selinux: improve error checking in sel_write_load()

[ Upstream commit 42c7732 ]

Move our existing input sanity checking to the top of sel_write_load()
and add a check to ensure the buffer size is non-zero.

Move a local variable initialization from the declaration to before it
is used.

Minor style adjustments.

Reported-by: Sam Sun <samsun1006219@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>
[cascardo: keep fsi initialization at its declaration point as it is used earlier]
[cascardo: keep check for 64MiB size limit]
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@igalia.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: pcmoore

security/selinux/selinuxfs.c

@@ -536,6 +536,16 @@ static ssize_t sel_write_load(struct file *file, const char __user *buf,
 	ssize_t length;
 	void *data = NULL;
 
+	/* no partial writes */
+	if (*ppos)
+		return -EINVAL;
+	/* no empty policies */
+	if (!count)
+		return -EINVAL;
+
+	if (count > 64 * 1024 * 1024)
+		return -EFBIG;
+
 	mutex_lock(&fsi->mutex);
 
 	length = avc_has_perm(&selinux_state,
@@ -544,23 +554,15 @@ static ssize_t sel_write_load(struct file *file, const char __user *buf,
 	if (length)
 		goto out;
 
-	/* No partial writes. */
-	length = -EINVAL;
-	if (*ppos != 0)
-		goto out;
-
-	length = -EFBIG;
-	if (count > 64 * 1024 * 1024)
-		goto out;
-
-	length = -ENOMEM;
 	data = vmalloc(count);
-	if (!data)
+	if (!data) {
+		length = -ENOMEM;
 		goto out;
-
-	length = -EFAULT;
-	if (copy_from_user(data, buf, count) != 0)
+	}
+	if (copy_from_user(data, buf, count) != 0) {
+		length = -EFAULT;
 		goto out;
+	}
 
 	length = security_load_policy(fsi->state, data, count);
 	if (length) {
@@ -579,6 +581,7 @@ static ssize_t sel_write_load(struct file *file, const char __user *buf,
 		"auid=%u ses=%u lsm=selinux res=1",
 		from_kuid(&init_user_ns, audit_get_loginuid(current)),
 		audit_get_sessionid(current));
+
 out:
 	mutex_unlock(&fsi->mutex);
 	vfree(data);