[PATCH net] ipv4: fix ip option filtering for locally generated fragments

[Jakub Kicinski <kuba@kernel.org>]

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>
---
 net/ipv4/ip_output.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 57c1d8431386..e331c8d4e6cf 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -825,15 +825,24 @@ 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;
-- 
2.31.1