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22b10cbb9a
This is a stable-only patch. The issue was inadvertently fixed in 6.17 [0]
as part of a refactoring, but this patch serves as a minimal targeted fix
for prior kernels.
Users of filemap_lock_folio() need to guard against the situation where
release_folio() has been invoked during reclaim but the folio was
ultimately not removed from the page cache. This patch covers one location
that was overlooked.
After acquiring the folio, use set_folio_extent_mapped() to ensure the
folio private state is valid. This is especially important in the subpage
case, where the private field is an allocated struct containing bitmap and
lock data.
Without this protection, the race below is possible:
[mm] page cache reclaim path [fs] relocation in subpage mode
shrink_folio_list()
folio_trylock() /* lock acquired */
filemap_release_folio()
mapping->a_ops->release_folio()
btrfs_release_folio()
__btrfs_release_folio()
clear_folio_extent_mapped()
btrfs_detach_subpage()
subpage = folio_detach_private(folio)
btrfs_free_subpage(subpage)
kfree(subpage) /* point A */
prealloc_file_extent_cluster()
filemap_lock_folio()
folio_try_get() /* inc refcount */
folio_lock() /* wait for lock */
if (...)
...
else if (!mapping || !__remove_mapping(..))
/*
* __remove_mapping() returns zero when
* folio_ref_freeze(folio, refcount) fails /* point B */
*/
goto keep_locked /* folio remains in cache */
keep_locked:
folio_unlock(folio) /* lock released */
/* lock acquired */
btrfs_subpage_clear_uptodate()
/* use-after-free */
subpage = folio_get_private(folio)
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.