AF_UNIX status report

[Joe Lowe]

On 2020-11-05 11:01, Ken Brown via Cygwin-developers wrote:
> On 11/5/2020 12:21 PM, Corinna Vinschen wrote:
>> On Nov  5 09:23, Ken Brown via Cygwin-developers wrote:
>>> OK, here's how I imagine this working:
>>>
>>> A process wants to send a file descriptor fd, so it creates a msghdr 
>>> with an
>>> SCM_RIGHTS cmsghdr and calls sendmsg.  The latter creates and sends 
>>> an admin
>>> packet A containing the fhandler for fd, and then it sends the original
>>> packet P.
>>>
>>> At the receiving end, recvmsg sees packet A first (recvmsg is always
>>> checking for admin packets anyway whenever it's called).  It stores the
>>> fhandler somewhere.  When it then reads packet P, it retrieves the 
>>> stored
>>> fhandler, fiddles with it (duplicating handles, etc.), and creates 
>>> the new
>>> file descriptor.
>>
>> Actually, this needs to be implemented in a source/dest-independent
>> manner.  Only the server of the named pipe can impersonate the client.
>> So the server side should do the job of duplicating the handles.  If the
>> sever is also the source of SCM_RIGHTS, it should send the fhandler with
>> already duplicated handles.
> 
> Ah, OK.  I was thinking of it differently.  Rather than having the 
> server impersonate the client, I was thinking that the sender would send 
> its winpid as part of its admin packet, which the receiver could then 
> use to get a handle to the sender's process.  The receiver could then 
> duplicate the handles.  But maybe your approach is better.  I'll have to 
> rethink it.

SCM_RIGHTS on *nix; fd are retained by message buffering in the kernel. 
A sending process can close an fd after sendmsg is called, before 
recvmsg is called in the receiving process.

SCM_RIGHTS on *nix; fd are not added to a receiving process fd table 
until the SCM_RIGHTS message is read. This is a consideration for DOS 
attacks.

So I expect it is necessary to create a temp copy of each fd being sent, 
so the sender can close the original. And I expect it is necessary to 
use handshake/acks between the two processes; so the DuplicateHandle() 
call can happen in the correct process and not until the SCM_RIGHTS 
message is read.

>>> Does this seem reasonable?  The main thing bothering me is the lack of
>>> atomicity.  I don't like the gap between the sending of the two 
>>> packets A
>>> and P, and similarly for the receiving.  I thought about using the 
>>> io_lock
>>> to at least make sure that the two packets are adjacent in the pipe, 
>>> but I
>>> don't know if we want to tie up the io_lock for that long.
>>>
>>> Also, the sending process might be sending several file descriptors 
>>> at once,
>>> so that there would be several admin packets to be sent (unless we 
>>> want to
>>> cram it all into one).
>>
>> We can safely assume that pipe packets up to 64K are sent and received
>> atomically.
>>
>> In most cases this shouldn't be much of a problem.  Most scenarios using
>> SCM_RIGHTS send no or only a minor payload.  Most scenarios share a
>> single or only a handful of descriptors.
>>
>> Apart from that, Linux also defines SCM_MAX_FD, the max. number of
>> descriptors in a single sendmsg call.  If the number of descriptors
>> is larger, sendmsg returns EINVAL.  SCM_MAX_FD is 253 on Linux, but
>>
>> What that means to us is, we can choose our own SCM_MAX_FD and just
>> return EINVAL if the number of descriptors is uncomfortably high.
>> The max. number of descriptors should be limited so that all descriptors
>> fit into 64K, or even 32K, just to leave space for payload.
>> Assuming a size of about 600 bytes per fhandler, 50 might be a good
>> candidate for SCM_MAX_FD.  I'd say even 32 would be sufficent for most
>> scenarios.
>>
>> The idea would be to create the packet on the source side with all
>> fhandlers in the ancilliary data block, followed by the payload.
>> This should typically fit in a 64K package.  If not, only the
>> payload needs to be split into multiple packages.  Do we really
>> need atomicity there?  Not sure, but only then we'd need an io_lock.

Joe L.