REDHAT-BUG-2346121: Integer Overflow

When performing a symlink lookup from a romfs filesystem, grub's romfs 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 maliciously 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 grubromfsreadsymlink() may cause a out-of-bounds writes when calling grubdiskread() function. This flaw may be leveraged to corrupt grub's internal critical data and may result in arbitrary code execution by-passing secure boot protections.