From: Eric Dumazet <edumazet@google.com>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: "David S. Miller" <davem@davemloft.net>,
linux-kernel <linux-kernel@vger.kernel.org>,
netdev <netdev@vger.kernel.org>,
Yuchung Cheng <ycheng@google.com>,
Jonathan Rajotte-Julien <joraj@efficios.com>
Subject: Re: [regression] TC_MD5SIG on established sockets
Date: Wed, 13 May 2020 12:49:57 -0700 [thread overview]
Message-ID: <CANn89i+GH2ukLZUcWYGquvKg66L9Vbto0FxyEt3pOJyebNxqBg@mail.gmail.com> (raw)
In-Reply-To: <341326348.19635.1589398715534.JavaMail.zimbra@efficios.com>
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@efficios.com> 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
next prev parent reply other threads:[~2020-05-13 19:50 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-13 19:38 [regression] TC_MD5SIG on established sockets Mathieu Desnoyers
2020-05-13 19:49 ` Eric Dumazet [this message]
2020-05-13 19:56 ` Eric Dumazet
2020-06-29 19:43 ` [regression] TCP_MD5SIG " Mathieu Desnoyers
2020-06-29 20:47 ` Eric Dumazet
2020-06-30 19:43 ` Linus Torvalds
2020-06-30 19:52 ` Linus Torvalds
2020-06-30 20:34 ` Mathieu Desnoyers
2020-06-30 20:39 ` Eric Dumazet
2020-06-30 20:44 ` David Miller
2020-06-30 20:56 ` Eric Dumazet
2020-06-30 21:17 ` Mathieu Desnoyers
2020-06-30 21:23 ` Eric Dumazet
2020-06-30 21:54 ` Eric Dumazet
2020-06-30 22:07 ` Eric Dumazet
2020-06-30 22:38 ` Eric Dumazet
2020-06-30 23:44 ` Mathieu Desnoyers
2020-07-01 0:01 ` Eric Dumazet
2020-07-01 2:02 ` Herbert Xu
2020-07-01 2:17 ` Eric Dumazet
2020-07-01 2:22 ` Herbert Xu
2020-07-01 2:30 ` Eric Dumazet
2020-07-01 2:39 ` Joe Perches
2020-07-01 2:58 ` Herbert Xu
2020-07-01 3:36 ` Eric Dumazet
2020-07-01 3:50 ` Herbert Xu
2020-07-01 12:19 ` Mathieu Desnoyers
2020-07-01 15:15 ` Eric Dumazet
2020-07-01 17:24 ` Eric Dumazet
2020-06-30 20:21 ` David Miller
2020-06-30 20:30 ` Eric Dumazet
2020-06-30 20:39 ` Mathieu Desnoyers
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