Re: [regression] TC_MD5SIG on established sockets

[Eric Dumazet]

On Wed, May 13, 2020 at 12:49 PM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>
> I do not think we want to transition sockets in the middle. since
> packets can be re-ordered in the network.
>
> MD5 is about security (and a loose form of it), so better make sure
> all packets have it from the beginning of the flow.
>
> A flow with TCP TS on can not suddenly be sending packets without TCP TS.
>
> Clearly, trying to support this operation is a can of worms, I do not
> want to maintain such atrocity.
>
> RFC can state whatever it wants, sometimes reality forces us to have
> sane operations.
>
> Thanks.


Also the RFC states :

"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"

It means the key can be changed, but this does not imply the option
can be turned on/off dynamically.



>
> On Wed, May 13, 2020 at 12:38 PM Mathieu Desnoyers
> <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
> >
> > 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