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

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, ksmbdsmb2checkmessage() 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 (SMB2HEADERSTRUCTURESIZE) 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 nextsmb2rcvhdroff. 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 ksmbdsmb2checkmessage (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 handleksmbdwork Call Trace: ... kasanreport (mm/kasan/report.c:595) ksmbdsmb2checkmessage (fs/smb/server/smb2misc.c:402) handleksmbdwork (fs/smb/server/server.c:119) processonework (kernel/workqueue.c:3314) workerthread (kernel/workqueue.c:3397) kthread (kernel/kthread.c:436) retfromfork (arch/x86/kernel/process.c:158) retfromforkasm (arch/x86/entry/entry64.S:245)

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