Re: [regression] TC_MD5SIG on established sockets
On Wed, May 13, 2020 at 12:49 PM Eric Dumazet 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
> 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
Re: [regression] TC_MD5SIG on established sockets
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
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
[regression] TC_MD5SIG on established sockets
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
