REDHAT-BUG-2346116: Medium severity GNU GRUB vulnerability

When performing a symlink lookup the grub's UFS module check the inode's data size to allocate the internal buffer for reading the file content however it misses to check if the symlink data size has overflown. If that happens grubmalloc() may be called with a smaller value than needed, as consequence when further reading the data from disk into the buffer grubufslookupsymlink() function will write past the end of the allocated size. An attack may leverage that by crafting a malicious filesystem and as a result it will corrupt data stored in the heap, it's possible that arbitrary code execution may be achieved through it and to be used to by-pass secure boot mechanisms.