On Wed, May 13, 2020 at 12:49 PM Eric Dumazet <eduma...@google.com> 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.desnoy...@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