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ksmbd: fix stack buffer overflow in multichannel session-key copy

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Commit 4b706360ff ("ksmbd: fix multichannel binding and enforce channel
limit") moved the binding-path session key out of the session-wide
sess->sess_key (CIFS_KEY_SIZE = 40) into a new per-channel buffer, and
sized both that buffer and the on-stack copy used during binding with
SMB2_NTLMV2_SESSKEY_SIZE (16):

	struct channel {
		char	sess_key[SMB2_NTLMV2_SESSKEY_SIZE];	/* 16 */
		...
	};

	ntlm_authenticate() / krb5_authenticate():
		char channel_key[SMB2_NTLMV2_SESSKEY_SIZE] = {};	/* 16 */
		char *auth_key = conn->binding ? channel_key : sess->sess_key;

The two writers that fill this destination still bound the copy length
against CIFS_KEY_SIZE (40), not against the 16-byte buffer:

	ksmbd_decode_ntlmssp_auth_blob() (NTLM key exchange):
		if (sess_key_len > CIFS_KEY_SIZE)	/* 40 */
			return -EINVAL;
		arc4_crypt(ctx_arc4, sess_key,
			   (char *)authblob + sess_key_off, sess_key_len);

	ksmbd_krb5_authenticate():
		if (resp->session_key_len > sizeof(sess->sess_key))	/* 40 */
			...
		memcpy(sess_key, resp->payload, resp->session_key_len);

On a binding SESSION_SETUP, auth_key points at the 16-byte channel_key,
so a client that supplies an NTLM EncryptedRandomSessionKey of up to 40
bytes (with NTLMSSP_NEGOTIATE_KEY_EXCH), or a Kerberos ticket whose
session key is longer than 16 bytes (a normal AES256 key is 32), writes
past the 16-byte stack buffer -- up to a 24-byte kernel stack overflow.
KASAN reports it as a stack-out-of-bounds write in arc4_crypt() called
from ksmbd_decode_ntlmssp_auth_blob().

The destinations must be able to hold the full session key the length
checks already permit. Size the per-channel key buffer and the two
on-stack channel_key buffers with CIFS_KEY_SIZE, matching sess->sess_key.

Fixes: 4b706360ff ("ksmbd: fix multichannel binding and enforce channel limit")
Signed-off-by: Gil Portnoy <dddhkts1@gmail.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>
This commit is contained in:
Gil Portnoy 2026-07-12 00:00:00 +00:00 committed by Steve French
parent 08b9452a33
commit 610346149d
2 changed files with 3 additions and 3 deletions
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2
fs/smb/server/mgmt/user_session.h
View File

@ -19,7 +19,7 @@
struct ksmbd_file_table;
struct channel {
char sess_key[SMB2_NTLMV2_SESSKEY_SIZE];
char sess_key[CIFS_KEY_SIZE];
__u8 smb3signingkey[SMB3_SIGN_KEY_SIZE];
struct ksmbd_conn *conn;
};

4
fs/smb/server/smb2pdu.c
View File

@ -1689,7 +1689,7 @@ static int ntlm_authenticate(struct ksmbd_work *work,
struct ksmbd_conn *conn = work->conn;
struct ksmbd_session *sess = work->sess;
struct ksmbd_user *user;
char channel_key[SMB2_NTLMV2_SESSKEY_SIZE] = {};
char channel_key[CIFS_KEY_SIZE] = {};
char *auth_key = conn->binding ? channel_key : sess->sess_key;
u64 prev_id;
bool binding = conn->binding;
@ -1826,7 +1826,7 @@ static int krb5_authenticate(struct ksmbd_work *work,
struct ksmbd_conn *conn = work->conn;
struct ksmbd_session *sess = work->sess;
char *in_blob, *out_blob;
char channel_key[SMB2_NTLMV2_SESSKEY_SIZE] = {};
char channel_key[CIFS_KEY_SIZE] = {};
char *auth_key = conn->binding ? channel_key : sess->sess_key;
u64 prev_sess_id;
bool binding = conn->binding;