CVE-2026-64567: btrfs: reject free space cache with more entries than pages

From: Greg Kroah-Hartman <[email protected]>

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

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

btrfs: reject free space cache with more entries than pages

When loading a v1 free space cache, __load_free_space_cache() takes
num_entries and num_bitmaps straight from the on-disk
btrfs_free_space_header. That header is stored in the tree_root under a key
with type 0, which the tree-checker has no case for, so neither count is
validated before the load trusts it.

The load loops num_entries times and maps the next page whenever the current
one runs out, going through io_ctl_check_crc() -> io_ctl_map_page(), which
does io_ctl->pages[io_ctl->index++]. But pages[] is allocated in
io_ctl_init() from the cache inode's i_size, not from num_entries:

	num_pages = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
	io_ctl->pages = kcalloc(num_pages, sizeof(struct page *), GFP_NOFS);

So if num_entries claims more records than the pages can hold, io_ctl->index
runs off the end of pages[]. The write side never hits this because
io_ctl_add_entry() and io_ctl_add_bitmap() both stop once
io_ctl->index >= io_ctl->num_pages; the read side just never had the same
check.

To trigger it, take a clean cache (num_entries = <N> here), set num_entries
in the header to 0x10000, and fix up the leaf checksum so it still passes
the tree-checker. The cache inode has i_size = 65536, so num_pages is 16 and
pages[] is a 16-pointer (kmalloc-128) array. The load now tries to read
65536 entries, io_ctl->index walks up to 16, and pages[16] is read past the
array:

  BUG: KASAN: slab-out-of-bounds in io_ctl_check_crc (fs/btrfs/free-space-cache.c:420 fs/btrfs/free-space-cache.c:565)
  Read of size 8 at addr ffff88800c833a80 by task kworker/u8:3/58
   io_ctl_check_crc (fs/btrfs/free-space-cache.c:420 fs/btrfs/free-space-cache.c:565)
   __load_free_space_cache (fs/btrfs/free-space-cache.c:655 fs/btrfs/free-space-cache.c:820)
   load_free_space_cache (fs/btrfs/free-space-cache.c:1017)
   caching_thread (fs/btrfs/block-group.c:880)
   btrfs_work_helper (fs/btrfs/async-thread.c:312)
   process_one_work
   worker_thread
   kthread
   ret_from_fork

free-space-cache.c:420 is io_ctl_map_page(), inlined into io_ctl_check_crc()
at line 565, which is why that is the frame KASAN names. The out-of-bounds
slot is then treated as a struct page and handed to crc32c(), so the bad
read turns into a GP fault.

Add the missing check to io_ctl_check_crc(), which is where both the entry
loop and the bitmap loop end up. When num_entries is too large the load now
fails like any corrupt cache: __load_free_space_cache() drops it and rebuilds
the free space from the extent tree, so a valid cache is never rejected.

The Linux kernel CVE team has assigned CVE-2026-64567 to this issue.


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

	Issue introduced in 3.2 with commit 5b0e95bf607ddd59b39f52d3d55e6581c817b530 and fixed in 6.6.148 with commit 33878ba25e2638bc0c61623d7a05c9ca2b74c039
	Issue introduced in 3.2 with commit 5b0e95bf607ddd59b39f52d3d55e6581c817b530 and fixed in 6.12.101 with commit 404a0b986e0b6e79738fdf1f0ebbbc43b9acd2a2
	Issue introduced in 3.2 with commit 5b0e95bf607ddd59b39f52d3d55e6581c817b530 and fixed in 6.18.42 with commit 5e1b2ca6b34939e70fb0785e8222b53cf060016f
	Issue introduced in 3.2 with commit 5b0e95bf607ddd59b39f52d3d55e6581c817b530 and fixed in 7.1.6 with commit f9fef131fa3f59b857217f522fa5ea430d1b707c
	Issue introduced in 3.2 with commit 5b0e95bf607ddd59b39f52d3d55e6581c817b530 and fixed in 7.2-rc4 with commit a2d8d5647ed854e38f941741aea45b9eb15a6350

Please see https://www.kernel.org for 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-2026-64567
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/btrfs/free-space-cache.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/33878ba25e2638bc0c61623d7a05c9ca2b74c039
	https://git.kernel.org/stable/c/404a0b986e0b6e79738fdf1f0ebbbc43b9acd2a2
	https://git.kernel.org/stable/c/5e1b2ca6b34939e70fb0785e8222b53cf060016f
	https://git.kernel.org/stable/c/f9fef131fa3f59b857217f522fa5ea430d1b707c
	https://git.kernel.org/stable/c/a2d8d5647ed854e38f941741aea45b9eb15a6350