ipv4: fix ip option filtering for locally generated fragments

BugLink: https://bugs.launchpad.net/bugs/1959879
[ Upstream commit 27a8caa59babb96c5890569e131bc0eb6d45daee ]

During IP fragmentation we sanitize IP options. This means overwriting
options which should not be copied with NOPs. Only the first fragment
has the original, full options.

ip_fraglist_prepare() copies the IP header and options from previous
fragment to the next one. Commit 19c3401a917b ("net: ipv4: place control
buffer handling away from fragmentation iterators") moved sanitizing
options before ip_fraglist_prepare() which means options are sanitized
and then overwritten again with the old values.

Fixing this is not enough, however, nor did the sanitization work
prior to aforementioned commit.

ip_options_fragment() (which does the sanitization) uses ipcb->opt.optlen
for the length of the options. ipcb->opt of fragments is not populated
(it's 0), only the head skb has the state properly built. So even when
called at the right time ip_options_fragment() does nothing. This seems
to date back all the way to v2.5.44 when the fast path for pre-fragmented
skbs had been introduced. Prior to that ip_options_build() would have been
called for every fragment (in fact ever since v2.5.44 the fragmentation
handing in ip_options_build() has been dead code, I'll clean it up in
-next).

In the original patch (see Link) caixf mentions fixing the handling
for fragments other than the second one, but I'm not sure how _any_
fragment could have had their options sanitized with the code
as it stood.

Tested with python (MTU on lo lowered to 1000 to force fragmentation):

  import socket
  s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
  s.setsockopt(socket.IPPROTO_IP, socket.IP_OPTIONS,
               bytearray([7,4,5,192, 20|0x80,4,1,0]))
  s.sendto(b'1'*2000, ('127.0.0.1', 1234))

Before:

IP (tos 0x0, ttl 64, id 1053, offset 0, flags [+], proto UDP (17), length 996, options (RR [bad length 4] [bad ptr 5] 192.148.4.1,,RA value 256))
    localhost.36500 > localhost.search-agent: UDP, length 2000
IP (tos 0x0, ttl 64, id 1053, offset 968, flags [+], proto UDP (17), length 996, options (RR [bad length 4] [bad ptr 5] 192.148.4.1,,RA value 256))
    localhost > localhost: udp
IP (tos 0x0, ttl 64, id 1053, offset 1936, flags [none], proto UDP (17), length 100, options (RR [bad length 4] [bad ptr 5] 192.148.4.1,,RA value 256))
    localhost > localhost: udp

After:

IP (tos 0x0, ttl 96, id 42549, offset 0, flags [+], proto UDP (17), length 996, options (RR [bad length 4] [bad ptr 5] 192.148.4.1,,RA value 256))
    localhost.51607 > localhost.search-agent: UDP, bad length 2000 > 960
IP (tos 0x0, ttl 96, id 42549, offset 968, flags [+], proto UDP (17), length 996, options (NOP,NOP,NOP,NOP,RA value 256))
    localhost > localhost: udp
IP (tos 0x0, ttl 96, id 42549, offset 1936, flags [none], proto UDP (17), length 100, options (NOP,NOP,NOP,NOP,RA value 256))
    localhost > localhost: udp

RA (20 | 0x80) is now copied as expected, RR (7) is "NOPed out".

Link: https://lore.kernel.org/netdev/20220107080559.122713-1-ooppublic@163.com/
Fixes: 19c3401a917b ("net: ipv4: place control buffer handling away from fragmentation iterators")
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: caixf <ooppublic@163.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 9bca57ef8b838f0139c34bfc342b9e0518cf8f09..ff38b46bd4b0f81fb7bdc6c028304fd33e1f612d 100644 (file)
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -826,15 +826,24 @@ int ip_do_fragment(struct net *net, struct sock *sk, struct sk_buff *skb,
                /* Everything is OK. Generate! */
                ip_fraglist_init(skb, iph, hlen, &iter);
 
-               if (iter.frag)
-                       ip_options_fragment(iter.frag);
-
                for (;;) {
                        /* Prepare header of the next frame,
                         * before previous one went down. */
                        if (iter.frag) {
+                               bool first_frag = (iter.offset == 0);
+
                                IPCB(iter.frag)->flags = IPCB(skb)->flags;
                                ip_fraglist_prepare(skb, &iter);
+                               if (first_frag && IPCB(skb)->opt.optlen) {
+                                       /* ipcb->opt is not populated for frags
+                                        * coming from __ip_make_skb(),
+                                        * ip_options_fragment() needs optlen
+                                        */
+                                       IPCB(iter.frag)->opt.optlen =
+                                               IPCB(skb)->opt.optlen;
+                                       ip_options_fragment(iter.frag);
+                                       ip_send_check(iter.iph);
+                               }
                        }
 
                        skb->tstamp = tstamp;