Re: POSIX permission mapping and NULL SIDs

[Andrey Repin <anrdaemon@yandex.ru>]

Greetings, Bill Zissimopoulos!

>>> 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)

Why not S-1-0-65535 ? It'll map to 0x1FFFF then without any special rules.

> 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



-- 
With best regards,
Andrey Repin
Monday, June 27, 2016 12:08:13

Sorry for my terrible english...