CVE-2024-38306 btrfs: protect folio::private when attaching extent buffer folios
In the Linux kernel, the following vulnerability has been resolved:

btrfs: protect folio::private when attaching extent buffer folios

[BUG]
Since v6.8 there are rare kernel crashes reported by various people,
the common factor is bad page status error messages like this:

  BUG: Bad page state in process kswapd0  pfn:d6e840
  page: refcount:0 mapcount:0 mapping:000000007512f4f2 index:0x2796c2c7c
  pfn:0xd6e840
  aops:btree_aops ino:1
  flags: 0x17ffffe0000008(uptodate|node=0|zone=2|lastcpupid=0x3fffff)
  page_type: 0xffffffff()
  raw: 0017ffffe0000008 dead000000000100 dead000000000122 ffff88826d0be4c0
  raw: 00000002796c2c7c 0000000000000000 00000000ffffffff 0000000000000000
  page dumped because: non-NULL mapping

[CAUSE]
Commit 09e6cef19c9f ("btrfs: refactor alloc_extent_buffer() to
allocate-then-attach method") changes the sequence when allocating a new
extent buffer.

Previously we always called grab_extent_buffer() under
mapping->i_private_lock, to ensure the safety on modification on
folio::private (which is a pointer to extent buffer for regular
sectorsize).

This can lead to the following race:

Thread A is trying to allocate an extent buffer at bytenr X, with 4
4K pages, meanwhile thread B is trying to release the page at X + 4K
(the second page of the extent buffer at X).

           Thread A                |                 Thread B
-----------------------------------+-------------------------------------
                                   | btree_release_folio()
				   | | This is for the page at X + 4K,
				   | | Not page X.
				   | |
alloc_extent_buffer()              | |- release_extent_buffer()
|- filemap_add_folio() for the     | |  |- atomic_dec_and_test(eb->refs)
|  page at bytenr X (the first     | |  |
|  page).                          | |  |
|  Which returned -EEXIST.         | |  |
|                                  | |  |
|- filemap_lock_folio()            | |  |
|  Returned the first page locked. | |  |
|                                  | |  |
|- grab_extent_buffer()            | |  |
|  |- atomic_inc_not_zero()        | |  |
|  |  Returned false               | |  |
|  |- folio_detach_private()       | |  |- folio_detach_private() for X
|     |- folio_test_private()      | |     |- folio_test_private()
      |  Returned true             | |     |  Returned true
      |- folio_put()               |       |- folio_put()

Now there are two puts on the same folio at folio X, leading to refcount
underflow of the folio X, and eventually causing the BUG_ON() on the
page->mapping.

The condition is not that easy to hit:

- The release must be triggered for the middle page of an eb
  If the release is on the same first page of an eb, page lock would kick
  in and prevent the race.

- folio_detach_private() has a very small race window
  It's only between folio_test_private() and folio_clear_private().

That's exactly when mapping->i_private_lock is used to prevent such race,
and commit 09e6cef19c9f ("btrfs: refactor alloc_extent_buffer() to
allocate-then-attach method") screwed that up.

At that time, I thought the page lock would kick in as
filemap_release_folio() also requires the page to be locked, but forgot
the filemap_release_folio() only locks one page, not all pages of an
extent buffer.

[FIX]
Move all the code requiring i_private_lock into
attach_eb_folio_to_filemap(), so that everything is done with proper
lock protection.

Furthermore to prevent future problems, add an extra
lockdep_assert_locked() to ensure we're holding the proper lock.

To reproducer that is able to hit the race (takes a few minutes with
instrumented code inserting delays to alloc_extent_buffer()):

  #!/bin/sh
  drop_caches () {
	  while(true); do
		  echo 3 > /proc/sys/vm/drop_caches
		  echo 1 > /proc/sys/vm/compact_memory
	  done
  }

  run_tar () {
	  while(true); do
		  for x in `seq 1 80` ; do
			  tar cf /dev/zero /mnt > /dev/null &
		  done
		  wait
	  done
  }

  mkfs.btrfs -f -d single -m single
---truncated---
CVSS v3.1 score: 7.8