Subject: CVE-2024-26726: btrfs: don't drop extent_map for free space inode on write error

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

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

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

btrfs: don't drop extent_map for free space inode on write error

While running the CI for an unrelated change I hit the following panic
with generic/648 on btrfs_holes_spacecache.

assertion failed: block_start != EXTENT_MAP_HOLE, in fs/btrfs/extent_io.c:1385
------------[ cut here ]------------
kernel BUG at fs/btrfs/extent_io.c:1385!
invalid opcode: 0000 [#1] PREEMPT SMP NOPTI
CPU: 1 PID: 2695096 Comm: fsstress Kdump: loaded Tainted: G        W          6.8.0-rc2+ #1
RIP: 0010:__extent_writepage_io.constprop.0+0x4c1/0x5c0
Call Trace:
 <TASK>
 extent_write_cache_pages+0x2ac/0x8f0
 extent_writepages+0x87/0x110
 do_writepages+0xd5/0x1f0
 filemap_fdatawrite_wbc+0x63/0x90
 __filemap_fdatawrite_range+0x5c/0x80
 btrfs_fdatawrite_range+0x1f/0x50
 btrfs_write_out_cache+0x507/0x560
 btrfs_write_dirty_block_groups+0x32a/0x420
 commit_cowonly_roots+0x21b/0x290
 btrfs_commit_transaction+0x813/0x1360
 btrfs_sync_file+0x51a/0x640
 __x64_sys_fdatasync+0x52/0x90
 do_syscall_64+0x9c/0x190
 entry_SYSCALL_64_after_hwframe+0x6e/0x76

This happens because we fail to write out the free space cache in one
instance, come back around and attempt to write it again.  However on
the second pass through we go to call btrfs_get_extent() on the inode to
get the extent mapping.  Because this is a new block group, and with the
free space inode we always search the commit root to avoid deadlocking
with the tree, we find nothing and return a EXTENT_MAP_HOLE for the
requested range.

This happens because the first time we try to write the space cache out
we hit an error, and on an error we drop the extent mapping.  This is
normal for normal files, but the free space cache inode is special.  We
always expect the extent map to be correct.  Thus the second time
through we end up with a bogus extent map.

Since we're deprecating this feature, the most straightforward way to
fix this is to simply skip dropping the extent map range for this failed
range.

I shortened the test by using error injection to stress the area to make
it easier to reproduce.  With this patch in place we no longer panic
with my error injection test.

The Linux kernel CVE team has assigned CVE-2024-26726 to this issue.

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

	Fixed in 6.1.79 with commit 02f2b95b00bf
	Fixed in 6.6.18 with commit 7bddf18f474f
	Fixed in 6.7.6 with commit a4b7741c8302
	Fixed in 6.8 with commit 5571e41ec6e5

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-2024-26726
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/inode.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/02f2b95b00bf57d20320ee168b30fb7f3db8e555
	https://git.kernel.org/stable/c/7bddf18f474f166c19f91b2baf67bf7c5eda03f7
	https://git.kernel.org/stable/c/a4b7741c8302e28073bfc6dd1c2e73598e5e535e
	https://git.kernel.org/stable/c/5571e41ec6e56e35f34ae9f5b3a335ef510e0ade