thr3ads.net - netfilter buglog - [Bug 1390] New: iptables -m string not working with --algo bm and OUTPUT chain under 5.3.x [Dec 2019]

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bugzilla-daemon at netfilter.org

2019-Dec-11 12:54 UTC

[Bug 1390] New: iptables -m string not working with --algo bm and OUTPUT chain under 5.3.x

https://bugzilla.netfilter.org/show_bug.cgi?id=1390

            Bug ID: 1390
           Summary: iptables -m string not working with --algo bm and
                    OUTPUT chain under 5.3.x
           Product: netfilter/iptables
           Version: unspecified
          Hardware: x86_64
                OS: All
            Status: NEW
          Severity: normal
          Priority: P5
         Component: unknown
          Assignee: netfilter-buglog at lists.netfilter.org
          Reporter: bassosimone at gmail.com

Under 5.3.x, `iptables -A OUTPUT -p tcp -m string --algo bm --string POST -j
DROP` does not drop outgoing packets containing "POST". This command
was
instead working as intended with 5.0.0.

I used this test case to reproduce the issue and gather info:

```
#!/bin/sh
set -ex
uname -r
iptables -F
iptables -L -v
iptables -A OUTPUT -p tcp -m string --algo bm --string POST -j DROP
lsmod | grep -e nft -e ipt
echo POST|nc -w10 example.com 80
```

Some interesting aspects I discovered while investigating w/ colleagues:

1. `iptables -A OUTPUT -p tcp -m string --algo kmp --string 'POST' -j
DROP`
works with 5.3.x

2. `iptables -A INPUT -p tcp -m string --algo bm --string 'title' -j
DROP`
works with 5.3.x

This is the output of the above script on 5.3.0:

```
+ uname -r
5.3.0-1009-gcp
+ iptables -F
+ iptables -L -v
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination 

Chain FORWARD (policy DROP 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination 

Chain OUTPUT (policy ACCEPT 1 packets, 132 bytes)
 pkts bytes target     prot opt in     out     source               destination 

Chain DOCKER (0 references)
 pkts bytes target     prot opt in     out     source               destination 

Chain DOCKER-ISOLATION-STAGE-1 (0 references)
 pkts bytes target     prot opt in     out     source               destination 

Chain DOCKER-ISOLATION-STAGE-2 (0 references)
 pkts bytes target     prot opt in     out     source               destination 

Chain DOCKER-USER (0 references)
 pkts bytes target     prot opt in     out     source               destination 
+ iptables -A OUTPUT -p tcp -m string --algo bm --string POST -j DROP
+ lsmod
+ grep -e nft -e ipt
iptable_nat            16384  1
nf_nat                 40960  2 iptable_nat,xt_MASQUERADE
iptable_filter         16384  1
ipt_REJECT             16384  0
nf_reject_ipv4         16384  1 ipt_REJECT
ip_tables              32768  2 iptable_filter,iptable_nat
x_tables               40960  9
xt_conntrack,iptable_filter,xt_LOG,xt_string,xt_tcpudp,xt_addrtype,ipt_REJECT,ip_tables,xt_MASQUERADE
+ echo POST
+ nc -w10 example.com 80
HTTP/1.0 501 Not Implemented
Content-Type: text/html
Content-Length: 357
Connection: close
Date: Wed, 11 Dec 2019 12:43:33 GMT
Server: ECSF (nyb/1D24)

<?xml version="1.0" encoding="iso-8859-1"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
         "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en"
lang="en">
    <head>
        <title>501 - Not Implemented</title>
    </head>
    <body>
        <h1>501 - Not Implemented</h1>
    </body>
</html>
```

This is instead the script output when run on 5.0.0:

```
+ uname -r
5.0.0-1013-gcp
+ iptables -F
+ iptables -L -v
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination 
Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination 
Chain OUTPUT (policy ACCEPT 2 packets, 248 bytes)
 pkts bytes target     prot opt in     out     source               destination 
+ iptables -A OUTPUT -p tcp -m string --algo bm --string POST -j DROP
+ grep -e nft -e ipt
+ lsmod
iptable_filter         16384  1
ip_tables              28672  1 iptable_filter
x_tables               40960  3 iptable_filter,xt_string,ip_tables
+ nc -w10 example.com 80
+ echo POST
```

The same issue has also been seen on the following 5.3.x hosts:

OS: Debian
Kernel: 5.3.0-2-amd64
lsmod | grep -e ipt -e nft | sort
----
nf_nat                 49152  1 nft_chain_nat
nfnetlink              16384  3 nft_compat,nf_tables
nf_tables             163840  5 nft_compat,nft_counter,nft_chain_nat
nft_chain_nat          16384  0
nft_compat             20480  3
nft_counter            16384  1
x_tables               49152  4 nft_compat,xt_LOG,xt_string,xt_tcpudp
----
nft ruleset: 
meta l4proto tcp tcp dport 80 # STRING match  "POST" ALGO name bm TO
65535
counter packets 0 bytes 0 log


OS: Fedora Core 31
Kernel: 5.3.15-300.fc31.x86_64
lsmod | grep -e ipt -e nft | sort
----
iptable_filter         16384  1
iptable_mangle         16384  1
iptable_nat            16384  1
iptable_raw            16384  1
ip_tables              32768  5
iptable_filter,iptable_security,iptable_raw,iptable_nat,iptable_mangle
iptable_security       16384  1
ipt_REJECT             16384  0
nf_nat                 49152  4
ip6table_nat,nf_nat_tftp,iptable_nat,xt_MASQUERADE
nf_reject_ipv4         16384  1 ipt_REJECT
----
iptables -L -v
----
Chain INPUT (policy ACCEPT 4 packets, 246 bytes)
 pkts bytes target     prot opt in     out     source               destination 

Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination 

Chain OUTPUT (policy ACCEPT 2 packets, 135 bytes)
 pkts bytes target     prot opt in     out     source               destination 
    0     0 DROP       tcp  --  any    any     anywhere             anywhere   
         STRING match  "POST" ALGO name bm TO 65535

[snipped many chains with 0 references]
----

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bugzilla-daemon at netfilter.org

2019-Dec-11 12:58 UTC

head link

[Bug 1390] iptables -m string not working with --algo bm and OUTPUT chain under 5.3.x

https://bugzilla.netfilter.org/show_bug.cgi?id=1390

Federico Ceratto <federico at debian.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |federico at debian.org

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bugzilla-daemon at netfilter.org

2020-Jan-06 13:27 UTC

head link

[Bug 1390] iptables -m string not working with --algo bm and OUTPUT chain under 5.3.x

https://bugzilla.netfilter.org/show_bug.cgi?id=1390

Simone Basso <bassosimone at gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P5                          |P3

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bugzilla-daemon at netfilter.org

2020-Jan-31 05:01 UTC

head link

[Bug 1390] iptables -m string not working with --algo bm and OUTPUT chain under 5.3.x

https://bugzilla.netfilter.org/show_bug.cgi?id=1390

Doug Smythies <dsmythies at telus.net> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dsmythies at telus.net

--- Comment #1 from Doug Smythies <dsmythies at telus.net> ---
Created attachment 584
  --> https://bugzilla.netfilter.org/attachment.cgi?id=584&action=edit
iptables example rules and packet counters

I confirm your issues with the bm algorithm and "POST".
I confirm that the otherwise same rule works with the kmp algorithm.
I deny that the otherwise same rule and "test" as the pattern works.

If an offset is introduced such that the search area only looks at the payload
portion of the packet (offset 52), then it works.

I started with kernel 5.5-rc6, but narrowed this down to between kernel 5.1 and
5.2-rc1, but do not have time to bisect the kernel.

In the attachment, output rule 9 was added after rule 10 had been traversed 38
times. Thereafter rule 9 was traversed.

By this crude experiment, it seems to be byte 48 that messes things up. On my
computer it seems to be 0X05 (based on a sample of 1).

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bugzilla-daemon at netfilter.org

2020-Feb-01 00:42 UTC

head link

[Bug 1390] iptables -m string not working with --algo bm and OUTPUT chain under 5.3.x

https://bugzilla.netfilter.org/show_bug.cgi?id=1390

--- Comment #2 from Doug Smythies <dsmythies at telus.net> ---
Created attachment 586
  --> https://bugzilla.netfilter.org/attachment.cgi?id=586&action=edit
Attachment demonsrates an issue that was fixed around the same time

As a side note:
Prior to kernel 5.2-rc1 there was an issue where using the "--to"
string match
parameter didn't seem to work exactly as the man page described.
Just mentioning that while this bug report is about something that got broken,
it seems that something also got fixed.

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