[PATCH] ksmbd: verify transform SessionId matches the decrypted header

[Dairui Zhang <zhangdairui@gmail.com>]

The decryption key for an encrypted request is selected by the
SessionId in the encryption transform header, but the request is
then authorized under the session named in the decrypted inner SMB2
header. Nothing compares the two, so on a connection carrying more
than one session a client can have a request decrypted with one
session's key and executed under another session's identity. Since
encrypted requests are also exempt from the signing requirement, the
AEAD tag is the only proof of session identity, and it is checked
against the wrong session.

Per MS-SMB2 the server must verify that the SessionId in the
transform header matches the one in the decrypted SMB2 header and
treat a mismatch as a protocol error. Validate the whole decrypted
message before dispatching it: the message must 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; each subsequent operation in a compound chain
must either set SMB2_FLAGS_RELATED_OPERATIONS or carry the same
SessionId; and NextCommand offsets must be 8-byte aligned and within
the message. When the encrypted payload is a compression transform,
the transform SessionId is saved during decryption and the same
validation runs on the decompressed message. (The 8-byte alignment
of compound responses is already handled by the existing
chained-response padding.)

Reported-by: Dairui Zhang <zhangdairui@gmail.com>
Assisted-by: LLM
Cc: stable@vger.kernel.org
Signed-off-by: Dairui Zhang <zhangdairui@gmail.com>
---
v2 -> v3:
- Per review, extend the check to the whole decrypted message before
  dispatch: first-op size/RELATED_OPERATIONS/SessionId rules and
  per-op compound-chain SessionId and NextCommand validation, shared
  between the plain and compression paths via a common helper.
v1 -> v2:
- Cover the compression-transform case.
- Reject a decrypted SMB2 message smaller than the fixed header
  before reading its SessionId.
---
 fs/smb/server/compress.c   | 17 +++++++++
 fs/smb/server/ksmbd_work.h |  4 ++
 fs/smb/server/smb2pdu.c    | 78 ++++++++++++++++++++++++++++++++++++++
 fs/smb/server/smb2pdu.h    |  2 +
 4 files changed, 101 insertions(+)

diff --git a/fs/smb/server/compress.c b/fs/smb/server/compress.c
index 5162fb8..b07b140 100644
--- a/fs/smb/server/compress.c
+++ b/fs/smb/server/compress.c
@@ -10,6 +10,7 @@
 
 #include "compress.h"
 #include "smb_common.h"
+#include "smb2pdu.h"
 #include "../common/compress/lz77.h"
 
 #define SMB_COMPRESS_MIN_LEN	PAGE_SIZE
@@ -125,6 +126,22 @@ int ksmbd_decompress_work_request(struct ksmbd_work *work)
 	if (rc)
 		return rc;
 
+	/*
+	 * The encryption transform SessionId was saved before the
+	 * compression transform was parsed; the decompressed message must
+	 * pass the same session binding validation as a plain decrypted
+	 * message.
+	 */
+	if (work->tr_sess_id) {
+		rc = ksmbd_check_transform_session(
+				(struct smb2_hdr *)smb_get_msg(out_buf),
+				get_rfc1002_len(out_buf), work->tr_sess_id);
+		if (rc) {
+			kvfree(out_buf);
+			return rc;
+		}
+	}
+
 	kvfree(work->request_buf);
 	work->request_buf = out_buf;
 	return 0;
diff --git a/fs/smb/server/ksmbd_work.h b/fs/smb/server/ksmbd_work.h
index 5f1d3eb..3e8bf2a 100644
--- a/fs/smb/server/ksmbd_work.h
+++ b/fs/smb/server/ksmbd_work.h
@@ -82,6 +82,10 @@ struct ksmbd_work {
 	/* Contiguous SMB2 compression transform owned by this work item. */
 	void				*compress_buf;
 
+	/* SessionId from the encryption transform header, for the
+	 * post-decompression SessionId check. Zero when unset. */
+	__u64				tr_sess_id;
+
 	unsigned char			state;
 	/* No response for cancelled request */
 	bool                            send_no_response:1;
diff --git a/fs/smb/server/smb2pdu.c b/fs/smb/server/smb2pdu.c
index 4cf7083..dbb7dcb 100644
--- a/fs/smb/server/smb2pdu.c
+++ b/fs/smb/server/smb2pdu.c
@@ -10852,6 +10852,62 @@ bool smb3_is_transform_hdr(void *buf)
 	return trhdr->ProtocolId == SMB2_TRANSFORM_PROTO_NUM;
 }
 
+/*
+ * Validate the session binding of a decrypted message against the
+ * encryption transform SessionId, per MS-SMB2:
+ *  - the message must 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;
+ *  - each following operation in a compound chain must either set
+ *    SMB2_FLAGS_RELATED_OPERATIONS or carry the same SessionId;
+ *  - NextCommand offsets must be 8-byte aligned and inside the message.
+ * Returns 0 on success, -ECONNABORTED on protocol error.
+ */
+int ksmbd_check_transform_session(struct smb2_hdr *hdr,
+				  unsigned int msg_len, __u64 tr_sess_id)
+{
+	struct smb2_hdr *in_hdr = hdr;
+	bool first = true;
+	u32 off = 0, next;
+
+	if (msg_len < sizeof(struct smb2_hdr)) {
+		pr_err_ratelimited("Decrypted message is smaller than SMB2 header\n");
+		return -ECONNABORTED;
+	}
+
+	for (;;) {
+		if (first) {
+			if (in_hdr->Flags & SMB2_FLAGS_RELATED_OPERATIONS) {
+				pr_err_ratelimited("RELATED_OPERATIONS set on first operation\n");
+				return -ECONNABORTED;
+			}
+			if (le64_to_cpu(in_hdr->SessionId) != tr_sess_id) {
+				pr_err_ratelimited("SessionId mismatch between transform and inner header\n");
+				return -ECONNABORTED;
+			}
+			first = false;
+		} else if (!(in_hdr->Flags & SMB2_FLAGS_RELATED_OPERATIONS) &&
+			   le64_to_cpu(in_hdr->SessionId) != tr_sess_id) {
+			pr_err_ratelimited("SessionId mismatch in compound chain\n");
+			return -ECONNABORTED;
+		}
+
+		next = le32_to_cpu(in_hdr->NextCommand);
+		if (!next)
+			return 0;
+		if (next % 8) {
+			pr_err_ratelimited("NextCommand %u is not 8-byte aligned\n", next);
+			return -ECONNABORTED;
+		}
+		off += next;
+		if (off + sizeof(struct smb2_hdr) > msg_len) {
+			pr_err_ratelimited("NextCommand %u is out of the message\n", next);
+			return -ECONNABORTED;
+		}
+		in_hdr = (struct smb2_hdr *)((u8 *)hdr + off);
+	}
+}
+
 int smb3_decrypt_req(struct ksmbd_work *work)
 {
 	char *buf = work->request_buf;
@@ -10860,6 +10916,7 @@ int smb3_decrypt_req(struct ksmbd_work *work)
 	unsigned int buf_data_size;
 	struct smb2_transform_hdr *tr_hdr = smb_get_msg(buf);
 	unsigned int original_msg_size;
+	__le32 proto;
 	int rc = 0;
 
 	if (pdu_length < sizeof(struct smb2_transform_hdr)) {
@@ -10890,6 +10947,27 @@ int smb3_decrypt_req(struct ksmbd_work *work)
 	if (rc)
 		return rc;
 
+	/*
+	 * The decryption key is selected by the transform header SessionId,
+	 * while the request is authorized under the session named in the
+	 * decrypted inner header. Per MS-SMB2 the two must match, so
+	 * validate the whole decrypted message before dispatching it. A
+	 * compression transform payload carries the session id only after
+	 * decompression, so save the transform SessionId for that check
+	 * instead.
+	 */
+	proto = ((struct smb2_hdr *)iov[1].iov_base)->ProtocolId;
+	if (proto == SMB2_PROTO_NUMBER) {
+		rc = ksmbd_check_transform_session(
+				(struct smb2_hdr *)iov[1].iov_base,
+				original_msg_size,
+				le64_to_cpu(tr_hdr->SessionId));
+		if (rc)
+			return rc;
+	} else if (proto == SMB2_COMPRESSION_TRANSFORM_ID) {
+		work->tr_sess_id = le64_to_cpu(tr_hdr->SessionId);
+	}
+
 	/* Drop the AEAD authentication tag from the inner RFC1002 frame. */
 	memmove(buf + 4, iov[1].iov_base, original_msg_size);
 	*(__be32 *)buf = cpu_to_be32(original_msg_size);
diff --git a/fs/smb/server/smb2pdu.h b/fs/smb/server/smb2pdu.h
index 1836259..2cdeb56 100644
--- a/fs/smb/server/smb2pdu.h
+++ b/fs/smb/server/smb2pdu.h
@@ -399,6 +399,8 @@ struct channel *lookup_chann_list(struct ksmbd_session *sess,
 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 smb2_hdr *hdr,
+				  unsigned int msg_len, __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