[PATCH net v2] tcp: devmem: don't write truncated dmabuf CMSGs to userspace

From: Stanislav Fomichev
Date: Wed Feb 19 2025 - 15:34:51 EST


Currently, we report -ETOOSMALL (err) only on the first iteration
(!sent). When we get put_cmsg error after a bunch of successful
put_cmsg calls, we don't signal the error at all. This might be
confusing on the userspace side which will see truncated CMSGs
but no MSG_CTRUNC signal.

Consider the following case:
- sizeof(struct cmsghdr) = 16
- sizeof(struct dmabuf_cmsg) = 24
- total cmsg size (CMSG_LEN) = 40 (16+24)

When calling recvmsg with msg_controllen=60, the userspace
will receive two(!) dmabuf_cmsg(s), the first one will
be a valid one and the second one will be silently truncated. There is no
easy way to discover the truncation besides doing something like
"cm->cmsg_len != CMSG_LEN(sizeof(dmabuf_cmsg))".

Introduce new put_devmem_cmsg wrapper that reports an error instead
of doing the truncation. Mina suggests that it's the intended way
this API should work.

Note that we might now report MSG_CTRUNC when the users (incorrectly)
call us with msg_control == NULL.

Fixes: 8f0b3cc9a4c1 ("tcp: RX path for devmem TCP")
Signed-off-by: Stanislav Fomichev <sdf@xxxxxxxxxxx>
---
net/ipv4/tcp.c | 25 +++++++++++++++++--------
1 file changed, 17 insertions(+), 8 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 0d704bda6c41..ba77beba60c4 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2394,6 +2394,16 @@ static int tcp_xa_pool_refill(struct sock *sk, struct tcp_xa_pool *p,
return k ? 0 : err;
}

+static int put_devmem_cmsg(struct msghdr *msg, int level, int type, int len,
+ void *data)
+{
+ /* Don't produce truncated CMSGs */
+ if (msg->msg_controllen < CMSG_LEN(len))
+ return -ETOOSMALL;
+
+ return put_cmsg(msg, level, type, len, data);
+}
+
/* On error, returns the -errno. On success, returns number of bytes sent to the
* user. May not consume all of @remaining_len.
*/
@@ -2438,10 +2448,10 @@ static int tcp_recvmsg_dmabuf(struct sock *sk, const struct sk_buff *skb,
*/
memset(&dmabuf_cmsg, 0, sizeof(dmabuf_cmsg));
dmabuf_cmsg.frag_size = copy;
- err = put_cmsg(msg, SOL_SOCKET, SO_DEVMEM_LINEAR,
- sizeof(dmabuf_cmsg), &dmabuf_cmsg);
+ err = put_devmem_cmsg(msg, SOL_SOCKET, SO_DEVMEM_LINEAR,
+ sizeof(dmabuf_cmsg),
+ &dmabuf_cmsg);
if (err || msg->msg_flags & MSG_CTRUNC) {
- msg->msg_flags &= ~MSG_CTRUNC;
if (!err)
err = -ETOOSMALL;
goto out;
@@ -2499,12 +2509,11 @@ static int tcp_recvmsg_dmabuf(struct sock *sk, const struct sk_buff *skb,
offset += copy;
remaining_len -= copy;

- err = put_cmsg(msg, SOL_SOCKET,
- SO_DEVMEM_DMABUF,
- sizeof(dmabuf_cmsg),
- &dmabuf_cmsg);
+ err = put_devmem_cmsg(msg, SOL_SOCKET,
+ SO_DEVMEM_DMABUF,
+ sizeof(dmabuf_cmsg),
+ &dmabuf_cmsg);
if (err || msg->msg_flags & MSG_CTRUNC) {
- msg->msg_flags &= ~MSG_CTRUNC;
if (!err)
err = -ETOOSMALL;
goto out;
--
2.48.1