From mboxrd@z Thu Jan 1 00:00:00 1970 From: ebiederm@xmission.com (Eric W. Biederman) Subject: Re: [PATCH] net: recvmsg: Unconditionally zero struct sockaddr_storage Date: Wed, 01 Nov 2017 07:48:36 -0500 Message-ID: <871slikvvf.fsf@xmission.com> References: <20171031161445.GA140874@beast> <1509471094.3828.26.camel@edumazet-glaptop3.roam.corp.google.com> Mime-Version: 1.0 Content-Type: text/plain Cc: Kees Cook , "David S. Miller" , Alexander Potapenko , Kostya Serebryany , Andrey Konovalov , Eric Dumazet , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, security@kernel.org To: Eric Dumazet Return-path: In-Reply-To: <1509471094.3828.26.camel@edumazet-glaptop3.roam.corp.google.com> (Eric Dumazet's message of "Tue, 31 Oct 2017 10:31:34 -0700") Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org Eric Dumazet writes: > On Tue, 2017-10-31 at 09:14 -0700, Kees Cook wrote: >> Some protocols do not correctly wipe the contents of the on-stack >> struct sockaddr_storage sent down into recvmsg() (e.g. SCTP), and leak >> kernel stack contents to userspace. This wipes it unconditionally before >> per-protocol handlers run. >> >> Note that leaks like this are mitigated by building with >> CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL=y >> >> Reported-by: Alexander Potapenko >> Cc: "David S. Miller" >> Cc: netdev@vger.kernel.org >> Signed-off-by: Kees Cook >> --- >> net/socket.c | 1 + >> 1 file changed, 1 insertion(+) >> >> diff --git a/net/socket.c b/net/socket.c >> index c729625eb5d3..34183f4fbdf8 100644 >> --- a/net/socket.c >> +++ b/net/socket.c >> @@ -2188,6 +2188,7 @@ static int ___sys_recvmsg(struct socket *sock, struct user_msghdr __user *msg, >> struct sockaddr __user *uaddr; >> int __user *uaddr_len = COMPAT_NAMELEN(msg); >> >> + memset(&addr, 0, sizeof(addr)); >> msg_sys->msg_name = &addr; >> > > This kind of patch comes every year. > > Standard answer is : We fix the buggy protocol, we do not make > everything slower just because we are lazy. > > struct sockaddr is 128 bytes, but IPV4 only uses a fraction of it. > > Also memset() is using long word stores, so next 4-byte or 2-byte stores > on same location hit a performance problem on x86. > > By adding all these defensive programming, we would give strong > incentives to bypass the kernel for networking. That would be bad. In this case it looks like the root cause is something in sctp not filling in the ipv6 sin6_scope_id. Which is not only a leak but a correctness bug. I ran the reproducer test program and while none of the leak checkers are telling me anything I have gotten as far as seeing that the returned length is correct and sometimes nonsense. Hmm. At a quick look it looks like all that is necessary is to do this: diff --git a/net/sctp/ipv6.c b/net/sctp/ipv6.c index 51c488769590..6301913d0516 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); Eric