CVE-2026-23177: mm, shmem: prevent infinite loop on truncate race

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

mm, shmem: prevent infinite loop on truncate race

When truncating a large swap entry, shmemfreeswap() returns 0 when the entry's index doesn't match the given index due to lookup alignment. The failure fallback path checks if the entry crosses the end border and aborts when it happens, so truncate won't erase an unexpected entry or range. But one scenario was ignored.

When index points to the middle of a large swap entry, and the large swap entry doesn't go across the end border, findgetentries() will return that large swap entry as the first item in the batch with indices[0] equal to index. The entry's base index will be smaller than indices[0], so shmemfreeswap() will fail and return 0 due to the "base < index" check. The code will then call shmemconfirmswap(), get the order, check if it crosses the END boundary (which it doesn't), and retry with the same index.

The next iteration will find the same entry again at the same index with same indices, leading to an infinite loop.

Fix this by retrying with a round-down index, and abort if the index is smaller than the truncate range.