[PATCH 6.18 141/198] ksmbd: fix mechToken leak when SPNEGO decode fails after token alloc

Greg Kroah-Hartman
2026-04-20T16:05:05Z

From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

commit ad0057fb91218914d6c98268718ceb9d59b388e1 upstream.

The kernel ASN.1 BER decoder calls action callbacks incrementally as it
walks the input. When ksmbd_decode_negTokenInit() reaches the mechToken
[2] OCTET STRING element, ksmbd_neg_token_alloc() allocates
conn->mechToken immediately via kmemdup_nul(). If a later element in
the same blob is malformed, then the decoder will return nonzero after
the allocation is already live. This could happen if mechListMIC [3]
overrunse the enclosing SEQUENCE.

decode_negotiation_token() then sets conn->use_spnego = false because
both the negTokenInit and negTokenTarg grammars failed. The cleanup at
the bottom of smb2_sess_setup() is gated on use_spnego:

if (conn->use_spnego && conn->mechToken) {
kfree(conn->mechToken);
conn->mechToken = NULL;
}

so the kfree is skipped, causing the mechToken to never be freed.

This codepath is reachable pre-authentication, so untrusted clients can
cause slow memory leaks on a server without even being properly
authenticated.

Fix this up by not checking check for use_spnego, as it's not required,
so the memory will always be properly freed. At the same time, always
free the memory in ksmbd_conn_free() incase some other failure path
forgot to free it.

Cc: Namjae Jeon <linkinjeon@kernel.org>
Cc: Steve French <smfrench@gmail.com>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Tom Talpey <tom@talpey.com>
Cc: linux-cifs@vger.kernel.org
Cc: <stable@kernel.org>
Assisted-by: gregkh_clanker_t1000
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/smb/server/connection.c | 1 +
fs/smb/server/smb2pdu.c | 2 +-
2 files changed, 2 insertions(+), 1 deletion(-)

--- a/fs/smb/server/connection.c
+++ b/fs/smb/server/connection.c
@@ -39,6 +39,7 @@ void ksmbd_conn_free(struct ksmbd_conn *
xa_destroy(&conn->sessions);
kvfree(conn->request_buf);
kfree(conn->preauth_info);
+ kfree(conn->mechToken);
if (atomic_dec_and_test(&conn->refcnt)) {
conn->transport->ops->free_transport(conn->transport);
kfree(conn);
--- a/fs/smb/server/smb2pdu.c
+++ b/fs/smb/server/smb2pdu.c
@@ -1924,7 +1924,7 @@ out_err:
else if (rc)
rsp->hdr.Status = STATUS_LOGON_FAILURE;

- if (conn->use_spnego && conn->mechToken) {
+ if (conn->mechToken) {
kfree(conn->mechToken);
conn->mechToken = NULL;
}