On Mon, Mar 13, 2023 at 7:40 AM Aleksandr Mikhalitsyn
[off-list ref] wrote:

+CC security subsystem folks

On Mon, Mar 13, 2023 at 12:32 PM Alexander Mikhalitsyn
[off-list ref] wrote:

quoted

Currently, kernel would set MSG_CTRUNC flag if msg_control buffer
wasn't provided and SO_PASSCRED was set or if there was pending SCM_RIGHTS.

For some reason we have no corresponding check for SO_PASSSEC.

In the recvmsg(2) doc we have:
       MSG_CTRUNC
              indicates that some control data was discarded due to lack
              of space in the buffer for ancillary data.

So, we need to set MSG_CTRUNC flag for all types of SCM.

This change can break applications those don't check MSG_CTRUNC flag.

Unless I'm missing something I don't think this will actually result
in a userspace visible change as put_cmsg() already has a number of
checks which set the MSG_CTRUNC flag if necessary (including if no
control buffer is passed, e.g. msg_control == NULL).

Regardless, it looks fine to me.

Acked-by: Paul Moore <paul@paul-moore.com>

quoted

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Leon Romanovsky <leon@kernel.org>
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Alexander Mikhalitsyn <redacted>

v2:
- commit message was rewritten according to Eric's suggestion
---
 include/net/scm.h | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/include/net/scm.h b/include/net/scm.h
index 1ce365f4c256..585adc1346bd 100644
--- a/include/net/scm.h
+++ b/include/net/scm.h

@@ -105,16 +105,27 @@ static inline void scm_passec(struct socket *sock, struct msghdr *msg, struct sc
                }
        }
 }
+
+static inline bool scm_has_secdata(struct socket *sock)
+{
+       return test_bit(SOCK_PASSSEC, &sock->flags);
+}
 #else
 static inline void scm_passec(struct socket *sock, struct msghdr *msg, struct scm_cookie *scm)
 { }
+
+static inline bool scm_has_secdata(struct socket *sock)
+{
+       return false;
+}
 #endif /* CONFIG_SECURITY_NETWORK */

 static __inline__ void scm_recv(struct socket *sock, struct msghdr *msg,
                                struct scm_cookie *scm, int flags)
 {
        if (!msg->msg_control) {
-               if (test_bit(SOCK_PASSCRED, &sock->flags) || scm->fp)
+               if (test_bit(SOCK_PASSCRED, &sock->flags) || scm->fp ||
+                   scm_has_secdata(sock))
                        msg->msg_flags |= MSG_CTRUNC;
                scm_destroy(scm);
                return;
--

2.34.1

-- 
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