_release(&ctx->psi.trigger, new);
-	cgroup_put(cgrp);
+
+out_unlock:
+	cgroup_kn_unlock(of->kn);
+	if (ret)
+		return ret;
 
 	return nbytes;
 }