From: ebiederm@xmission.com (Eric W. Biederman) Date: Thu, 16 Nov 2017 04:17:48 +0000 Subject: [PATCH net] net/sctp: Always set scope_id in sctp_inet6_skb_msgname Message-Id: <871skyzwk3.fsf_-_@xmission.com> List-Id: References: <20171031161445.GA140874@beast> <1509471094.3828.26.camel@edumazet-glaptop3.roam.corp.google.com> <871slikvvf.fsf@xmission.com> In-Reply-To: (Alexander Potapenko's message of "Wed, 15 Nov 2017 09:22:39 +0100") MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: "David S. Miller" Cc: Kees Cook , Eric Dumazet , Kostya Serebryany , Andrey Konovalov , Eric Dumazet , Network Development , LKML , security@kernel.org, Alexander Potapenko , linux-sctp@vger.kernel.org, Neil Horman , Vlad Yasevich 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. Cc: stable@vger.kernel.org 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 Tested-by: Alexander Potapenko Signed-off-by: "Eric W. Biederman" --- 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 a6dfa86c0201..3b18085e3b10 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); -- 2.14.1