Subject: [PATCH 22/22] net/sctp: Always set scope_id in sctp_inet6_skb_msgname

From: "Eric W. Biederman" <[email-masked]>
Date: Wed, 15 Nov 2017 22:17:48 -0600

[ 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.

Fixes: 372f525b495c ("SCTP:  Resync with LKSCTP tree.")
History-tree: https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
Reported-by: Alexander Potapenko <[email-masked]>
Tested-by: Alexander Potapenko <[email-masked]>
Signed-off-by: "Eric W. Biederman" <[email-masked]>
Signed-off-by: David S. Miller <[email-masked]>
---
 net/sctp/ipv6.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/net/sctp/ipv6.c b/net/sctp/ipv6.c
index 1344e3a411ae..edb462b0b73b 100644
--- a/net/sctp/ipv6.c
+++ b/net/sctp/ipv6.c
@@ -807,9 +807,10 @@ static void sctp_inet6_skb_msgname(struct sk_buff *skb, char *msgname,
 		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);
-- 
2.13.6