AZL-83273

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

ext4: validate pidx bounds in ext4extcorrectindexes

ext4extcorrectindexes() walks up the extent tree correcting index entries when the first extent in a leaf is modified. Before accessing path[k].pidx->eiblock, there is no validation that pidx falls within the valid range of index entries for that level.

If the on-disk extent header contains a corrupted or crafted ehentries value, pidx can point past the end of the allocated buffer, causing a slab-out-of-bounds read.

Fix this by validating path[k].pidx against EXTLAST_INDEX() at both access sites: before the while loop and inside it. Return -EFSCORRUPTED if the index pointer is out of range, consistent with how other bounds violations are handled in the ext4 extent tree code.