CVE-2026-64578: ksmbd: validate compound request size before reading StructureSize2

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

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

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

ksmbd: validate compound request size before reading StructureSize2

When ksmbd validates a compound (chained) SMB2 request,
ksmbd_smb2_check_message() reads pdu->StructureSize2 without first
checking that the compound element is large enough to contain it.
StructureSize2 is a 2-byte field at offset 64
(__SMB2_HEADER_STRUCTURE_SIZE) from the start of each element.

The compound-walking logic only guarantees that a full 64-byte SMB2
header is present for the trailing element: when NextCommand is 0, len is
reduced to the number of bytes remaining after next_smb2_rcv_hdr_off. A
remote client can craft a compound request whose last element has exactly
64 bytes, so the 2-byte StructureSize2 read at offset 64 extends one byte
past the receive buffer, producing a slab-out-of-bounds read.

  BUG: KASAN: slab-out-of-bounds in ksmbd_smb2_check_message (fs/smb/server/smb2misc.c:402)
  Read of size 2 at addr ffff888012ae31ac by task kworker/0:1/14
  The buggy address is located 172 bytes inside of allocated 173-byte region
  Workqueue: ksmbd-io handle_ksmbd_work
  Call Trace:
   ...
   kasan_report (mm/kasan/report.c:595)
   ksmbd_smb2_check_message (fs/smb/server/smb2misc.c:402)
   handle_ksmbd_work (fs/smb/server/server.c:119)
   process_one_work (kernel/workqueue.c:3314)
   worker_thread (kernel/workqueue.c:3397)
   kthread (kernel/kthread.c:436)
   ret_from_fork (arch/x86/kernel/process.c:158)
   ret_from_fork_asm (arch/x86/entry/entry_64.S:245)

Reject any compound element that is too small to hold StructureSize2
before dereferencing it.

The Linux kernel CVE team has assigned CVE-2026-64578 to this issue.


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

	Issue introduced in 5.15 with commit e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9 and fixed in 6.6.148 with commit 2c307126ed8e7adddab82b8e31d962d3a2156ab1
	Issue introduced in 5.15 with commit e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9 and fixed in 6.12.101 with commit f7550a91ab211726f59cb137523b7a9eae1ac6eb
	Issue introduced in 5.15 with commit e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9 and fixed in 6.18.42 with commit f0e337e7db67cc1c832958bbb6c4026bdceacfdb
	Issue introduced in 5.15 with commit e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9 and fixed in 7.1.6 with commit ea128f06d2fb2186f0cac0c9f3e953e4d1f5c29a
	Issue introduced in 5.15 with commit e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9 and fixed in 7.2-rc4 with commit 15b38176fd1530372905c602fde51fe89ec8c877

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-2026-64578
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/smb/server/smb2misc.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/2c307126ed8e7adddab82b8e31d962d3a2156ab1
	https://git.kernel.org/stable/c/f7550a91ab211726f59cb137523b7a9eae1ac6eb
	https://git.kernel.org/stable/c/f0e337e7db67cc1c832958bbb6c4026bdceacfdb
	https://git.kernel.org/stable/c/ea128f06d2fb2186f0cac0c9f3e953e4d1f5c29a
	https://git.kernel.org/stable/c/15b38176fd1530372905c602fde51fe89ec8c877