Namjae, Tom, Re-checked v5 against the updated MS-SMB2 text. The ~0 exemption was my mistake - the spec has no wildcard for non-related operations. Dropped it in v6. The other checks already match, decompression included. No other changes. From a320c66f1aeb773cf65d2489fae9f982eaf3873f Mon Sep 17 00:00:00 2001 From: Dairui Zhang Date: Tue, 29 Sep 2026 22:30:00 +0800 Subject: [PATCH v6] ksmbd: verify transform SessionId matches the decrypted header When an encrypted request comes in, the decryption key is picked by the SessionId in the transform header, but the request is then authorized under whatever session the decrypted inner header names. Nothing ever compares the two, so on a connection with more than one session a client can get a request decrypted with one session's key and run it as another session. Encrypted requests are exempt from signing too, so a valid AEAD tag is the only proof of session identity, and it is checked against the wrong session. MS-SMB2 says the server must check that the transform SessionId matches the one in the decrypted SMB2 header and treat a mismatch as a protocol error. Add that check in __handle_ksmbd_work() after decryption, before dispatching: the message has to be at least one fixed SMB2 header, the first operation must not set SMB2_FLAGS_RELATED_OPERATIONS and its SessionId must match the transform SessionId, and every following non-related operation in a compound chain must carry the same SessionId. NextCommand offsets must be 8-byte aligned and stay inside the message; the bounds check is done by subtraction so it cannot wrap. When the decrypted payload is a compression transform, the existing decompression step runs first and the decompressed SMB2 message is checked instead, since a compression transform header carries no SessionId. Fixes: e2f34481b24d ("cifsd: add server-side procedures for SMB3") Reported-by: Dairui Zhang Assisted-by: LLM Cc: stable@vger.kernel.org Signed-off-by: Dairui Zhang --- v4 -> v5: - Move the check into __handle_ksmbd_work(), after the existing decompression step, instead of rejecting encrypted compression transforms in smb3_decrypt_req(), per Namjae's review. The transform SessionId is saved before decryption. - Use subtraction for the compound-walk bounds check; the addition form could wrap on 32-bit. - Add the Fixes tag and drop the incorrect claims about the decompression step and ProtocolId check from the commit message. v5 -> v6: - Drop the SessionId ~0 exemption for subsequent non-related operations, per Tom Talpey's review. --- fs/smb/server/server.c | 16 ++++++++++++ fs/smb/server/smb2pdu.c | 54 ++++++++++++++++++++++++++++++++++++++++ fs/smb/server/smb2pdu.h | 1 + 3 files changed, 71 insertions(+) diff --git a/fs/smb/server/server.c b/fs/smb/server/server.c --- a/fs/smb/server/server.c +++ b/fs/smb/server/server.c @@ -187,6 +187,16 @@ if (conn->ops->is_transform_hdr && conn->ops->is_transform_hdr(work->request_buf)) { + u64 tr_sess_id; + + if (get_rfc1002_len(work->request_buf) < + sizeof(struct smb2_transform_hdr)) { + ksmbd_conn_abort(conn); + return; + } + tr_sess_id = le64_to_cpu(((struct smb2_transform_hdr *) + smb_get_msg(work->request_buf))->SessionId); + rc = conn->ops->decrypt_req(work); if (rc < 0) { ksmbd_conn_abort(conn); @@ -215,6 +225,12 @@ ksmbd_conn_abort(conn); return; } + + rc = ksmbd_check_transform_session(work, tr_sess_id); + if (rc < 0) { + ksmbd_conn_abort(conn); + return; + } } if (conn->ops->allocate_rsp_buf(work)) diff --git a/fs/smb/server/smb2pdu.c b/fs/smb/server/smb2pdu.c --- a/fs/smb/server/smb2pdu.c +++ b/fs/smb/server/smb2pdu.c @@ -12256,6 +12256,60 @@ return trhdr->ProtocolId == SMB2_TRANSFORM_PROTO_NUM; } +/* + * The decryption key is selected by the transform SessionId, but + * the request is authorized under the session named in the + * decrypted SMB2 header. Per MS-SMB2 the two must match, so check + * the decrypted message and its compound chain before dispatch. + */ +int ksmbd_check_transform_session(struct ksmbd_work *work, u64 tr_sess_id) +{ + struct smb2_hdr *hdr = smb_get_msg(work->request_buf); + size_t msg_len = get_rfc1002_len(work->request_buf); + size_t off = 0; + bool first = true; + + if (msg_len < sizeof(*hdr)) { + pr_err_ratelimited("Decrypted message is smaller than SMB2 header\n"); + return -ECONNABORTED; + } + + for (;;) { + u64 sid = le64_to_cpu(hdr->SessionId); + u32 next; + + if (first) { + if (hdr->Flags & SMB2_FLAGS_RELATED_OPERATIONS) { + pr_err_ratelimited("RELATED_OPERATIONS set on first operation\n"); + return -ECONNABORTED; + } + if (sid != tr_sess_id) { + pr_err_ratelimited("SessionId mismatch between transform and inner header\n"); + return -ECONNABORTED; + } + first = false; + } else if (!(hdr->Flags & SMB2_FLAGS_RELATED_OPERATIONS) && + sid != tr_sess_id) { + pr_err_ratelimited("SessionId mismatch in compound chain\n"); + return -ECONNABORTED; + } + + next = le32_to_cpu(hdr->NextCommand); + if (!next) + return 0; + if (next % 8) { + pr_err_ratelimited("NextCommand %u is not 8-byte aligned\n", next); + return -ECONNABORTED; + } + if (next > msg_len - off - sizeof(*hdr)) { + pr_err_ratelimited("NextCommand %u is out of the message\n", next); + return -ECONNABORTED; + } + off += next; + hdr = (struct smb2_hdr *)((u8 *)smb_get_msg(work->request_buf) + off); + } +} + int smb3_decrypt_req(struct ksmbd_work *work) { char *buf = work->request_buf; diff --git a/fs/smb/server/smb2pdu.h b/fs/smb/server/smb2pdu.h --- a/fs/smb/server/smb2pdu.h +++ b/fs/smb/server/smb2pdu.h @@ -416,6 +416,7 @@ void smb3_preauth_hash_rsp(struct ksmbd_work *work); bool smb3_is_transform_hdr(void *buf); int smb3_decrypt_req(struct ksmbd_work *work); +int ksmbd_check_transform_session(struct ksmbd_work *work, u64 tr_sess_id); int smb3_encrypt_resp(struct ksmbd_work *work); bool smb3_11_final_sess_setup_resp(struct ksmbd_work *work); int smb2_set_rsp_credits(struct ksmbd_work *work); -- 2.53.0