Linux: erofs: disable LZ4 rolling decompression for now

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

erofs: disable LZ4 rolling decompression for now

LZ4 rolling decompression [1] was introduced to reduce the memory footprint of temporary pages:

For many cases, it is needed for users to read small data within a compressed extent (pcluster), either due to random small read, or since uptodate folios (typically order-0) cannot be reused for decompression again since decompression algorithm refills already-uptodate folios.

Rolling decompression works because LZ4 is LZ77-based and only refers to the most recent 64 KiB of decompressed data, so in theory only a bounded rolling window of temporary pages is needed when decompressing.

It can save a lot of temporary memory, e.g. 601,960-byte data can be compressed into a 256k LZ4 compressed extent, which means it needs 146 extra pages per request in the worst case if rolling decompression is disabled.

However, the upstream LZ4 implementation is not under EROFS' control: For example, the literal copy memmove() may still copy long literals backward on x86 based on the address comparison even when the source and destination ranges do not overlap (IOWs, inline decompression doesn't need to be considered here). That breaks the rolling assumption and makes the optimization broken.

Disable it for now to make sure the data correctness first since EROFS is used everywhere now: The rolling window approach can be revived once we either ensure that the official LZ4 code always copies forward for non-overlapping ranges or maintain our own LZ4 implementation in EROFS.

The main impact is a higher runtime memory footprint; However, recent commit 0f6273ab4637 ("erofs: add a reserved buffer pool for lz4 decompression") helps mitigate this when enabled but it's still not perfect.

[1] https://www.usenix.org/conference/atc19/presentation/gao § 3.3 Decompression