1a6a0951fc
When we check the tcp options of a packet and it doesn't match the current
fingerprint, the tcp packet option pointer must be restored to its initial
value in order to do the proper tcp options check for the next fingerprint.
Here we can see an example.
Assumming the following fingerprint base with two lines:
S10:64:1:60:M*,S,T,N,W6: Linux:3.0::Linux 3.0
S20:64:1:60:M*,S,T,N,W7: Linux:4.19:arch:Linux 4.1
Where TCP options are the last field in the OS signature, all of them overlap
except by the last one, ie. 'W6' versus 'W7'.
In case a packet for Linux 4.19 kicks in, the osf finds no matching because the
TCP options pointer is updated after checking for the TCP options in the first
line.
Therefore, reset pointer back to where it should be.
Fixes: 11eeef41d5f6 ("netfilter: passive OS fingerprint xtables match")
Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.