REDHAT-BUG-2346120: Integer Overflow

When reading data from a jfs filesystem, grub's jfs filesystem module uses user-controlled parameters from the filesystem geometry to determine the internal buffers size, however it misses to properly check for integer overflows. A maliciouly crafted filesystem may lead some of those buffer size calculation to overflow, causing it to perform a grubmalloc() operation with a smaller size than expected. As a result the grubjfslookupsymlink() function will write past of the internal buffer length during grubjfsreadfile(). This flaw may be leveraged to corrupt grub's internal critical data and may result in arbitrary code execution by-passing secure boot protections.