On Wed, Jul 30, 2025 at 4:59 PM Chuck Lever <chuck.lever@xxxxxxxxxx> wrote:
>
> On 7/30/25 4:08 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. Scott found that 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@xxxxxxxxxx>
> > Fixes: 5e052dda121e ("SUNRPC: Recognize control messages in server-side TCP socket code")
> > Suggested-by: Trond Myklebust <trondmy@xxxxxxxxxxxxxxx>
> > Suggested-by: Scott Mayhew <smayhew@xxxxxxxxxx>
> > Signed-off-by: Olga Kornievskaia <okorniev@xxxxxxxxxx>
> > ---
> > 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) {
> > + 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;
>
> Fwiw, I've already pulled 1/4 into nfsd-testing. But 4/4 might not apply
> until the others are in the tree. We might want these to go through a
> single tree.
>
> Or, can we delay 4/4 until 6.18 ?
Delaying 4/4 until 6.18 sounds fine to me.
>
>
> --
> Chuck Lever
>
