Note: Versions mentioned in the description apply only to the upstream kernel-debug-devel package and not the kernel-debug-devel package as distributed by RHEL. See How to fix? for RHEL:8 relevant fixed versions and status.

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.

• https://access.redhat.com/security/cve/CVE-2024-49884
• https://git.kernel.org/stable/c/5d949ea75bb529ea6342e83465938a3b0ac51238
• https://git.kernel.org/stable/c/8fe117790b37c84c651e2bad9efc0e7fda73c0e3
• https://git.kernel.org/stable/c/915ac3630488af0ca194dc63b86d99802b4f6e18
• https://git.kernel.org/stable/c/a5401d4c3e2a3d25643c567d26e6de327774a2c9
• https://git.kernel.org/stable/c/c26ab35702f8cd0cdc78f96aa5856bfb77be798f
• https://git.kernel.org/stable/c/cafcc1bd62934547c76abf46c6d0d54f135006fe
• https://git.kernel.org/stable/c/e52f933598b781d291b9297e39c463536da0e185
• https://git.kernel.org/stable/c/393a46f60ea4f249dc9d496d4eb2d542f5e11ade