drivers/base: Fix length checks in create_syslog_header()/dev_vprintk_emit()

snprintf() returns the number of bytes that could have been written
(excluding the null), not the actual number of bytes written.  Given a
long enough subsystem or device name, these functions will advance
beyond the end of the on-stack buffer in dev_vprintk_exit(), resulting
in an information leak or stack corruption.  I don't know whether such
a long name is currently possible.

In case snprintf() returns a value >= the buffer size, do not add
structured logging information.  Also WARN if this happens, so we can
fix the driver or increase the buffer size.
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/base/core.c

@@ -2007,6 +2007,8 @@ create_syslog_header(const struct device *dev, char *hdr, size_t hdrlen)
 		return 0;
 
 	pos += snprintf(hdr + pos, hdrlen - pos, "SUBSYSTEM=%s", subsys);
+	if (pos >= hdrlen)
+		goto overflow;
 
 	/*
 	 * Add device identifier DEVICE=:
@@ -2038,7 +2040,14 @@ create_syslog_header(const struct device *dev, char *hdr, size_t hdrlen)
 				"DEVICE=+%s:%s", subsys, dev_name(dev));
 	}
 
+	if (pos >= hdrlen)
+		goto overflow;
+
 	return pos;
+
+overflow:
+	dev_WARN(dev, "device/subsystem name too long");
+	return 0;
 }
 
 int dev_vprintk_emit(int level, const struct device *dev,