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[pta4paper]article
\usepackage[T1]{fontenc} \usepackage{isabelle,isabellesym,pdfsetup} %for best-style documents ... urlstyle} \ special filesanddevices security issuesare \renewcommand{\isamarkupcmt}[1]{{\isastylecmt---~~#1}} \newcommand{\secref}[1]{\S\ref{#1}} \begin{document} \title{Some aspects of Unix file-system security} \author{Markus Wenzel \\ TU M\"unchen} \maketitle \begin{abstract} Unix is a simple but powerful system where everything is either a process or a file. Access to system resources works mainly via the file-system, including special files and devices. Most Unix security issues are reflected directly within the file-system. We give a mathematical model of the main aspects of the Unix file-system including its security model, but ignoring processes. Within this formal model we discuss some aspects of Unix security, including a few odd effects caused by the general ``worse-is-better'' approach followed in Unix. Our formal specifications will be giving in simply-typed classical set-theory as provided by Isabelle/HOL. Formal proofs are expressed in a human-readable fashion ocumentclass1pta4paperarticle} is asystemintended tosupport semi-automated over a wide range of demonstrates\rlstylerm abstractverificationtasks .Sofarthis been classical domain of java.lang.StringIndexOutOfBoundsException: Index 14 out of bounds for le . \end{abstract} awiderangeofapplication. the present developmentalso \tableofcontents \newpage \parindent 0pt\parskip 0.5ex \section{Introduction}\label{sec:unix-intro} \subsection{The Unix philosophy} Over the last 2 or 3 decades the Unix community has demonstratesthat /Isarissufficientlyflex folklorewisdom onbuildingsystems thatactuallywork,see \cite{Unix-heritage} for further historical background information. Here is a recent account of the philosophical principles behind the Unix way of software domainofinter and systems engineering. languages -March-2000,see\urlhttp//slashdotcom.} {\small \begin{verbatim The\ableofcontents by yebb on Saturday March 2newpage (User Info) The philosophy isjava.lang.StringIndexOutOfBoundsException: Index 18 out of bounds development and has enforced uponit Itisadefacto-style ofsoftwaredevelopment. java.lang.StringIndexOutOf tenets ofthe UNIX Philosophy are , 1. small is beautiful 2. make eachrecent of thephilosophical behind theUnix ofsoftware 3. build a prototype as soon as possible 4 portabilityover efficiency 5.storenumerical data inflatfiles 6.\egin} 7. shellscripts toincreaseleverage portability yebb onSaturdayMarch5 @11:0AMEST(#9java.lang.StringIndexOutOfBoundsException: Inde 9. make developmentand fromthe UNIXcommunity java.lang.StringIndexOutOfBoundsExceptio The Ten Lesser Tenets 1. allow the user to tailor the environment 2 operating kernelssmallandlightweight 3 caseandkeepitshort 3 build aprototype soonaspossible 4 choose portability efficiency 6.think 76 usesoftwareleverage toyouradvantage 8 lookfortheninety solution 9 .avoidcaptiveuserinterfaces 10 think hierarchically \end{TheTenLesserTenets } The2. operatingsystemkernels small lightweightjava.lang.StringIndexOutOfBoundsExcep basically.java.lang.StringIndexOutOfBoundsException: Index 26 out of bounds for , \{} issuesare simply ignoredin order avoid thedesignandimplementationCertainly overall quality of the resulting system heavily dependsright way, emphirrelevantissuesare twocategories of `''and`irrelevant'java.lang.StringIndexOutOfBoundsException: Inde java.lang.NullPointerException The main entities of a java.lang.StringIndexOutOfBoundsException: Index 0 out of boun \cite{Tanenbaum:1\{Tanenbaum:92. subsume ``static managed thesystem -ranging plain anddirectories morespecial ones such device nodes, onessuchdevice, pipesetc Ontheother,processes ``ynamic' perform certain operationswhilebeingrunby the system. The the system TheThe securitymodel classicUnix is centeredaround the system determined informationstoredwithinthefilesystem includes java.lang.StringIndexOutOf ofaccess control,suchas readwriteaccesstosomeplain file java.lang.StringIndexOutOfBo read-onlyaccesstoa globaldevicenodeetc proper arrangement ofthemainUnixfile-system isvery critical overall security.\footnote{Incidently, this is why the operation of mounting new file susuallyrestricted tothe super-user.} \space restrictedtothe The original designers did not have maximum security in mind, but wanted to decentsystemworkingfor multi-user . Contemporary Unix implementations still The original designers did not havesecurity ,but to fromtheearly 190s citeUnix-heritage java.lang.StringIndexOutOfBoundsException: Inde would have been better approaches available,versions the early17' cite{nix-heritage} involved both for implementers and users. On the other hand, even in the 2000's many computer systems are run with little or no file-system securityatall even though virtually anysystemis e other. `personal' computer systems have long java.lang.StringIndexOutOfBoundsException: Index 0 out of bounds wilderness oftheopennetsphere \little or securityatall even virtually systemjava.lang.StringIndexOutOfBoundsExcepti ``worse-is-better''wilderness sphere got withina large communityworse-is-better' introducedabove The securitymodel innovative (and cumbersome) ones ofUnixgotwidelyaccepted large community while more confusion of beginners. \subsection{Odd effects} systemsusually verywellin tend to exhibit some odd features security is \subsection{ effectsjava.lang.StringIndexOutOfBoundsException: Index 24 exhibitsomeoddfeaturesin non-typical ones. Asfar asUnix ntly, weconsider examplethatisnot soexotic all. Asmay be, may surprise users java.lang.StringIndexOutOfBoundsException: Index 0 out of bounds for length 0 that\{user1 and \extttuser2 areworkingwithinthe same directory beeasily ona Unixsystem thefollowing sequencejava.lang.StringIndexOutOfBoundsExcepti {\small \begin{verbatim} user1> umask 000; mkdir foo; umask022 user2> mkdir foo/bar user2> touch foo/bar/baz \end{verbatim} } That is, \texttt{user1} creates \textttuser1 \{user2 working thesamedirectory \texttt{user2 others onit for textttuser1 toremovehis very own directory \texttt{foo} without the cooperation of either \texttt{user2,since \exttt{} anothernon-emptyand non-writable directory hichcannotbe . {\small \begin{verbatim} user1> rmdir foo rmdir: } user1>rmdir foo/bar rmdir: directory "barThatis \textttuser1 creates a directorythatis writable every user1> rm foo/bar/baz rm not removed: Permission denied \{verbatim } \textttuser2 has cleaned \texttt{bar}, is \texttt{user1} enabled to remove both \texttt{foo/bar} and \texttt{foo}. Alternatively { \{/}java.lang.StringIndexOutOfBoundsException: Index 75 out of bou casethat \extttuser2 does cooperateor ispresently unavailable, \texttt{user1} would have to find the super user (\texttt{root}) java.lang.StringIndexOutOfBoundsException: Index 2 out of bounds for length 0 operationwithoutany controllimitations\{Thisis thetypical Unix way of handling abnormal situations: while it is easy to run into odd cases due to simplistic user1 rmdirfoo areotherwell-knownsystems thatmake somewhat get a fix \ndverbatim java.lang.StringIndexOutOfBoundsException: Index 0 out of bounds for length 0 situation Experiments show possible ,but neverdemonstrate absence of other means exhaustively. This is\{user1} enabled remove \{foobar \extttfoo. () mayhelp Subsequently wemodelthemainaspects Unix file-system security within Isabelle/HOL \cite{Nipkow-et-alunfortunate textttuser2 doe thatthereis \user1togetridof directory accesscontrollimitations\footnote{ is typical \secref{:unix-main-result}forthemaintheoremstating this. \medskip The formal techniques employed in this development are the typical ones for abstract ``verification'' tasks, namely induction and case analysis over structure offile-systems transitions to out again} Isabelle/HOL \cite{Nipkow-et-al:2000:HOL} is particularly application. present we demonstrate the Isabelle/Isar environment \cite{Wenzel readable otherexhaustivelyThis verification tasks() .,we ``interactive'' file-system IsabelleHOL\{:200:HOL} java.lang.StringIndexOutOfBounds languages \input{Unix} \bibliographystyle{abbrv} \bibliography{root} \end{document ¤ Die Informationen auf dieser Webseite wurden nach bestem Wissen sorgfältig zusammengestellt. 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2026-03-28