Subject: CVE-2024-49868: btrfs: fix a NULL pointer dereference when failed to start a new trasacntion

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

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

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

btrfs: fix a NULL pointer dereference when failed to start a new trasacntion

[BUG]
Syzbot reported a NULL pointer dereference with the following crash:

  FAULT_INJECTION: forcing a failure.
   start_transaction+0x830/0x1670 fs/btrfs/transaction.c:676
   prepare_to_relocate+0x31f/0x4c0 fs/btrfs/relocation.c:3642
   relocate_block_group+0x169/0xd20 fs/btrfs/relocation.c:3678
  ...
  BTRFS info (device loop0): balance: ended with status: -12
  Oops: general protection fault, probably for non-canonical address 0xdffffc00000000cc: 0000 [#1] PREEMPT SMP KASAN NOPTI
  KASAN: null-ptr-deref in range [0x0000000000000660-0x0000000000000667]
  RIP: 0010:btrfs_update_reloc_root+0x362/0xa80 fs/btrfs/relocation.c:926
  Call Trace:
   <TASK>
   commit_fs_roots+0x2ee/0x720 fs/btrfs/transaction.c:1496
   btrfs_commit_transaction+0xfaf/0x3740 fs/btrfs/transaction.c:2430
   del_balance_item fs/btrfs/volumes.c:3678 [inline]
   reset_balance_state+0x25e/0x3c0 fs/btrfs/volumes.c:3742
   btrfs_balance+0xead/0x10c0 fs/btrfs/volumes.c:4574
   btrfs_ioctl_balance+0x493/0x7c0 fs/btrfs/ioctl.c:3673
   vfs_ioctl fs/ioctl.c:51 [inline]
   __do_sys_ioctl fs/ioctl.c:907 [inline]
   __se_sys_ioctl+0xf9/0x170 fs/ioctl.c:893
   do_syscall_x64 arch/x86/entry/common.c:52 [inline]
   do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
   entry_SYSCALL_64_after_hwframe+0x77/0x7f

[CAUSE]
The allocation failure happens at the start_transaction() inside
prepare_to_relocate(), and during the error handling we call
unset_reloc_control(), which makes fs_info->balance_ctl to be NULL.

Then we continue the error path cleanup in btrfs_balance() by calling
reset_balance_state() which will call del_balance_item() to fully delete
the balance item in the root tree.

However during the small window between set_reloc_contrl() and
unset_reloc_control(), we can have a subvolume tree update and created a
reloc_root for that subvolume.

Then we go into the final btrfs_commit_transaction() of
del_balance_item(), and into btrfs_update_reloc_root() inside
commit_fs_roots().

That function checks if fs_info->reloc_ctl is in the merge_reloc_tree
stage, but since fs_info->reloc_ctl is NULL, it results a NULL pointer
dereference.

[FIX]
Just add extra check on fs_info->reloc_ctl inside
btrfs_update_reloc_root(), before checking
fs_info->reloc_ctl->merge_reloc_tree.

That DEAD_RELOC_TREE handling is to prevent further modification to the
reloc tree during merge stage, but since there is no reloc_ctl at all,
we do not need to bother that.

The Linux kernel CVE team has assigned CVE-2024-49868 to this issue.

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

	Fixed in 5.10.227 with commit d73d48acf36f
	Fixed in 5.15.168 with commit 37fee9c220b9
	Fixed in 6.1.113 with commit d13249c0df7a
	Fixed in 6.6.55 with commit 7ad0c5868f2f
	Fixed in 6.10.14 with commit dc02c1440705
	Fixed in 6.11.3 with commit 39356ec0e319
	Fixed in 6.12-rc2 with commit c3b47f49e831

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-2024-49868
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/relocation.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/d73d48acf36f57362df7e4f9d76568168bf5e944
	https://git.kernel.org/stable/c/37fee9c220b92c3b7bf22b51c51dde5364e7590b
	https://git.kernel.org/stable/c/d13249c0df7aab885acb149695f82c54c0822a70
	https://git.kernel.org/stable/c/7ad0c5868f2f0418619089513d95230c66cb7eb4
	https://git.kernel.org/stable/c/dc02c1440705e3451abd1c2c8114a5c1bb188e9f
	https://git.kernel.org/stable/c/39356ec0e319ed07627b3a0f402d0608546509e6
	https://git.kernel.org/stable/c/c3b47f49e83197e8dffd023ec568403bcdbb774b