[Patch] Allow to disable root privileges with CYGWIN=noroot

[Christian Franke]

Hi Corinna,

Corinna Vinschen wrote:
> New patch attached.  I made the test a bit more foolproof, hopefully.
> And a restricted token does not require to load the user's registry hive,
> nor should Cygwin try to enable the backup/restore permissions in the
> new token.  That spoils the idea of a restricted token a bit...
> ...
>
>   

Thanks!

> +  bool request_restricted_uid_switch =
> +     uid == myself->uid
> +     && (   (cygheap->user.external_token != NO_IMPERSONATION
> +	     && IsTokenRestricted (cygheap->user.external_token))
> +	 || (cygheap->user.external_token == NO_IMPERSONATION
> +	     && cygheap->user.issetuid ()
> +	     && IsTokenRestricted (cygheap->user.curr_primary_token)));
>   


Unfortunately this does not work for a typical use case: an admin 
process creates a restricted token with standard user rights. The 
function IsTokenRestricted() returns TRUE only if the token contains 
'restricted SIDs'.
(http://msdn.microsoft.com/en-us/library/aa379137(VS.85).aspx)

Test with tokens returned by SaferComputeTokenFromLevel():
(http://msdn.microsoft.com/en-us/library/ms972827.aspx)

SAFER_LEVELID_NORMALUSER:  IsTokenRestricted()=FALSE
SAFER_LEVELID_CONSTRAINED: IsTokenRestricted()=TRUE
SAFER_LEVELID_UNTRUSTED:   IsTokenRestricted()=TRUE

BTW: Only NORMALUSER is works for Cygwin. Using DropMyRights.exe to 
start of a Cygwin process with a CONTRAINED token results in:

5 [sig] true 3788 C:\cygwin-1.7\bin\true.exe:
   *** fatal error - couldn't create signal pipe, Win32 error 5


There is apparently no function to check whether a token is a result of 
CreateRestrictedToken() or SaferComputeTokenFromLevel().

Would'nt it be easier to add a new function 
'cygwin_set_restricted_token(token)' instead of the test of the token type?

Christian