Re: [regression] TC_MD5SIG on established sockets

[Eric Dumazet]

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.

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