CVE-2026-99275: smb: client: reject short SESSION_SETUP responses

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

smb: client: reject short SESSIONSETUP responses

smb2checkmessage() accepts frames shorter than struct smb2pdu when the SMB2 status is non-zero, before the minimum-PDU check added by b4694f269e66d. A 64-byte STATUSMOREPROCESSINGREQUIRED SESSIONSETUP response is therefore accepted even though it cannot contain the SecurityBufferOffset and SecurityBufferLength fields.

The parser then reads those fields from stale bytes left in the reused receive buffer by an earlier frame. With a long stale SecurityBufferLength, decodentlmsspchallenge() copies up to about 64 KB from beyond the receive object and sends the data back to the server in the NTLMv2 AUTHENTICATE target info.

Reject short MPR SESSIONSETUP responses before the short-error path, and validate SecurityBufferOffset + SecurityBufferLength against the received response length before decoding the challenge.

The ordering bypass is not new: the short-error early return predates b4694f269e66d, and the unvalidated SecurityBufferLength read predates the 2016 raw NTLMSSP split in 166cea4dc3a4, so all supported stable trees are affected. Reproduced on 6.6.87.2; the path is unchanged on mainline.