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From:   Ben Hutchings <ben@decadent.org.uk>
To:     linux-kernel@vger.kernel.org, stable@vger.kernel.org
CC:     akpm@linux-foundation.org, "David S. Miller" <davem@davemloft.net>,
        "Alexander Potapenko" <glider@google.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>
Date:   Sun, 11 Feb 2018 04:20:06 +0000
Message-ID: <lsq.1518322806.501018586@decadent.org.uk>
Subject: [PATCH 3.2 43/79] net/sctp: Always set scope_id in
 sctp_inet6_skb_msgname
In-Reply-To: <lsq.1518322802.943358572@decadent.org.uk>
Sender: linux-kernel-owner@vger.kernel.org
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3.2.99-rc1 review patch.  If anyone has any objections, please let me know.

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

From: "Eric W. Biederman" <ebiederm@xmission.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@google.com>
Tested-by: Alexander Potapenko <glider@google.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.2: 
 - Adjust context
 - Add braces]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
--- a/net/sctp/ipv6.c
+++ b/net/sctp/ipv6.c
@@ -781,6 +781,8 @@ 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;
 		}
 	}