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From:	Bill Zissimopoulos <billziss AT navimatics DOT com>
To:	"cygwin AT cygwin DOT com" <cygwin AT cygwin DOT com>
Subject:	Re: POSIX permission mapping and NULL SIDs
Date:	Sun, 26 Jun 2016 20:46:58 +0000
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On 6/24/16, 2:59 PM, "Corinna Vinschen" <cygwin-owner AT cygwin DOT com on behalf
of corinna-cygwin AT cygwin DOT com> wrote:


>>>If you want some specific mapping we can arrange that, but it must not
>> >be the NULL SID.  If you know you're communicating with a Cygwin
>>process,
>> >what about using an arbitrary, unused SID like S-1-0-42?
>> 
>> I am inclined to try S-1-5-7 (Anonymous). But I do not know if that is a
>> bad choice for some reason or other.
>
>I thought about Anonymous myself when I wrote my reply to your OP.  I
>refrained from mentioning it because it might have some unexpected side
>effect we're not aware about.

I ended up implementing this a couple of days ago. I was just spending a
lazy Sunday morning and then it hit me: this is an exceptionally bad idea.

The problem is that Windows uses the Anonymous identity for accounts who
have not logged in using a password (as per Erik Soderquist’s email
regarding IIS behavior). Files in FUSE file systems that have a UID that
cannot be mapped to a SID, will suddenly be owned by that Anonymous user!

Obviously this is a huge security hole. I intend to fix this ASAP, but I
am now back to where we started. The obvious SID to use is the NULL SID,
but that is already used by Cygwin for other purposes.

>> The main reason that I am weary of using an unused SID is that Microsoft
>> may decide to assign some special powers to it in a future release (e.g.
>> GodMode SID). But I agree that this is rather unlikely in the S-1-0-X
>> namespace.
>
>I think it's very unlikely.  We could chose any RID value we like and
>the chance for collision is nil.  When I created the new implementation
>for POSIX ACLs, I toyed around with this already and used a special
>Cygwin SID within the NULL SID AUTHORITY.  I'm not entirely sure why I
>changed this to the NULL SID deny ACE.  I think I disliked the fact that
>almost every Cygwin ACL would contain a mysterious "unknown SID".

Ideally we should choose a SID that:

(1) Is very unlikely to be used by Microsoft at any point in the future.
(2) Cannot be associated to a user logon for any reason (see problem with
Anonymous SID) above.
(3) Maps to a reasonable UID in Cygwin.

I propose the following SID/UID mapping:

    S-1-0-99 <=> UID 0xffffffff (32-bit -1)

This is a SID in the S-1-0 (Null Authority) namespace (same one that
contains the NULL SID), which is unlikely to be used by Microsoft. So it
likely satisfies (1).

For the same reason (that it is a new/unused SID in the S-1-0) namespace,
I think it also satisfies (2).

If we follow the rules from Cygwin’s "POSIX accounts, permission, and
security” document [IDMAP], the SID S-1-0-99 maps to 0x10063. But we can
make a special rule for this SID to map it to a different UID. Mapping it
to -1 may be the easiest option, but perhaps we can also consider mapping
it to 0xfffffffe (-2).

Bill

[IDMAP] https://cygwin.com/cygwin-ug-net/ntsec.html

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