linux-cve-announce - CVE-2025-40128: btrfs: fix symbolic link reading when bs

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Message-ID: <2025111253-CVE-2025-40128-2dd9@gregkh>
Date: Wed, 12 Nov 2025 19:24:04 +0900
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2025-40128: btrfs: fix symbolic link reading when bs > ps

From: Greg Kroah-Hartman <gregkh@...nel.org>

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

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

btrfs: fix symbolic link reading when bs > ps

[BUG DURING BS > PS TEST]
When running the following script on a btrfs whose block size is larger
than page size, e.g. 8K block size and 4K page size, it will trigger a
kernel BUG:

  # mkfs.btrfs -s 8k $dev
  # mount $dev $mnt
  # mkdir $mnt/dir
  # ln -s dir $mnt/link
  # ls $mnt/link

The call trace looks like this:

  BTRFS warning (device dm-2): support for block size 8192 with page size 4096 is experimental, some features may be missing
  BTRFS info (device dm-2): checking UUID tree
  BTRFS info (device dm-2): enabling ssd optimizations
  BTRFS info (device dm-2): enabling free space tree
  ------------[ cut here ]------------
  kernel BUG at /home/adam/linux/include/linux/highmem.h:275!
  Oops: invalid opcode: 0000 [#1] SMP
  CPU: 8 UID: 0 PID: 667 Comm: ls Tainted: G           OE       6.17.0-rc4-custom+ #283 PREEMPT(full)
  Tainted: [O]=OOT_MODULE, [E]=UNSIGNED_MODULE
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS unknown 02/02/2022
  RIP: 0010:zero_user_segments.constprop.0+0xdc/0xe0 [btrfs]
  Call Trace:
   <TASK>
   btrfs_get_extent.cold+0x85/0x101 [btrfs 7453c70c03e631c8d8bfdd4264fa62d3e238da6f]
   btrfs_do_readpage+0x244/0x750 [btrfs 7453c70c03e631c8d8bfdd4264fa62d3e238da6f]
   btrfs_read_folio+0x9c/0x100 [btrfs 7453c70c03e631c8d8bfdd4264fa62d3e238da6f]
   filemap_read_folio+0x37/0xe0
   do_read_cache_folio+0x94/0x3e0
   __page_get_link.isra.0+0x20/0x90
   page_get_link+0x16/0x40
   step_into+0x69b/0x830
   path_lookupat+0xa7/0x170
   filename_lookup+0xf7/0x200
   ? set_ptes.isra.0+0x36/0x70
   vfs_statx+0x7a/0x160
   do_statx+0x63/0xa0
   __x64_sys_statx+0x90/0xe0
   do_syscall_64+0x82/0xae0
   entry_SYSCALL_64_after_hwframe+0x4b/0x53
   </TASK>

Please note bs > ps support is still under development and the
enablement patch is not even in btrfs development branch.

[CAUSE]
Btrfs reuses its data folio read path to handle symbolic links, as the
symbolic link target is stored as an inline data extent.

But for newly created inodes, btrfs only set the minimal order if the
target inode is a regular file.

Thus for above newly created symbolic link, it doesn't properly respect
the minimal folio order, and triggered the above crash.

[FIX]
Call btrfs_set_inode_mapping_order() unconditionally inside
btrfs_create_new_inode().

For symbolic links this will fix the crash as now the folio will meet
the minimal order.

For regular files this brings no change.

For directory/bdev/char and all the other types of inodes, they won't
go through the data read path, thus no effect either.

The Linux kernel CVE team has assigned CVE-2025-40128 to this issue.


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

	Issue introduced in 6.17 with commit cc38d178ff33543cdb0bd58cfbb9a7c41372ff75 and fixed in 6.17.3 with commit 3ea252a5c48dd3a4e1f7d0c53d3b0f7b648becc9
	Issue introduced in 6.17 with commit cc38d178ff33543cdb0bd58cfbb9a7c41372ff75 and fixed in 6.18-rc1 with commit 67378b754608a3524d125bfa5744508a49fe48be

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-2025-40128
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/3ea252a5c48dd3a4e1f7d0c53d3b0f7b648becc9
	https://git.kernel.org/stable/c/67378b754608a3524d125bfa5744508a49fe48be