Subject: CVE-2024-26727: btrfs: do not ASSERT() if the newly created subvolume already got read

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

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

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

btrfs: do not ASSERT() if the newly created subvolume already got read

[BUG]
There is a syzbot crash, triggered by the ASSERT() during subvolume
creation:

 assertion failed: !anon_dev, in fs/btrfs/disk-io.c:1319
 ------------[ cut here ]------------
 kernel BUG at fs/btrfs/disk-io.c:1319!
 invalid opcode: 0000 [#1] PREEMPT SMP KASAN
 RIP: 0010:btrfs_get_root_ref.part.0+0x9aa/0xa60
  <TASK>
  btrfs_get_new_fs_root+0xd3/0xf0
  create_subvol+0xd02/0x1650
  btrfs_mksubvol+0xe95/0x12b0
  __btrfs_ioctl_snap_create+0x2f9/0x4f0
  btrfs_ioctl_snap_create+0x16b/0x200
  btrfs_ioctl+0x35f0/0x5cf0
  __x64_sys_ioctl+0x19d/0x210
  do_syscall_64+0x3f/0xe0
  entry_SYSCALL_64_after_hwframe+0x63/0x6b
 ---[ end trace 0000000000000000 ]---

[CAUSE]
During create_subvol(), after inserting root item for the newly created
subvolume, we would trigger btrfs_get_new_fs_root() to get the
btrfs_root of that subvolume.

The idea here is, we have preallocated an anonymous device number for
the subvolume, thus we can assign it to the new subvolume.

But there is really nothing preventing things like backref walk to read
the new subvolume.
If that happens before we call btrfs_get_new_fs_root(), the subvolume
would be read out, with a new anonymous device number assigned already.

In that case, we would trigger ASSERT(), as we really expect no one to
read out that subvolume (which is not yet accessible from the fs).
But things like backref walk is still possible to trigger the read on
the subvolume.

Thus our assumption on the ASSERT() is not correct in the first place.

[FIX]
Fix it by removing the ASSERT(), and just free the @anon_dev, reset it
to 0, and continue.

If the subvolume tree is read out by something else, it should have
already get a new anon_dev assigned thus we only need to free the
preallocated one.

The Linux kernel CVE team has assigned CVE-2024-26727 to this issue.

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

	Issue introduced in 5.9 with commit 2dfb1e43f57d and fixed in 5.10.210 with commit 3f5d47eb163b
	Issue introduced in 5.9 with commit 2dfb1e43f57d and fixed in 5.15.149 with commit e31546b0f34a
	Issue introduced in 5.9 with commit 2dfb1e43f57d and fixed in 6.1.79 with commit 66b317a2fc45
	Issue introduced in 5.9 with commit 2dfb1e43f57d and fixed in 6.6.18 with commit 833775656d44
	Issue introduced in 5.9 with commit 2dfb1e43f57d and fixed in 6.7.6 with commit 5a172344bfda
	Issue introduced in 5.9 with commit 2dfb1e43f57d and fixed in 6.8 with commit e03ee2fe873e
	Issue introduced in 5.8.3 with commit 917d608fe375

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-2024-26727
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/disk-io.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/3f5d47eb163bceb1b9e613c9003bae5fefc0046f
	https://git.kernel.org/stable/c/e31546b0f34af21738c4ceac47d662c00ee6382f
	https://git.kernel.org/stable/c/66b317a2fc45b2ef66527ee3f8fa08fb5beab88d
	https://git.kernel.org/stable/c/833775656d447c545133a744a0ed1e189ce61430
	https://git.kernel.org/stable/c/5a172344bfdabb46458e03708735d7b1a918c468
	https://git.kernel.org/stable/c/e03ee2fe873eb68c1f9ba5112fee70303ebf9dfb