From: Jakub Kicinski kuba@kernel.org
[ 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 --- 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 e77afaecc9818..4f76e8183f403 100644 --- a/net/ipv4/ip_output.c +++ b/net/ipv4/ip_output.c @@ -834,15 +834,24 @@ 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;