diff mbox series

[net] ipv4, ipv6: Fix handling of transhdrlen in __ip{,6}_append_data()

Message ID 75315.1695139973@warthog.procyon.org.uk
State New
Headers
Series [net] ipv4, ipv6: Fix handling of transhdrlen in __ip{,6}_append_data() |

Commit Message

David Howells Sept. 19, 2023, 4:12 p.m. UTC 
  Including the transhdrlen in length is a problem when the packet is
partially filled (e.g. something like send(MSG_MORE) happened previously)
when appending to an IPv4 or IPv6 packet as we don't want to repeat the
transport header or account for it twice.  This can happen under some
circumstances, such as splicing into an L2TP socket.

The symptom observed is a warning in __ip6_append_data():

    WARNING: CPU: 1 PID: 5042 at net/ipv6/ip6_output.c:1800 __ip6_append_data.isra.0+0x1be8/0x47f0 net/ipv6/ip6_output.c:1800

that occurs when MSG_SPLICE_PAGES is used to append more data to an already
partially occupied skbuff.  The warning occurs when 'copy' is larger than
the amount of data in the message iterator.  This is because the requested
length includes the transport header length when it shouldn't.  This can be
triggered by, for example:

        sfd = socket(AF_INET6, SOCK_DGRAM, IPPROTO_L2TP);
        bind(sfd, ...); // ::1
        connect(sfd, ...); // ::1 port 7
        send(sfd, buffer, 4100, MSG_MORE);
        sendfile(sfd, dfd, NULL, 1024);

Fix this by pushing the addition of transhdrlen into length down into
__ip_append_data() and __ip6_append_data(), making it conditional on the
write queue being empty (otherwise we just clear transhdrlen).

Notes:

 (1) The length supplied by userspace in ping_*_sendmsg() includes the
     transport header len in the size, so we need to subtract it as the
     iterator has been advanced to copy out the header - however we still
     have to return the fact that we use it from sys_sendmsg(), so we can't
     just preemptively decrease len at the beginning of the function.

 (2) Raw sockets include the transport header in the payload and pass down
     a zero transhdrlen, which I just leave as is.

 (3) A smaller fix could be to make ip{,6}_append_data() subtract
     transhdrlen from length if it is going to set transhdrlen to 0 - but
     it would seem better just not to include it in the first place.

Reported-by: syzbot+62cbf263225ae13ff153@syzkaller.appspotmail.com
Link: https://lore.kernel.org/r/0000000000001c12b30605378ce8@google.com/
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Eric Dumazet <edumazet@google.com>
cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
cc: "David S. Miller" <davem@davemloft.net>
cc: David Ahern <dsahern@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: Jakub Kicinski <kuba@kernel.org>
cc: netdev@vger.kernel.org
cc: bpf@vger.kernel.org
cc: syzkaller-bugs@googlegroups.com
---
 net/ipv4/icmp.c       |    3 +--
 net/ipv4/ip_output.c  |    7 +++++--
 net/ipv4/ping.c       |    3 ++-
 net/ipv4/udp.c        |    8 +++-----
 net/ipv6/icmp.c       |    6 ++----
 net/ipv6/ip6_output.c |   16 +++++++++-------
 net/ipv6/ping.c       |    3 ++-
 net/ipv6/udp.c        |    8 +++-----
 net/l2tp/l2tp_ip6.c   |    4 +---
 9 files changed, 28 insertions(+), 30 deletions(-)

Comments

Willem de Bruijn Sept. 19, 2023, 8:32 p.m. UTC | #1 
On Tue, Sep 19, 2023 at 12:12 PM David Howells <dhowells@redhat.com> wrote:
>
>
> Including the transhdrlen in length is a problem when the packet is
> partially filled (e.g. something like send(MSG_MORE) happened previously)
> when appending to an IPv4 or IPv6 packet as we don't want to repeat the
> transport header or account for it twice.  This can happen under some
> circumstances, such as splicing into an L2TP socket.
>
> The symptom observed is a warning in __ip6_append_data():
>
>     WARNING: CPU: 1 PID: 5042 at net/ipv6/ip6_output.c:1800 __ip6_append_data.isra.0+0x1be8/0x47f0 net/ipv6/ip6_output.c:1800
>
> that occurs when MSG_SPLICE_PAGES is used to append more data to an already
> partially occupied skbuff.  The warning occurs when 'copy' is larger than
> the amount of data in the message iterator.  This is because the requested
> length includes the transport header length when it shouldn't.  This can be
> triggered by, for example:
>
>         sfd = socket(AF_INET6, SOCK_DGRAM, IPPROTO_L2TP);
>         bind(sfd, ...); // ::1
>         connect(sfd, ...); // ::1 port 7
>         send(sfd, buffer, 4100, MSG_MORE);
>         sendfile(sfd, dfd, NULL, 1024);
>
> Fix this by pushing the addition of transhdrlen into length down into
> __ip_append_data() and __ip6_append_data(), making it conditional on the
> write queue being empty (otherwise we just clear transhdrlen).

I'm afraid that we might start to dig an ever deeping hole.

The proposed fix is non-trivial, and changes not just the new path
that observes the issue (MSG_SPLICE_PAGES), but also the other more
common paths that exercise __ip6_append_data.

There is significant risk to introduce an unintended side effect
requiring a follow-up fix. Because this function is notoriously
complex, multiplexing a lot of behavior: with and without transport
headers, edge cases like fragmentation, MSG_MORE, absence of
scatter-gather, ....

Does the issue discovered only affect MSG_SPLICE_PAGES or can it
affect other paths too? If the first, it possible to create a more
targeted fix that can trivially be seen to not affect code prior to
introduction of splice pages?
David Howells Sept. 20, 2023, 8:35 a.m. UTC | #2 
Willem de Bruijn <willemdebruijn.kernel@gmail.com> wrote:

> The proposed fix is non-trivial, and changes not just the new path
> that observes the issue (MSG_SPLICE_PAGES), but also the other more
> common paths that exercise __ip6_append_data.

I realise that.  I broke ping/ping6 briefly, but I corrected that (I
subtracted the ICMP header len from length after copying it out, but forgot
that it needed adding back on for the return value of sendmsg()).  But I don't
think there are that many callers - however, you might be right that this is
too big for a fix.

> There is significant risk to introduce an unintended side effect
> requiring a follow-up fix. Because this function is notoriously
> complex, multiplexing a lot of behavior: with and without transport
> headers, edge cases like fragmentation, MSG_MORE, absence of
> scatter-gather, ....

The problem is that the bug isn't in __ip{,6}_append_data(), I think, it's
actually higher up in ip{,6}_append_data().  I think I see *why* length has
transhdrlen handed into it: because ping and raw sockets come with that
pre-added-in by userspace.

I would actually like to eliminate the length argument entirely and use the
length in the iterator - but that doesn't work in all cases as sometimes there
isn't a msghdr struct.  (And, besides, that's too big a change for a fix).

I think the simplest fix, then, is just to make ip{,6}_append_data() subtract
transhdrlen from length before clearing transhdrlen when there's already a
packet in the queue from MSG_MORE/cork that will be appended to.

> Does the issue discovered only affect MSG_SPLICE_PAGES or can it
> affect other paths too? If the first, it possible to create a more
> targeted fix that can trivially be seen to not affect code prior to
> introduction of splice pages?

It may also affect MSG_ZEROCOPY in interesting ways.  msg_zerocopy_realloc()
looks suspicious as it does things with 'size' bytes from the buffer that
doesn't have 'size' bytes of data in it (because size (aka length) includes
transhdrlen).

I would guess that we don't notice issues with ping sockets because people
don't often use MSG_MORE/corking with them.

Raw sockets shouldn't exhibit this bug as they set transhdrlen to 0 up front,
but I can't help but wonder what the consequences are as some bits of
__ip*_append_data() change behaviour if they see transhdrlen==0 :-/

David
diff mbox series

Patch

diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
index b8607763d113..e1140597e971 100644
--- a/net/ipv4/icmp.c
+++ b/net/ipv4/icmp.c
@@ -368,8 +368,7 @@  static void icmp_push_reply(struct sock *sk,
 	struct sk_buff *skb;
 
 	if (ip_append_data(sk, fl4, icmp_glue_bits, icmp_param,
-			   icmp_param->data_len+icmp_param->head_len,
-			   icmp_param->head_len,
+			   icmp_param->data_len, icmp_param->head_len,
 			   ipc, rt, MSG_DONTWAIT) < 0) {
 		__ICMP_INC_STATS(sock_net(sk), ICMP_MIB_OUTERRORS);
 		ip_flush_pending_frames(sk);
diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 4ab877cf6d35..80735bb34a73 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -974,6 +974,11 @@  static int __ip_append_data(struct sock *sk,
 	bool paged, extra_uref = false;
 	u32 tskey = 0;
 
+	if (skb_queue_empty(&sk->sk_write_queue))
+		length += transhdrlen;
+	else
+		transhdrlen = 0;
+
 	skb = skb_peek_tail(queue);
 
 	exthdrlen = !skb ? rt->dst.header_len : 0;
@@ -1353,8 +1358,6 @@  int ip_append_data(struct sock *sk, struct flowi4 *fl4,
 		err = ip_setup_cork(sk, &inet->cork.base, ipc, rtp);
 		if (err)
 			return err;
-	} else {
-		transhdrlen = 0;
 	}
 
 	return __ip_append_data(sk, fl4, &sk->sk_write_queue, &inet->cork.base,
diff --git a/net/ipv4/ping.c b/net/ipv4/ping.c
index 75e0aee35eb7..237f25a015f7 100644
--- a/net/ipv4/ping.c
+++ b/net/ipv4/ping.c
@@ -819,7 +819,8 @@  static int ping_v4_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	pfh.wcheck = 0;
 	pfh.family = AF_INET;
 
-	err = ip_append_data(sk, &fl4, ping_getfrag, &pfh, len,
+	err = ip_append_data(sk, &fl4, ping_getfrag, &pfh,
+			     len - sizeof(struct icmphdr),
 			     sizeof(struct icmphdr), &ipc, &rt,
 			     msg->msg_flags);
 	if (err)
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index f39b9c844580..b16da49a8478 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1042,7 +1042,6 @@  int udp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	DECLARE_SOCKADDR(struct sockaddr_in *, usin, msg->msg_name);
 	struct flowi4 fl4_stack;
 	struct flowi4 *fl4;
-	int ulen = len;
 	struct ipcm_cookie ipc;
 	struct rtable *rt = NULL;
 	int free = 0;
@@ -1084,7 +1083,6 @@  int udp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 		}
 		release_sock(sk);
 	}
-	ulen += sizeof(struct udphdr);
 
 	/*
 	 *	Get and verify the address.
@@ -1238,7 +1236,7 @@  int udp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	if (!corkreq) {
 		struct inet_cork cork;
 
-		skb = ip_make_skb(sk, fl4, getfrag, msg, ulen,
+		skb = ip_make_skb(sk, fl4, getfrag, msg, len,
 				  sizeof(struct udphdr), &ipc, &rt,
 				  &cork, msg->msg_flags);
 		err = PTR_ERR(skb);
@@ -1268,8 +1266,8 @@  int udp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	up->pending = AF_INET;
 
 do_append_data:
-	up->len += ulen;
-	err = ip_append_data(sk, fl4, getfrag, msg, ulen,
+	up->len += sizeof(struct udphdr) + len;
+	err = ip_append_data(sk, fl4, getfrag, msg, len,
 			     sizeof(struct udphdr), &ipc, &rt,
 			     corkreq ? msg->msg_flags|MSG_MORE : msg->msg_flags);
 	if (err)
diff --git a/net/ipv6/icmp.c b/net/ipv6/icmp.c
index 93a594a901d1..55ae1966fa75 100644
--- a/net/ipv6/icmp.c
+++ b/net/ipv6/icmp.c
@@ -614,8 +614,7 @@  void icmp6_send(struct sk_buff *skb, u8 type, u8 code, __u32 info,
 	idev = __in6_dev_get(skb->dev);
 
 	if (ip6_append_data(sk, icmpv6_getfrag, &msg,
-			    len + sizeof(struct icmp6hdr),
-			    sizeof(struct icmp6hdr),
+			    len, sizeof(struct icmp6hdr),
 			    &ipc6, &fl6, (struct rt6_info *)dst,
 			    MSG_DONTWAIT)) {
 		ICMP6_INC_STATS(net, idev, ICMP6_MIB_OUTERRORS);
@@ -801,8 +800,7 @@  static enum skb_drop_reason icmpv6_echo_reply(struct sk_buff *skb)
 			goto out_dst_release;
 
 	if (ip6_append_data(sk, icmpv6_getfrag, &msg,
-			    skb->len + sizeof(struct icmp6hdr),
-			    sizeof(struct icmp6hdr), &ipc6, &fl6,
+			    skb->len, sizeof(struct icmp6hdr), &ipc6, &fl6,
 			    (struct rt6_info *)dst, MSG_DONTWAIT)) {
 		__ICMP6_INC_STATS(net, idev, ICMP6_MIB_OUTERRORS);
 		ip6_flush_pending_frames(sk);
diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 54fc4c711f2c..21bae77489c9 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -1490,6 +1490,11 @@  static int __ip6_append_data(struct sock *sk,
 	unsigned int wmem_alloc_delta = 0;
 	bool paged, extra_uref = false;
 
+	if (skb_queue_empty(&sk->sk_write_queue))
+		length += transhdrlen;
+	else
+		transhdrlen = 0;
+
 	skb = skb_peek_tail(queue);
 	if (!skb) {
 		exthdrlen = opt ? opt->opt_flen : 0;
@@ -1868,7 +1873,6 @@  int ip6_append_data(struct sock *sk,
 {
 	struct inet_sock *inet = inet_sk(sk);
 	struct ipv6_pinfo *np = inet6_sk(sk);
-	int exthdrlen;
 	int err;
 
 	if (flags&MSG_PROBE)
@@ -1884,13 +1888,11 @@  int ip6_append_data(struct sock *sk,
 			return err;
 
 		inet->cork.fl.u.ip6 = *fl6;
-		exthdrlen = (ipc6->opt ? ipc6->opt->opt_flen : 0);
-		length += exthdrlen;
-		transhdrlen += exthdrlen;
-	} else {
-		transhdrlen = 0;
 	}
 
+	/* Add space for extensions */
+	transhdrlen += (ipc6->opt ? ipc6->opt->opt_flen : 0);
+
 	return __ip6_append_data(sk, &sk->sk_write_queue, &inet->cork,
 				 &np->cork, sk_page_frag(sk), getfrag,
 				 from, length, transhdrlen, flags, ipc6);
@@ -2095,7 +2097,7 @@  struct sk_buff *ip6_make_skb(struct sock *sk,
 
 	err = __ip6_append_data(sk, &queue, cork, &v6_cork,
 				&current->task_frag, getfrag, from,
-				length + exthdrlen, transhdrlen + exthdrlen,
+				length, transhdrlen + exthdrlen,
 				flags, ipc6);
 	if (err) {
 		__ip6_flush_pending_frames(sk, &queue, cork, &v6_cork);
diff --git a/net/ipv6/ping.c b/net/ipv6/ping.c
index 5831aaa53d75..fa2701241724 100644
--- a/net/ipv6/ping.c
+++ b/net/ipv6/ping.c
@@ -174,7 +174,8 @@  static int ping_v6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 		ipc6.hlimit = ip6_sk_dst_hoplimit(np, &fl6, dst);
 
 	lock_sock(sk);
-	err = ip6_append_data(sk, ping_getfrag, &pfh, len,
+	err = ip6_append_data(sk, ping_getfrag, &pfh,
+			      len - sizeof(struct icmp6hdr),
 			      sizeof(struct icmp6hdr), &ipc6, &fl6, rt,
 			      MSG_DONTWAIT);
 
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 86b5d509a468..9dd71f542c9f 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -1331,7 +1331,6 @@  int udpv6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	struct ipcm6_cookie ipc6;
 	int addr_len = msg->msg_namelen;
 	bool connected = false;
-	int ulen = len;
 	int corkreq = READ_ONCE(up->corkflag) || msg->msg_flags&MSG_MORE;
 	int err;
 	int is_udplite = IS_UDPLITE(sk);
@@ -1416,7 +1415,6 @@  int udpv6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 		}
 		release_sock(sk);
 	}
-	ulen += sizeof(struct udphdr);
 
 	memset(fl6, 0, sizeof(*fl6));
 
@@ -1567,7 +1565,7 @@  int udpv6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	if (!corkreq) {
 		struct sk_buff *skb;
 
-		skb = ip6_make_skb(sk, getfrag, msg, ulen,
+		skb = ip6_make_skb(sk, getfrag, msg, len,
 				   sizeof(struct udphdr), &ipc6,
 				   (struct rt6_info *)dst,
 				   msg->msg_flags, &cork);
@@ -1594,8 +1592,8 @@  int udpv6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 do_append_data:
 	if (ipc6.dontfrag < 0)
 		ipc6.dontfrag = np->dontfrag;
-	up->len += ulen;
-	err = ip6_append_data(sk, getfrag, msg, ulen, sizeof(struct udphdr),
+	up->len += sizeof(struct udphdr) + len;
+	err = ip6_append_data(sk, getfrag, msg, len, sizeof(struct udphdr),
 			      &ipc6, fl6, (struct rt6_info *)dst,
 			      corkreq ? msg->msg_flags|MSG_MORE : msg->msg_flags);
 	if (err)
diff --git a/net/l2tp/l2tp_ip6.c b/net/l2tp/l2tp_ip6.c
index ed8ebb6f5909..d5d1c3b68200 100644
--- a/net/l2tp/l2tp_ip6.c
+++ b/net/l2tp/l2tp_ip6.c
@@ -499,7 +499,6 @@  static int l2tp_ip6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	struct ipcm6_cookie ipc6;
 	int addr_len = msg->msg_namelen;
 	int transhdrlen = 4; /* zero session-id */
-	int ulen;
 	int err;
 
 	/* Rough check on arithmetic overflow,
@@ -507,7 +506,6 @@  static int l2tp_ip6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	 */
 	if (len > INT_MAX - transhdrlen)
 		return -EMSGSIZE;
-	ulen = len + transhdrlen;
 
 	/* Mirror BSD error message compatibility */
 	if (msg->msg_flags & MSG_OOB)
@@ -629,7 +627,7 @@  static int l2tp_ip6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 back_from_confirm:
 	lock_sock(sk);
 	err = ip6_append_data(sk, ip_generic_getfrag, msg,
-			      ulen, transhdrlen, &ipc6,
+			      len, transhdrlen, &ipc6,
 			      &fl6, (struct rt6_info *)dst,
 			      msg->msg_flags);
 	if (err)