> From: Syzop [mailto:syzat_private] > Sent: Thursday, August 30, 2001 5:59 AM > To: c0nceptat_private > Cc: VULN-DEVat_private; FOCUS-MSat_private > Subject: Re: Windows NT does not check permissions after HANDLEs are > open > > c0nceptat_private wrote: > > > The check against the ACL only occurs when the HANDLE > > is first opened, however. If a HANDLE is opened and > > permissions on the objecect subsiquently change, the original > > requestor of the object retains the original access-permissions. > Isn't this normal? Yes, it's pretty typical for discretionary access control (DAC) mechanisms to perform access checking when a subject first requests access to an object (eg. opens a file) and grant that access (if allowed by the DAC) for the subject's lifetime or until it's specifically relinquished (eg. by closing the handle). In principle, a reference monitor should check every access, but "every access" is pretty vague. Should the monitor check every operation a subject performs on an object - eg. every read and write on a file - against the current ACL? That would be expensive, so instead most systems establish a boundary at which a subject (eg. a program, running with the rights of a particular user, who may belong to one or more groups) is granted access to an object for any number of subsequent operations (generally of certain kinds - eg. you open a file for reading or writing or read-write). Once the subject is past that boundary the monitor remembers its decision for that {subject, object, operation(s)} tuple and doesn't need to revisit the ACL. This is indeed an issue for timely revocation of access, but it's a well-known one, and arguably not much of a threat in most cases. See for example the aside about revocation in capability-based DAC implementations in section 7.1 of the Rainbow book on DAC mechanisms.[1] [1] http://security.isu.edu/isl/dac.html Michael Wojcik Principal Software Systems Developer, Micro Focus Department of English, Miami University
This archive was generated by hypermail 2b30 : Thu Aug 30 2001 - 09:14:48 PDT