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 <[email protected]> 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
