Re: [PATCH][next] mptcp: Avoid NULL dereference in mptcp_getsockopt_subflow_addrs()

From: Mat Martineau
Date: Thu Sep 30 2021 - 19:14:39 EST


On Mon, 20 Sep 2021, Mat Martineau wrote:

On Mon, 20 Sep 2021, Tim Gardner wrote:

Coverity complains of a possible NULL dereference in
mptcp_getsockopt_subflow_addrs():

861 } else if (sk->sk_family == AF_INET6) {
3. returned_null: inet6_sk returns NULL. [show details]
4. var_assigned: Assigning: np = NULL return value from inet6_sk.
862 const struct ipv6_pinfo *np = inet6_sk(sk);

Fix this by checking for NULL.

Cc: Mat Martineau <mathew.j.martineau@xxxxxxxxxxxxxxx>
Cc: Matthieu Baerts <matthieu.baerts@xxxxxxxxxxxx>
Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
Cc: netdev@xxxxxxxxxxxxxxx
Cc: mptcp@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Tim Gardner <tim.gardner@xxxxxxxxxxxxx>

[ I'm not at all sure this is the right thing to do since the final result is to
return garbage to user space in mptcp_getsockopt_subflow_addrs(). Is this one
of those cases where inet6_sk() can't fail ?]

Hi Tim -

Thanks for noticing this and proposing a fix.

As you commented, this isn't the right change to merge since mptcp_getsockopt_subflow_addrs() would copy garbage.

This block of code already checks that CONFIG_IPV6 is enabled, so the question is whether sk_fullsock() would return false because the subflow is in TCP_TIME_WAIT or TCP_NEW_SYN_RECV. The caller is iterating over sockets in the MPTCP socket's conn_list, which does not contain request_socks (so there are no sockets in the TCP_NEW_SYN_RECV state).

TCP subflow sockets are normally removed from the conn_list before they are closed by their parent MPTCP socket, but I need to double-check for corner cases. I created a github issue to track this: https://github.com/multipath-tcp/mptcp_net-next/issues/231


Tim,

Could you submit a v2 of this patch? Paolo took a look and the condition should not happen, but adding the NULL check would be a good idea and returning early as your patch does is ok. The data copied after the early return will be zeroed and look like the address family is AF_UNSPEC.

Could you add a

Fixes: https://github.com/multipath-tcp/mptcp_net-next/issues/231

tag and make the one change below?


---
net/mptcp/sockopt.c | 3 +++
1 file changed, 3 insertions(+)

diff --git a/net/mptcp/sockopt.c b/net/mptcp/sockopt.c
index 8137cc3a4296..c89f2bedce79 100644
--- a/net/mptcp/sockopt.c
+++ b/net/mptcp/sockopt.c
@@ -861,6 +861,9 @@ static void mptcp_get_sub_addrs(const struct sock *sk, struct mptcp_subflow_addr
} else if (sk->sk_family == AF_INET6) {
const struct ipv6_pinfo *np = inet6_sk(sk);

+ if (!np)

This could be

if (WARN_ON_ONCE(!np))

(as suggested by Paolo) to make it clear the condition is unexpected.

+ return;
+
a->sin6_local.sin6_family = AF_INET6;
a->sin6_local.sin6_port = inet->inet_sport;

--
2.33.0

Thanks,

--
Mat Martineau
Intel