CVE-2017-15115: Use After Free (CWE-416)
value: The sctp_do_peeloff function in net/sctp/socket.c in the Linux kernel before 4.14 does not check whether the intended netns is used in a peel-off action, which allows local users to cause a denial of service (use-after-free and system crash) or possibly have unspecified other impact via crafted system calls.
CWE description: The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.
CVSS v3 base score: 7.8
Published: 2017-11-15T21:29Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: net/sctp/socket.c
Commit date: 2017-10-19 13:16:07 +0100
Commit message: sctp: do not peel off an assoc from one netns to another one

Now when peeling off an association to the sock in another netns, all
transports in this assoc are not to be rehashed and keep use the old
key in hashtable.

As a transport uses sk->net as the hash key to insert into hashtable,
it would miss removing these transports from hashtable due to the new
netns when closing the sock and all transports are being freeed, then
later an use-after-free issue could be caused when looking up an asoc
and dereferencing those transports.

This is a very old issue since very beginning, ChunYu found it with
syzkaller fuzz testing with this series:

  socket$inet6_sctp()
  bind$inet6()
  sendto$inet6()
  unshare(0x40000000)
  getsockopt$inet_sctp6_SCTP_GET_ASSOC_ID_LIST()
  getsockopt$inet_sctp6_SCTP_SOCKOPT_PEELOFF()

This patch is to block this call when peeling one assoc off from one
netns to another one, so that the netns of all transport would not
go out-sync with the key in hashtable.

Note that this patch didn't fix it by rehashing transports, as it's
difficult to handle the situation when the tuple is already in use
in the new netns. Besides, no one would like to peel off one assoc
to another netns, considering ipaddrs, ifaces, etc. are usually
different.

Reported-by: ChunYu Wang <chunwang@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Diff: diff --git a/net/sctp/socket.c b/net/sctp/socket.c
index d4730ada7f3233..17841ab30798ec 100644
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c
@@ -4904,10 +4904,14 @@ int sctp_do_peeloff(struct sock *sk, sctp_assoc_t id, struct socket **sockp)
 	struct sctp_association *asoc = sctp_id2assoc(sk, id);
 	struct sctp_sock *sp = sctp_sk(sk);
 	struct socket *sock;
 	int err = 0;
 
+	/* Do not peel off from one netns to another one. */
+	if (!net_eq(current->nsproxy->net_ns, sock_net(sk)))
+		return -EINVAL;
+
 	if (!asoc)
 		return -EINVAL;
 
 	/* If there is a thread waiting on more sndbuf space for
 	 * sending on this asoc, it cannot be peeled.

Fixed code: 	/* Do not peel off from one netns to another one. */
	if (!net_eq(current->nsproxy->net_ns, sock_net(sk)))
		return -EINVAL;