net/sctp: Always set scope_id in sctp_inet6_skb_msgname

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>

diff --git a/net/sctp/ipv6.c b/net/sctp/ipv6.c
index a6dfa86c02016e3ff81f10f729a56e6673affc68..3b18085e3b10253f3f81be7a6747b50ef9357db2 100644 (file)
--- 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);