AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

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

erofs: fix managed cache race for unaligned extents

After unaligned compressed extents were introduced, the following race
could occur:

[Thread 1]                                   [Thread 2]
(z_erofs_fill_bio_vec)
<handle a Z_EROFS_PREALLOCATED_FOLIO folio>
...
filemap_add_folio (1)
                                             (z_erofs_bind_cache)
                                             <the same folio is found..>
                                             ..
                                             ..
folio_attach_private (2)
                                             filemap_add_folio (3) again

Since (1) is executed but (2) hasn't been executed yet, it's possible
that another thread finds the same managed folio in z_erofs_bind_cache()
for a different pcluster and calls filemap_add_folio() again since
folio->private is still Z_EROFS_PREALLOCATED_FOLIO.

Fix this by explicitly clearing folio->private before making the folio
visible in the managed cache so that another pcluster can simply wait
on the locked managed folio as what we did for other shared cases [1].

This only impacts unaligned data compression (`-E48bit` with zstd,
for example).

[1] Commit 9e2f9d34dd12 ("erofs: handle overlapped pclusters out of
 crafted images properly") was originally introduced to handle crafted
 overlapped extents, but it addresses unaligned extents as well.

https://git.kernel.org/stable/c/038166f873c4caf6e85cfd4ea0c5a5ba297b4e8b

https://git.kernel.org/stable/c/425d32d6288d7d845e486af9419bbedccd8c9103

https://git.kernel.org/stable/c/649932fc3815eda2f24eb4de4b3a5e94886ee0b9

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