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Date:   Fri, 1 Oct 2021 10:13:06 -0400
From:   "J. Bruce Fields" <bfields@fieldses.org>
To:     Volodymyr Khomenko <volodymyr@vastdata.com>
Cc:     linux-nfs@vger.kernel.org
Subject: Re: GSSAPI fix for pynfs nfs4.1 client code
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On Fri, Oct 01, 2021 at 09:49:50AM +0300, Volodymyr Khomenko wrote:
> > So, I can verify that --security=krb5 works after this patch but not
> > before, good.  But why is that?  As you say, the server is supposed to
> > ignore the sequence number on context creation requests.  And 0 is valid
> > sequence number as far as I know.
> 
> By design of RPCGSS we have a 'last seen seq_number' counter on the
> server side per each GSS context
> and we must not accept any packet that was already seen before (we
> also have a bitmask of several last requests for this).
> This 'last seen counter' is unsigned int32 (the same as seq_num) so we
> can't just init it to -1 so next seq_num=0 will be valid.
> The most obvious implementation is to init it last_seen_seq_num to 0
> so the very 1st packet after NFS4 NULL must be 1 to differ from last
> seen seq_number.

Note in theory gssapi mechanisms can require multiple round trips (in
the GSS_PROC_CONTINUE_INIT case), so this wouldn't actually avoid
duplicate sequence numbers.

In any case, the rfc is unambiguous here: "In a creation request, the
seq_num and service fields are undefined and both must be ignored by the
server."

> A better implementation (theoretically) can set this counter to
> 'undefined' state by additional flag, but this is  more
> resource-consuming
> (you need to process is_inited flag + last_seen_seq_num instead of
> just one counter).
> If the last seen seq_number is undefined during GSS initialization,
> then strictly speaking we can send ANY seq_num for the very 1st
> request after NFS4 NULL.

Right, again, from RFC 2203, " The seq_num field can start at any value
below MAXSEQ."

It can be implemented without the need for an is_inited flag.

The initial sequence number of 0 really did find an actual bug in the
server, so pynfs is definitely doing its job in this case!

--b.