CVE-2023-52497: erofs: fix lz4 inplace decompression

CVE-2023-52497: erofs: fix lz4 inplace decompression

[Lee Jones]

Description
===========

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

erofs: fix lz4 inplace decompression

Currently EROFS can map another compressed buffer for inplace
decompression, that was used to handle the cases that some pages of
compressed data are actually not in-place I/O.

However, like most simple LZ77 algorithms, LZ4 expects the compressed
data is arranged at the end of the decompressed buffer and it
explicitly uses memmove() to handle overlapping:
  __________________________________________________________
 |_ direction of decompression --> ____ |_ compressed data _|

Although EROFS arranges compressed data like this, it typically maps two
individual virtual buffers so the relative order is uncertain.
Previously, it was hardly observed since LZ4 only uses memmove() for
short overlapped literals and x86/arm64 memmove implementations seem to
completely cover it up and they don't have this issue.  Juhyung reported
that EROFS data corruption can be found on a new Intel x86 processor.
After some analysis, it seems that recent x86 processors with the new
FSRM feature expose this issue with "rep movsb".

Let's strictly use the decompressed buffer for lz4 inplace
decompression for now.  Later, as an useful improvement, we could try
to tie up these two buffers together in the correct order.

The Linux kernel CVE team has assigned CVE-2023-52497 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 5.3 with commit 598162d05080 and fixed in 6.1.76 with commit 33bf23c9940d
	Issue introduced in 5.3 with commit 598162d05080 and fixed in 6.6.15 with commit f36d200a80a3
	Issue introduced in 5.3 with commit 598162d05080 and fixed in 6.7.3 with commit bffc4cc334c5
	Issue introduced in 5.3 with commit 598162d05080 and fixed in 6.8-rc1 with commit 3c12466b6b7b

Please see https://www.kernel.org or a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2023-52497
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	fs/erofs/decompressor.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/33bf23c9940dbd3a22aad7f0cda4c84ed5701847
	https://git.kernel.org/stable/c/f36d200a80a3ca025532ed60dd1ac21b620e14ae
	https://git.kernel.org/stable/c/bffc4cc334c5bb31ded54bc3cfd651735a3cb79e
	https://git.kernel.org/stable/c/3c12466b6b7bf1e56f9b32c366a3d83d87afb4de