Patch "net/sctp: Always set scope_id in sctp_inet6_skb_msgname" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

net/sctp: Always set scope_id in sctp_inet6_skb_msgname

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
net-sctp-always-set-scope_id-in-sctp_inet6_skb_msgname.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "Eric W. Biederman" <[email protected]>
Date: Wed, 15 Nov 2017 22:17:48 -0600
Subject: net/sctp: Always set scope_id in sctp_inet6_skb_msgname

From: "Eric W. Biederman" <[email protected]>

[ Upstream commit 7c8a61d9ee1df0fb4747879fa67a99614eb62fec ]

Alexandar Potapenko while testing the kernel with KMSAN and syzkaller discovered that in some configurations sctp would leak 4 bytes of kernel stack.

Working with his reproducer I discovered that those 4 bytes that are leaked is the scope id of an ipv6 address returned by recvmsg.

With a little code inspection and a shrewd guess I discovered that sctp_inet6_skb_msgname only initializes the scope_id field for link local ipv6 addresses to the interface index the link local address pertains to instead of initializing the scope_id field for all ipv6 addresses.

That is almost reasonable as scope_id's are meaniningful only for link local addresses. Set the scope_id in all other cases to 0 which is not a valid interface index to make it clear there is nothing useful in the scope_id field.

There should be no danger of breaking userspace as the stack leak guaranteed that previously meaningless random data was being returned.

History-tree: https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git

---
 net/sctp/ipv6.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

@@ -806,9 +806,10 @@ static void sctp_inet6_skb_msgname(struc
 		addr->v6.sin6_flowinfo = 0;
 		addr->v6.sin6_port = sh->source;
 		addr->v6.sin6_addr = ipv6_hdr(skb)->saddr;
-		if (ipv6_addr_type(&addr->v6.sin6_addr) & IPV6_ADDR_LINKLOCAL) {
+		if (ipv6_addr_type(&addr->v6.sin6_addr) & IPV6_ADDR_LINKLOCAL)
 			addr->v6.sin6_scope_id = sctp_v6_skb_iif(skb);
-		}
+		else
+			addr->v6.sin6_scope_id = 0;
 	}
 
 	*addr_len = sctp_v6_addr_to_user(sctp_sk(skb->sk), addr);

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/net-sctp-always-set-scope_id-in-sctp_inet6_skb_msgname.patch