linux-kernel - [PATCH 3.16 085/136] net/sctp: Always set scope_id in sctp_inet6_skb

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Message-ID: <lsq.1518323471.400637311@decadent.org.uk>
Date:   Sun, 11 Feb 2018 04:31:11 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "David S. Miller" <davem@...emloft.net>,
        "Alexander Potapenko" <glider@...gle.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: [PATCH 3.16 085/136] net/sctp: Always set scope_id in
 sctp_inet6_skb_msgname

3.16.54-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: "Eric W. Biederman" <ebiederm@...ssion.com>

commit 7c8a61d9ee1df0fb4747879fa67a99614eb62fec upstream.

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 <glider@...gle.com>
Tested-by: Alexander Potapenko <glider@...gle.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
[bwh: Backported to 3.16: 
 - Adjust context
 - Add braces]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
--- a/net/sctp/ipv6.c
+++ b/net/sctp/ipv6.c
@@ -787,6 +787,8 @@ static void sctp_inet6_skb_msgname(struc
 		if (ipv6_addr_type(&addr->v6.sin6_addr) & IPV6_ADDR_LINKLOCAL) {
 			struct sctp_ulpevent *ev = sctp_skb2event(skb);
 			addr->v6.sin6_scope_id = ev->iif;
+		} else {
+			addr->v6.sin6_scope_id = 0;
 		}
 	}