In the Linux kernel, the following vulnerability has been resolved:<\/p>\n
erofs: disable LZ4 rolling decompression for now<\/p>\n
LZ4 rolling decompression [1] was introduced to reduce the memory
\nfootprint of temporary pages:<\/p>\n
For many cases, it is needed for users to read small data within
\n a compressed extent (pcluster), either due to random small read, or
\n since uptodate folios (typically order-0) cannot be reused for
\n decompression again since decompression algorithm refills
\n already-uptodate folios.<\/p>\n
Rolling decompression works because LZ4 is LZ77-based and only refers
\nto the most recent 64 KiB of decompressed data, so in theory only a
\nbounded rolling window of temporary pages is needed when decompressing.<\/p>\n
It can save a lot of temporary memory, e.g.
\n 601,960-byte data can be compressed into a 256k LZ4 compressed extent,
\n which means it needs 146 extra pages per request in the worst case if
\n rolling decompression is disabled.<\/p>\n
However, the upstream LZ4 implementation is not under EROFS’ control:
\nFor example, the literal copy memmove() may still **copy long literals
\nbackward** on x86 based on the address comparison even when the source
\nand destination ranges do not overlap (IOWs, inline decompression
\ndoesn’t need to be considered here). That breaks the rolling assumption
\nand makes the optimization broken.<\/p>\n
Disable it for now to make sure the data correctness first since EROFS
\nis used everywhere now: The rolling window approach can be revived once
\nwe either ensure that the official LZ4 code always copies forward for
\nnon-overlapping ranges or maintain our own LZ4 implementation in EROFS.<\/p>\n
The main impact is a higher runtime memory footprint; However, recent
\ncommit 0f6273ab4637 (“erofs: add a reserved buffer pool for lz4
\ndecompression”) helps mitigate this when enabled but it’s still not
\nperfect.<\/p>\n
[1] https:\/\/www.usenix.org\/conference\/atc19\/presentation\/gao
\n \u00a7 3.3 Decompression<\/p>\n","protected":false},"excerpt":{"rendered":"
In the Linux kernel, the following vulnerability has been resolved:<\/p>\n
erofs: disable LZ4 rolling decompression for now<\/p>\n
LZ4 rolling decompression [1] was introduced to reduce the memory
\nfootprint of te<\/p>\n","protected":false},"template":"","meta":[],"categories":[],"tags":[],"class_list":["post-29222","vuln","type-vuln","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=\/wp\/v2\/vuln\/29222","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=\/wp\/v2\/vuln"}],"about":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=\/wp\/v2\/types\/vuln"}],"wp:attachment":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=29222"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.54iter.cn\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=29222"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.54iter.cn\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=29222"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}