Subject: CVE-2024-49884: ext4: fix slab-use-after-free in ext4_split_extent_at()

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

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

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

ext4: fix slab-use-after-free in ext4_split_extent_at()

We hit the following use-after-free:

==================================================================
BUG: KASAN: slab-use-after-free in ext4_split_extent_at+0xba8/0xcc0
Read of size 2 at addr ffff88810548ed08 by task kworker/u20:0/40
CPU: 0 PID: 40 Comm: kworker/u20:0 Not tainted 6.9.0-dirty #724
Call Trace:
 <TASK>
 kasan_report+0x93/0xc0
 ext4_split_extent_at+0xba8/0xcc0
 ext4_split_extent.isra.0+0x18f/0x500
 ext4_split_convert_extents+0x275/0x750
 ext4_ext_handle_unwritten_extents+0x73e/0x1580
 ext4_ext_map_blocks+0xe20/0x2dc0
 ext4_map_blocks+0x724/0x1700
 ext4_do_writepages+0x12d6/0x2a70
[...]

Allocated by task 40:
 __kmalloc_noprof+0x1ac/0x480
 ext4_find_extent+0xf3b/0x1e70
 ext4_ext_map_blocks+0x188/0x2dc0
 ext4_map_blocks+0x724/0x1700
 ext4_do_writepages+0x12d6/0x2a70
[...]

Freed by task 40:
 kfree+0xf1/0x2b0
 ext4_find_extent+0xa71/0x1e70
 ext4_ext_insert_extent+0xa22/0x3260
 ext4_split_extent_at+0x3ef/0xcc0
 ext4_split_extent.isra.0+0x18f/0x500
 ext4_split_convert_extents+0x275/0x750
 ext4_ext_handle_unwritten_extents+0x73e/0x1580
 ext4_ext_map_blocks+0xe20/0x2dc0
 ext4_map_blocks+0x724/0x1700
 ext4_do_writepages+0x12d6/0x2a70
[...]
==================================================================

The flow of issue triggering is as follows:

ext4_split_extent_at
  path = *ppath
  ext4_ext_insert_extent(ppath)
    ext4_ext_create_new_leaf(ppath)
      ext4_find_extent(orig_path)
        path = *orig_path
        read_extent_tree_block
          // return -ENOMEM or -EIO
        ext4_free_ext_path(path)
          kfree(path)
        *orig_path = NULL
  a. If err is -ENOMEM:
  ext4_ext_dirty(path + path->p_depth)
  // path use-after-free !!!
  b. If err is -EIO and we have EXT_DEBUG defined:
  ext4_ext_show_leaf(path)
    eh = path[depth].p_hdr
    // path also use-after-free !!!

So when trying to zeroout or fix the extent length, call ext4_find_extent()
to update the path.

In addition we use *ppath directly as an ext4_ext_show_leaf() input to
avoid possible use-after-free when EXT_DEBUG is defined, and to avoid
unnecessary path updates.

The Linux kernel CVE team has assigned CVE-2024-49884 to this issue.

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

	Issue introduced in 3.18 with commit dfe5080939ea and fixed in 5.10.227 with commit e52f933598b7
	Issue introduced in 3.18 with commit dfe5080939ea and fixed in 5.15.168 with commit cafcc1bd6293
	Issue introduced in 3.18 with commit dfe5080939ea and fixed in 6.1.113 with commit a5401d4c3e2a
	Issue introduced in 3.18 with commit dfe5080939ea and fixed in 6.6.55 with commit 8fe117790b37
	Issue introduced in 3.18 with commit dfe5080939ea and fixed in 6.10.14 with commit 5d949ea75bb5
	Issue introduced in 3.18 with commit dfe5080939ea and fixed in 6.11.3 with commit 915ac3630488
	Issue introduced in 3.18 with commit dfe5080939ea and fixed in 6.12-rc1 with commit c26ab35702f8

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-2024-49884
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/ext4/extents.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/e52f933598b781d291b9297e39c463536da0e185
	https://git.kernel.org/stable/c/cafcc1bd62934547c76abf46c6d0d54f135006fe
	https://git.kernel.org/stable/c/a5401d4c3e2a3d25643c567d26e6de327774a2c9
	https://git.kernel.org/stable/c/8fe117790b37c84c651e2bad9efc0e7fda73c0e3
	https://git.kernel.org/stable/c/5d949ea75bb529ea6342e83465938a3b0ac51238
	https://git.kernel.org/stable/c/915ac3630488af0ca194dc63b86d99802b4f6e18
	https://git.kernel.org/stable/c/c26ab35702f8cd0cdc78f96aa5856bfb77be798f