powerpc/pseries/papr-sysparm: Validate buffer object lengths

The ability to get and set system parameters will be exposed to user
space, so let's get a little more strict about malformed
papr_sysparm_buf objects.

* Create accessors for the length field of struct papr_sysparm_buf.
The length is always stored in MSB order and this is better than
spreading the necessary conversions all over.

* Reject attempts to submit invalid buffers to RTAS.

* Warn if RTAS returns a buffer with an invalid length, clamping the
returned length to a safe value that won't overrun the buffer.

These are meant as precautionary measures to mitigate both firmware
and kernel bugs in this area, should they arise, but I am not aware of
any.

Signed-off-by: Nathan Lynch <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://msgid.link/[e-mail]

arch/powerpc/platforms/pseries/papr-sysparm.c

···
 	kfree(buf);
 }
 
+static size_t papr_sysparm_buf_get_length(const struct papr_sysparm_buf *buf)
+{
+	return be16_to_cpu(buf->len);
+}
+
+static void papr_sysparm_buf_set_length(struct papr_sysparm_buf *buf, size_t length)
+{
+	WARN_ONCE(length > sizeof(buf->val),
+		  "bogus length %zu, clamping to safe value", length);
+	length = min(sizeof(buf->val), length);
+	buf->len = cpu_to_be16(length);
+}
+
+/*
+ * For use on buffers returned from ibm,get-system-parameter before
+ * returning them to callers. Ensures the encoded length of valid data
+ * cannot overrun buf->val[].
+ */
+static void papr_sysparm_buf_clamp_length(struct papr_sysparm_buf *buf)
+{
+	papr_sysparm_buf_set_length(buf, papr_sysparm_buf_get_length(buf));
+}
+
+/*
+ * Perform some basic diligence on the system parameter buffer before
+ * submitting it to RTAS.
+ */
+static bool papr_sysparm_buf_can_submit(const struct papr_sysparm_buf *buf)
+{
+	/*
+	 * Firmware ought to reject buffer lengths that exceed the
+	 * maximum specified in PAPR, but there's no reason for the
+	 * kernel to allow them either.
+	 */
+	if (papr_sysparm_buf_get_length(buf) > sizeof(buf->val))
+		return false;
+
+	return true;
+}
+
 /**
  * papr_sysparm_get() - Retrieve the value of a PAPR system parameter.
  * @param: PAPR system parameter token as described in
···
 	if (token == RTAS_UNKNOWN_SERVICE)
 		return -ENOENT;
 
+	if (!papr_sysparm_buf_can_submit(buf))
+		return -EINVAL;
+
 	work_area = rtas_work_area_alloc(sizeof(*buf));
 
 	memcpy(rtas_work_area_raw_buf(work_area), buf, sizeof(*buf));
···
 	case 0:
 		ret = 0;
 		memcpy(buf, rtas_work_area_raw_buf(work_area), sizeof(*buf));
+		papr_sysparm_buf_clamp_length(buf);
 		break;
 	case -3: /* parameter not implemented */
 		ret = -EOPNOTSUPP;
···
 
 	if (token == RTAS_UNKNOWN_SERVICE)
 		return -ENOENT;
+
+	if (!papr_sysparm_buf_can_submit(buf))
+		return -EINVAL;
 
 	work_area = rtas_work_area_alloc(sizeof(*buf));