Subject: [regression] TC_MD5SIG on established sockets

From: Mathieu Desnoyers <[email-masked]>
Date: Wed, 13 May 2020 15:38:35 -0400 (EDT)

Hi,

I am reporting a regression with respect to use of TCP_MD5SIG/TCP_MD5SIG_EXT
on established sockets. It is observed by a customer.

This issue is introduced by this commit:

commit 721230326891 "tcp: md5: reject TCP_MD5SIG or TCP_MD5SIG_EXT on established sockets"

The intent of this commit appears to be to fix a use of uninitialized value in
tcp_parse_options(). The change introduced by this commit is to disallow setting
the TCP_MD5SIG{,_EXT} socket options on an established socket.

The justification for this change appears in the commit message:

   "I believe this was caused by a TCP_MD5SIG being set on live
    flow.
    
    This is highly unexpected, since TCP option space is limited.
    
    For instance, presence of TCP MD5 option automatically disables
    TCP TimeStamp option at SYN/SYNACK time, which we can not do
    once flow has been established.
    
    Really, adding/deleting an MD5 key only makes sense on sockets
    in CLOSE or LISTEN state."

However, reading through RFC2385 [1], this justification does not appear
correct. Quoting to the RFC:

   "This password never appears in the connection stream, and the actual
    form of the password is up to the application. It could even change
    during the lifetime of a particular connection so long as this change
    was synchronized on both ends"

The paragraph above clearly underlines that changing the MD5 signature of
a live TCP socket is allowed.

I also do not understand why it would be invalid to transition an established
TCP socket from no-MD5 to MD5, or transition from MD5 to no-MD5. Quoting the
RFC:

  "The total header size is also an issue.  The TCP header specifies
   where segment data starts with a 4-bit field which gives the total
   size of the header (including options) in 32-byte words.  This means
   that the total size of the header plus option must be less than or
   equal to 60 bytes -- this leaves 40 bytes for options."

The paragraph above seems to be the only indication that some TCP options
cannot be combined on a given TCP socket: if the resulting header size does
not fit. However, I do not see anything in the specification preventing any
of the following use-cases on an established TCP socket:

- Transition from no-MD5 to MD5,
- Transition from MD5 to no-MD5,
- Changing the MD5 key associated with a socket.

As long as the resulting combination of options does not exceed the available
header space.

Can we please fix this KASAN report in a way that does not break user-space
applications expectations about Linux' implementation of RFC2385 ?

Thanks,

Mathieu

[1] RFC2385: https://tools.ietf.org/html/rfc2385

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com