xfs: don't walk off the end of a directory data block

In the Linux kernel, the following vulnerability has been resolved:

xfs: don't walk off the end of a directory data block

This adds sanity checks for xfs_dir2_data_unused and xfs_dir2_data_entry

to make sure don't stray beyond valid memory region. Before patching, the

loop simply checks that the start offset of the dup and dep is within the

range. So in a crafted image, if last entry is xfs_dir2_data_unused, we

can change dup->length to dup->length-1 and leave 1 byte of space. In the

next traversal, this space will be considered as dup or dep. We may

encounter an out of bound read when accessing the fixed members.

In the patch, we make sure that the remaining bytes large enough to hold

an unused entry before accessing xfs_dir2_data_unused and

xfs_dir2_data_unused is XFS_DIR2_DATA_ALIGN byte aligned. We also make

sure that the remaining bytes large enough to hold a dirent with a

single-byte name before accessing xfs_dir2_data_entry.