@stable@vger.kernel.org I apologize that git send-mail went to stable email. Please ignore this email as this is an internal discussion for now. But will be public soon.

On Tue, Jul 29, 2025 at 1:32 PM Chuck Lever chuck.lever@oracle.com wrote:

On 7/29/25 12:40 PM, Olga Kornievskaia wrote:

...

Scott Mayhew discovered a security exploit in NFS over TLS in tls_alert_recv() due to its assumption it can read data from the msg iterator's kvec..

kTLS implementation splits TLS non-data record payload between the control message buffer (which includes the type such as TLS aler or TLS cipher change) and the rest of the payload (say TLS alert's level/description) which goes into the msg payload buffer.

This patch proposes to rework how control messages are setup and used by sock_recvmsg().

If no control message structure is setup, kTLS layer will read and process TLS data record types. As soon as it encounters a TLS control message, it would return an error. At that point, NFS can setup a kvec backed msg buffer and read in the control message such as a TLS alert. Msg iterator can advance the kvec pointer as a part of the copy process thus we need to revert the iterator before calling into the tls_alert_recv.

Reported-by: Scott Mayhew smayhew@redhat.com Fixes: 5e052dda121e ("SUNRPC: Recognize control messages in server-side TCP socket code") Suggested-by: Trond Myklebust trondmy@hammerspace.com Cc: Stable@vger.kernel.org Signed-off-by: Olga Kornievskaia okorniev@redhat.com

---

net/sunrpc/svcsock.c | 43 +++++++++++++++++++++++++++++++++++-------- 1 file changed, 35 insertions(+), 8 deletions(-)

diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c index 46c156b121db..e2c5e0e626f9 100644 --- a/net/sunrpc/svcsock.c +++ b/net/sunrpc/svcsock.c @@ -257,20 +257,47 @@ svc_tcp_sock_process_cmsg(struct socket *sock, struct msghdr *msg, }

static int -svc_tcp_sock_recv_cmsg(struct svc_sock *svsk, struct msghdr *msg) +svc_tcp_sock_recv_cmsg(struct socket *sock, unsigned int *msg_flags) { union { struct cmsghdr cmsg; u8 buf[CMSG_SPACE(sizeof(u8))]; } u;

• struct socket *sock = svsk->sk_sock;
  

• u8 alert[2];
  

• struct kvec alert_kvec = {
  

•         .iov_base = alert,
  

•         .iov_len = sizeof(alert),
  

• };
  

• struct msghdr msg = {
  

•         .msg_flags = *msg_flags,
  

•         .msg_control = &u,
  

•         .msg_controllen = sizeof(u),
  

• };
  

• int ret;
  

• iov_iter_kvec(&msg.msg_iter, ITER_DEST, &alert_kvec, 1,
  

•               alert_kvec.iov_len);
  

• ret = sock_recvmsg(sock, &msg, MSG_DONTWAIT);
  

• if (ret > 0 &&
  

•     tls_get_record_type(sock->sk, &u.cmsg) == TLS_RECORD_TYPE_ALERT) {
  

•         iov_iter_revert(&msg.msg_iter, ret);
  

•         ret = svc_tcp_sock_process_cmsg(sock, &msg, &u.cmsg, -EAGAIN);
  

• }
  

• return ret;
  

+}

+static int +svc_tcp_sock_recvmsg(struct svc_sock *svsk, struct msghdr *msg) +{ int ret;

• struct socket *sock = svsk->sk_sock;
  
  

• msg->msg_control = &u;
  

• msg->msg_controllen = sizeof(u);
  ret = sock_recvmsg(sock, msg, MSG_DONTWAIT);
  

• if (unlikely(msg->msg_controllen != sizeof(u)))
  

•         ret = svc_tcp_sock_process_cmsg(sock, msg, &u.cmsg, ret);
  

• if (msg->msg_flags & MSG_CTRUNC) {
  

Nit: can we leave the unlikely() here?

Will do.

...

•         msg->msg_flags &= ~(MSG_CTRUNC | MSG_EOR);
  

•         if (ret == 0 || ret == -EIO)
  

•                 ret = svc_tcp_sock_recv_cmsg(sock, &msg->msg_flags);
  

• }
  return ret;
  

}

@@ -321,7 +348,7 @@ static ssize_t svc_tcp_read_msg(struct svc_rqst *rqstp, size_t buflen, iov_iter_advance(&msg.msg_iter, seek); buflen -= seek; }

• len = svc_tcp_sock_recv_cmsg(svsk, &msg);
  

• len = svc_tcp_sock_recvmsg(svsk, &msg);
  if (len > 0)
          svc_flush_bvec(bvec, len, seek);
  
  

@@ -1018,7 +1045,7 @@ static ssize_t svc_tcp_read_marker(struct svc_sock *svsk, iov.iov_base = ((char *)&svsk->sk_marker) + svsk->sk_tcplen; iov.iov_len = want; iov_iter_kvec(&msg.msg_iter, ITER_DEST, &iov, 1, want);

•         len = svc_tcp_sock_recv_cmsg(svsk, &msg);
  

•         len = svc_tcp_sock_recvmsg(svsk, &msg);
          if (len < 0)
                  return len;
          svsk->sk_tcplen += len;
  

-- Chuck Lever