The function takes the two KEXINIT packets in their string form,
together with a list of mappings from names to known algorithm
implementations, and returns the selected one of each kind, along with
all the other necessary auxiliary stuff.
I've introduced a new POD struct type 'ssh_ttymodes' which stores an
encoding of everything you can specify in the "pty-req" packet or the
SSH-1 equivalent. This allows me to split up
write_ttymodes_to_packet_from_conf() into two separate functions, one
to parse all the ttymode data out of a Conf (and a Seat for fallback)
and return one of those structures, and the other to write it into an
SSH packet.
While I'm at it, I've moved the special case of terminal speeds into
the same mechanism, simplifying the call sites in both versions of the
SSH protocol.
The new master definition of all terminal modes lives in a header
file, with an ifdef around each item, so that later on I'll be able to
include it in a context that only enumerates the modes supported by
the particular target Unix platform.
This gets another big pile of logic out of ssh2connection and puts it
somewhere more central. Now the only thing left in ssh2connection is
the formatting and parsing of the various channel requests; the logic
deciding which ones to issue and what to do about them is devolved to
the Channel implementation, as it properly should be.
This is a new vtable-based abstraction which is passed to a backend in
place of Frontend, and it implements only the subset of the Frontend
functions needed by a backend. (Many other Frontend functions still
exist, notably the wide range of things called by terminal.c providing
platform-independent operations on the GUI terminal window.)
The purpose of making it a vtable is that this opens up the
possibility of creating a backend as an internal implementation detail
of some other activity, by providing just that one backend with a
custom Seat that implements the methods differently.
For example, this refactoring should make it feasible to directly
implement an SSH proxy type, aka the 'jump host' feature supported by
OpenSSH, aka 'open a secondary SSH session in MAINCHAN_DIRECT_TCP
mode, and then expose the main channel of that as the Socket for the
primary connection'. (Which of course you can already do by spawning
'plink -nc' as a separate proxy process, but this would permit it in
the _same_ process without anything getting confused.)
I've centralised a full set of stub methods in misc.c for the new
abstraction, which allows me to get rid of several annoying stubs in
the previous code. Also, while I'm here, I've moved a lot of
duplicated modalfatalbox() type functions from application main
program files into wincons.c / uxcons.c, which I think saves
duplication overall. (A minor visible effect is that the prefixes on
those console-based fatal error messages will now be more consistent
between applications.)
LogContext is now the owner of the logevent() function that back ends
and so forth are constantly calling. Previously, logevent was owned by
the Frontend, which would store the message into its list for the GUI
Event Log dialog (or print it to standard error, or whatever) and then
pass it _back_ to LogContext to write to the currently open log file.
Now it's the other way round: LogContext gets the message from the
back end first, writes it to its log file if it feels so inclined, and
communicates it back to the front end.
This means that lots of parts of the back end system no longer need to
have a pointer to a full-on Frontend; the only thing they needed it
for was logging, so now they just have a LogContext (which many of
them had to have anyway, e.g. for logging SSH packets or session
traffic).
LogContext itself also doesn't get a full Frontend pointer any more:
it now talks back to the front end via a little vtable of its own
called LogPolicy, which contains the method that passes Event Log
entries through, the old askappend() function that decides whether to
truncate a pre-existing log file, and an emergency function for
printing an especially prominent message if the log file can't be
created. One minor nice effect of this is that console and GUI apps
can implement that last function subtly differently, so that Unix
console apps can write it with a plain \n instead of the \r\n
(harmless but inelegant) that the old centralised implementation
generated.
One other consequence of this is that the LogContext has to be
provided to backend_init() so that it's available to backends from the
instant of creation, rather than being provided via a separate API
call a couple of function calls later, because backends have typically
started doing things that need logging (like making network
connections) before the call to backend_provide_logctx. Fortunately,
there's no case in the whole code base where we don't already have
logctx by the time we make a backend (so I don't actually remember why
I ever delayed providing one). So that shortens the backend API by one
function, which is always nice.
While I'm tidying up, I've also moved the printf-style logeventf() and
the handy logevent_and_free() into logging.c, instead of having copies
of them scattered around other places. This has also let me remove
some stub functions from a couple of outlying applications like
Pageant. Finally, I've removed the pointless "_tag" at the end of
LogContext's official struct name.
The sshverstring quasi-frontend is passed a Frontend pointer at setup
time, so that it can generate Event Log entries containing the local
and remote version strings and the results of remote bug detection.
I'm promoting that field of sshverstring to a field of the public BPP
structure, so now all BPPs have the right to talk directly to the
frontend if they want to. This means I can move all the log messages
of the form 'Initialised so-and-so cipher/MAC/compression' down into
the BPPs themselves, where they can live exactly alongside the actual
initialisation of those primitives.
It also means BPPs will be able to log interesting things they detect
at any point in the packet stream, which is about to come in useful
for another purpose.
I haven't needed these until now, but I'm about to need to inspect the
entire contents of a packet queue before deciding whether to process
the first item on it.
I've changed the single 'vtable method' in packet queues from get(),
which returned the head of the queue and optionally popped it, to
after() which does the same bug returns the item after a specified
tree node. So if you pass the special end node to after(), then it
behaves like get(), but now you can also use it to retrieve the
successor of a packet.
(Orthogonality says that you can also _pop_ the successor of a packet
by calling after() with prev != pq.end and pop == TRUE. I don't have a
use for that one yet.)
All the main backend structures - Ssh, Telnet, Pty, Serial etc - now
describe structure types themselves rather than pointers to them. The
same goes for the codebase-wide trait types Socket and Plug, and the
supporting types SockAddr and Pinger.
All those things that were typedefed as pointers are older types; the
newer ones have the explicit * at the point of use, because that's
what I now seem to be preferring. But whichever one of those is
better, inconsistently using a mixture of the two styles is worse, so
let's make everything consistent.
A few types are still implicitly pointers, such as Bignum and some of
the GSSAPI types; generally this is either because they have to be
void *, or because they're typedefed differently on different
platforms and aren't always pointers at all. Can't be helped. But I've
got rid of the main ones, at least.
The check_termination function in ssh2connection is supposed to be
called whenever it's possible that we've run out of (a) channels, and
(b) sharing downstreams. I've been calling it on every channel close,
but apparently completely forgot to add a callback from sshshare.c
that also arranges to call it when we run out of downstreams.
In commit 8cb68390e I managed to copy the packet contexts inaccurately
from the old implementation of ssh2_pkt_type, and listed the ECDH KEX
packets against SSH2_PKTCTX_DHGEX instead of SSH2_PKTCTX_ECDHKEX,
which led to them appearing as "unknown" in packet log files.
I've tried to separate out as many individually coherent changes from
this work as I could into their own commits, but here's where I run
out and have to commit the rest of this major refactoring as a
big-bang change.
Most of ssh.c is now no longer in ssh.c: all five of the main
coroutines that handle layers of the SSH-1 and SSH-2 protocols now
each have their own source file to live in, and a lot of the
supporting functions have moved into the appropriate one of those too.
The new abstraction is a vtable called 'PacketProtocolLayer', which
has an input and output packet queue. Each layer's main coroutine is
invoked from the method ssh_ppl_process_queue(), which is usually
(though not exclusively) triggered automatically when things are
pushed on the input queue. In SSH-2, the base layer is the transport
protocol, and it contains a pair of subsidiary queues by which it
passes some of its packets to the higher SSH-2 layers - first userauth
and then connection, which are peers at the same level, with the
former abdicating in favour of the latter at the appropriate moment.
SSH-1 is simpler: the whole login phase of the protocol (crypto setup
and authentication) is all in one module, and since SSH-1 has no
repeat key exchange, that setup layer abdicates in favour of the
connection phase when it's done.
ssh.c itself is now about a tenth of its old size (which all by itself
is cause for celebration!). Its main job is to set up all the layers,
hook them up to each other and to the BPP, and to funnel data back and
forth between that collection of modules and external things such as
the network and the terminal. Once it's set up a collection of packet
protocol layers, it communicates with them partly by calling methods
of the base layer (and if that's ssh2transport then it will delegate
some functionality to the corresponding methods of its higher layer),
and partly by talking directly to the connection layer no matter where
it is in the stack by means of the separate ConnectionLayer vtable
which I introduced in commit 8001dd4cb, and to which I've now added
quite a few extra methods replacing services that used to be internal
function calls within ssh.c.
(One effect of this is that the SSH-1 and SSH-2 channel storage is now
no longer shared - there are distinct struct types ssh1_channel and
ssh2_channel. That means a bit more code duplication, but on the plus
side, a lot fewer confusing conditionals in the middle of half-shared
functions, and less risk of a piece of SSH-1 escaping into SSH-2 or
vice versa, which I remember has happened at least once in the past.)
The bulk of this commit introduces the five new source files, their
common header sshppl.h and some shared supporting routines in
sshcommon.c, and rewrites nearly all of ssh.c itself. But it also
includes a couple of other changes that I couldn't separate easily
enough:
Firstly, there's a new handling for socket EOF, in which ssh.c sets an
'input_eof' flag in the BPP, and that responds by checking a flag that
tells it whether to report the EOF as an error or not. (This is the
main reason for those new BPP_READ / BPP_WAITFOR macros - they can
check the EOF flag every time the coroutine is resumed.)
Secondly, the error reporting itself is changed around again. I'd
expected to put some data fields in the public PacketProtocolLayer
structure that it could set to report errors in the same way as the
BPPs have been doing, but in the end, I decided propagating all those
data fields around was a pain and that even the BPPs shouldn't have
been doing it that way. So I've reverted to a system where everything
calls back to functions in ssh.c itself to report any connection-
ending condition. But there's a new family of those functions,
categorising the possible such conditions by semantics, and each one
has a different set of detailed effects (e.g. how rudely to close the
network connection, what exit status should be passed back to the
whole application, whether to send a disconnect message and/or display
a GUI error box).
I don't expect this to be immediately perfect: of course, the code has
been through a big upheaval, new bugs are expected, and I haven't been
able to do a full job of testing (e.g. I haven't tested every auth or
kex method). But I've checked that it _basically_ works - both SSH
protocols, all the different kinds of forwarding channel, more than
one auth method, Windows and Linux, connection sharing - and I think
it's now at the point where the easiest way to find further bugs is to
let it out into the wild and see what users can spot.
This means that someone putting things on a packet queue doesn't need
to separately hold a pointer to someone who needs notifying about it,
or remember to call the notification function every time they push
things on the queue. It's all taken care of automatically, without
having to put extra stuff at the call sites.
The precise semantics are that the callback will be scheduled whenever
_new_ packets appear on the queue, but not when packets are removed.
(Because the expectation is that the callback is notifying whoever is
consuming the queue.)
This paves the way for me to reorganise ssh.c in a way that will mean
I don't have a ConnectionLayer available yet at the time I have to
create the connshare. The constructor function now takes a mere
Frontend, for generating setup-time Event Log messages, and there's a
separate ssh_connshare_provide_connlayer() function I can call later
once I have the ConnectionLayer to provide.
NFC for the moment: the new provide_connlayer function is called
immediately after ssh_connection_sharing_init.
This is essentially trivial, because the only thing it needed from the
Ssh structure was the Conf. So the version in sshcommon.c just takes
an actual Conf as an argument, and now it doesn't need access to the
big structure definition any more.
This is a new idea I've had to make memory-management of PktIn even
easier. The idea is that a PktIn is essentially _always_ an element of
some linked-list queue: if it's not one of the queues by which packets
move through ssh.c, then it's a special 'free queue' which holds
packets that are unowned and due to be freed.
pq_pop() on a PktInQueue automatically relinks the packet to the free
queue, and also triggers an idempotent callback which will empty the
queue and really free all the packets on it. Hence, you can pop a
packet off a real queue, parse it, handle it, and then just assume
it'll get tidied up at some point - the only constraint being that you
have to finish with it before returning to the application's main loop.
The exception is that it's OK to pq_push() the packet back on to some
other PktInQueue, because a side effect of that will be to _remove_ it
from the free queue again. (And if _all_ the incoming packets get that
treatment, then when the free-queue handler eventually runs, it may
find it has nothing to do - which is harmless.)
Now I've got a list macro defining all the packet types we recognise,
I can use it to write a test for 'is this a recognised code?', and use
that in turn to centralise detection of completely unrecognised codes
into the binary packet protocol, where any such messages will be
handled entirely internally and never even be seen by the next level
up. This lets me remove another big pile of boilerplate in ssh.c.
This allows me to share just one definition of the packet types
between the enum declarations in ssh.h and the string translation
functions in sshcommon.c. No functional change.
The style of list macro is slightly unusual; instead of the
traditional 'X-macro' in which LIST(X) expands to invocations of the
form X(list element), this is an 'X-y macro', where LIST(X,y) expands
to invocations of the form X(y, list element). That style makes it
possible to wrap the list macro up in another macro and pass a
parameter through from the wrapper to the per-element macro. I'm not
using that facility just yet, but I will in the next commit.
Moved the typedef of BinaryPacketProtocol into defs.h on the general
principle that it's just the kind of thing that ought to go there;
also removed the declaration of pq_base_init from ssh.h on the grounds
that there's never been any such function! (At least, not in public
source control - it existed in an early draft of commit 6e24b7d58.)
I've removed the encrypted_len fields from PktIn and PktOut, which
were used to communicate from the BPP to ssh.c how much each packet
contributed to the amount of data encrypted with a given set of cipher
keys. It seems more sensible to have the BPP itself keep that counter
- especially since only one of the three BPPs even needs to count it
at all. So now there's a new DataTransferStats structure which the BPP
updates, and ssh.c only needs to check it for overflow and reset the
limits.
That function _did_ depend on ssh.c's internal facilities, namely the
layout of 'struct ssh_channel'. In place of that, it now takes an
extra integer argument telling it where to find the channel id in
whatever data structure you give it a tree of - so now I can split up
the SSH-1 and SSH-2 channel handling without losing the services of
that nice channel-number allocator.
While I'm at it, I've brought it all into a single function: the
parsing of data from Conf, the list of modes, and even the old
callback system for writing to the destination buffer is now a simple
if statement that formats mode parameters as byte or uint32 depending
on SSH version. Also, the terminal speeds and the end byte are part of
the same setup, so it's all together in one place instead of scattered
all over ssh.c.
It's really just a concatenator for a pair of linked lists, but
unhelpfully restricted in which of the lists it replaces with the
output. Better to have a three-argument function that puts the output
wherever you like, whether it overlaps either or neither one of the
inputs.
Some upcoming restructuring I've got planned will need to pass output
packets back and forth on queues, as well as input ones. So here's a
change that arranges that we can have a PktInQueue and a PktOutQueue,
sharing most of their implementation via a PacketQueueBase structure
which links together the PacketQueueNode fields in the two packet
structures.
There's a tricksy bit of macro manoeuvring to get all of this
type-checked, so that I can't accidentally link a PktOut on to a
PktInQueue or vice versa. It works by having the main queue functions
wrapped by macros; when receiving a packet structure on input, they
type-check it against the queue structure and then automatically look
up its qnode field to pass to the underlying PacketQueueBase function;
on output, they translate a returned PacketQueueNode back to its
containing packet type by calling a 'get' function pointer.
This should make it easier to do formatted-string based logging
outside ssh.c, because I can wrap up a local macro in any source file
I like that expands to logevent_and_free(wherever my Frontend is,
dupprintf(macro argument)).
It caused yet another stub function to be needed in testbn, but there
we go.
(Also, while I'm here, removed a redundant declaration of logevent
itself from ssh.h. The one in putty.h is all we need.)
This is a vtable that wraps up all the functionality required from the
SSH connection layer by associated modules like port forwarding and
connection sharing. This extra layer of indirection adds nothing
useful right now, but when I later separate the SSH-1 and SSH-2
connection layer implementations, it will be convenient for each one
to be able to implement this vtable in terms of its own internal data
structures.
To simplify this vtable, I've moved a lot of the logging duties
relating to connection sharing out of ssh.c into sshshare.c: now it
handles nearly all the logging itself relating to setting up
connection sharing in the first place and downstreams connecting and
disconnecting. The only exception is the 'Reusing a shared connection'
announcement in the console window, which is now done in ssh.c by
detecting downstream status immediately after setup.
The tree234 storing currently active port forwardings - both local and
remote - now lives in portfwd.c, as does the complicated function that
updates it based on a Conf listing the new set of desired forwardings.
Local port forwardings are passed to ssh.c via the same route as
before - once the listening port receives a connection and portfwd.c
knows where it should be directed to (in particular, after the SOCKS
exchange, if any), it calls ssh_send_port_open.
Remote forwardings are now initiated by calling ssh_rportfwd_alloc,
which adds an entry to the rportfwds tree (which _is_ still in ssh.c,
and still confusingly sorted by a different criterion depending on SSH
protocol version) and sends out the appropriate protocol request.
ssh_rportfwd_remove cancels one again, sending a protocol request too.
Those functions look enough like ssh_{alloc,remove}_sharing_rportfwd
that I've merged those into the new pair as well - now allocating an
rportfwd allows you to specify either a destination host/port or a
sharing context, and returns a handy pointer you can use to cancel the
forwarding later.
Clients outside ssh.c - all implementations of Channel - will now not
see the ssh_channel data type itself, but only a subobject of the
interface type SshChannel. All the sshfwd_* functions have become
methods in that interface type's vtable (though, wrapped in the usual
kind of macros, the call sites look identical).
This paves the way for me to split up the SSH-1 and SSH-2 connection
layers and have each one lay out its channel bookkeeping structure as
it sees fit; as long as they each provide an implementation of the
sshfwd_ method family, the types behind that need not look different.
A minor good effect of this is that the sshfwd_ methods are no longer
global symbols, so they don't have to be stubbed in Unix Pageant to
get it to compile.
This was mildly fiddly because there's a single vtable structure that
implements two distinct interface types, one for compression and one
for decompression - and I have actually confused them before now
(commit d4304f1b7), so I think it's important to make them actually be
separate types!
hmacmd5_do_hmac and hmac_sha1_simple should be consistently referring
to input memory blocks as 'const void *', but one had pointlessly
typed the pointer as 'const unsigned char *' and the other had missed
out the consts.
The new version of ssh_hash has the same nice property as ssh2_mac,
that I can make the generic interface object type function directly as
a BinarySink so that clients don't have to call h->sink() and worry
about the separate sink object they get back from that.
This piece of tidying-up has come out particularly well in terms of
saving tedious repetition and boilerplate. I've managed to remove
three pointless methods from every MAC implementation by means of
writing them once centrally in terms of the implementation-specific
methods; another method (hmacmd5_sink) vanished because I was able to
make the interface type 'ssh2_mac' be directly usable as a BinarySink
by way of a new delegation system; and because all the method
implementations can now find their own vtable, I was even able to
merge a lot of keying and output functions that had previously
differed only in length parameters by having them look up the lengths
in whatever vtable they were passed.
This is more or less the same job as the SSH-1 case, only more
extensive, because we have a wider range of ciphers.
I'm a bit disappointed about the AES case, in particular, because I
feel as if it ought to have been possible to arrange to combine this
layer of vtable dispatch with the subsidiary one that selects between
hardware and software implementations of the underlying cipher. I may
come back later and have another try at that, in fact.
The interchangeable system of SSH-1 ciphers previously followed the
same pattern as the backends and the public-key algorithms, in that
all the clients would maintain two separate pointers, one to the
vtable and the other to the individual instance / context. Now I've
merged them, just as I did with those other two, so that you only cart
around a single pointer, which has a vtable pointer inside it and a
type distinguishing it from an instance of any of the other
interchangeable sets of algorithms.
ATOFFSET in dialog.h became obsolete when the old 'struct Config' gave
way to the new Conf, because its only use was to identify fields in
struct Config for the generic control handlers to update.
And lenof in ssh.h is redundant because there's also a copy in misc.h.
(Which is already included _everywhere_ that lenof is used - I didn't
even need to add any instances of #include "misc.h" after removing the
copy in ssh.h.)
This was a particularly confusing piece of type-danger, because three
different types were passed outside sshshare.c as 'void *' and only
human vigilance prevented one coming back as the wrong one. Now they
all keep their opaque structure tags when they move through other
parts of the code.
There's now an interface called 'Channel', which handles the local
side of an SSH connection-layer channel, in terms of knowing where to
send incoming channel data to, whether to close the channel, etc.
Channel and the previous 'struct ssh_channel' mutually refer. The
latter contains all the SSH-specific parts, and as much of the common
logic as possible: in particular, Channel doesn't have to know
anything about SSH packet formats, or which SSH protocol version is in
use, or deal with all the fiddly stuff about window sizes - with the
exception that x11fwd.c's implementation of it does have to be able to
ask for a small fixed initial window size for the bodgy system that
distinguishes upstream from downstream X forwardings.
I've taken the opportunity to move the code implementing the detailed
behaviour of agent forwarding out of ssh.c, now that all of it is on
the far side of a uniform interface. (This also means that if I later
implement agent forwarding directly to a Unix socket as an
alternative, it'll be a matter of changing just the one call to
agentf_new() that makes the Channel to plug into a forwarding.)
This is another major source of unexplained 'void *' parameters
throughout the code.
In particular, the currently unused testback.c actually gave the wrong
pointer type to its internal store of the frontend handle - it cast
the input void * to a Terminal *, from which it got implicitly cast
back again when calling from_backend, and nobody noticed. Now it uses
the right type internally as well as externally.
Nearly every part of the code that ever handles a full backend
structure has historically done it using a pair of pointer variables,
one pointing at a constant struct full of function pointers, and the
other pointing to a 'void *' state object that's passed to each of
those.
While I'm modernising the rest of the code, this seems like a good
time to turn that into the same more or less type-safe and less
cumbersome system as I'm using for other parts of the code, such as
Socket, Plug, BinaryPacketProtocol and so forth: the Backend structure
contains a vtable pointer, and a system of macro wrappers handles
dispatching through that vtable.
Now when we construct a packet containing sensitive data, we just set
a field saying '... and make it take up at least this much space, to
disguise its true size', and nothing in the rest of the system worries
about that flag until ssh2bpp.c acts on it.
Also, I've changed the strategy for doing the padding. Previously, we
were following the real packet with an SSH_MSG_IGNORE to make up the
size. But that was only a partial defence: it works OK against passive
traffic analysis, but an attacker proxying the TCP stream and
dribbling it out one byte at a time could still have found out the
size of the real packet by noting when the dribbled data provoked a
response. Now I put the SSH_MSG_IGNORE _first_, which should defeat
that attack.
But that in turn doesn't work when we're doing compression, because we
can't predict the compressed sizes accurately enough to make that
strategy sensible. Fortunately, compression provides an alternative
strategy anyway: if we've got zlib turned on when we send one of these
sensitive packets, then we can pad out the compressed zlib data as
much as we like by adding empty RFC1951 blocks (effectively chaining
ZLIB_PARTIAL_FLUSHes). So both strategies should now be dribble-proof.
The return value wasn't used to indicate failure; it only indicated
whether any compression was being done at all or whether the
compression method was ssh_comp_none, and we can tell the latter just
as well by the fact that its init function returns a null context
pointer.
sshbpp.h now defines a classoid that encapsulates both directions of
an SSH binary packet protocol - that is, a system for reading a
bufchain of incoming data and turning it into a stream of PktIn, and
another system for taking a PktOut and turning it into data on an
outgoing bufchain.
The state structure in each of those files contains everything that
used to be in the 'rdpkt2_state' structure and its friends, and also
quite a lot of bits and pieces like cipher and MAC states that used to
live in the main Ssh structure.
One minor effect of this layer separation is that I've had to extend
the packet dispatch table by one, because the BPP layer can no longer
directly trigger sending of SSH_MSG_UNIMPLEMENTED for a message too
short to have a type byte. Instead, I extend the PktIn type field to
use an out-of-range value to encode that, and the easiest way to make
that trigger an UNIMPLEMENTED message is to have the dispatch table
contain an entry for it.
(That's a system that may come in useful again - I was also wondering
about inventing a fake type code to indicate network EOF, so that that
could be propagated through the layers and be handled by whichever one
currently knew best how to respond.)
I've also moved the packet-censoring code into its own pair of files,
partly because I was going to want to do that anyway sooner or later,
and mostly because it's called from the BPP code, and the SSH-2
version in particular has to be called from both the main SSH-2 BPP
and the bare unencrypted protocol used for connection sharing. While I
was at it, I took the opportunity to merge the outgoing and incoming
censor functions, so that the parts that were common between them
(e.g. CHANNEL_DATA messages look the same in both directions) didn't
need to be repeated.
ssh.c has been an unmanageably huge monolith of a source file for too
long, and it's finally time I started breaking it up into smaller
pieces. The first step is to move some declarations - basic types like
packets and packet queues, standard constants, enums, and the
coroutine system - into headers where other files can see them.
Simon Tatham