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Annotation of src/share/man/man7/setuid.7, Revision 1.5.12.1

1.5.12.1! tls         1: .\" $NetBSD$
1.1       wiz         2: .\"
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                      6: .\" This code is derived from software contributed to The NetBSD Foundation
                      7: .\" by Henry Spencer <henry@spsystems.net>.
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1.3       christos   30: .Dd February 26, 2009
1.5       joerg      31: .Dt SETUID 7
1.1       wiz        32: .Os
                     33: .Sh NAME
                     34: .Nm setuid
                     35: .Nd checklist for security of setuid programs
                     36: .Sh DESCRIPTION
                     37: .Em Please note :
                     38: This manual page was written long ago, and is in need of updating to
                     39: match today's systems.
                     40: We think it is valuable enough to include, even though parts of it
                     41: are outdated.
                     42: A carefully-researched updated version
                     43: would be very useful, if anyone is feeling enthusiastic...
                     44: .Pp
                     45: Writing a secure setuid (or setgid) program is tricky.
                     46: There are a number of possible ways of subverting such a program.
                     47: The most conspicuous security holes occur when a setuid program is
                     48: not sufficiently careful to avoid giving away access to resources
                     49: it legitimately has the use of.
                     50: Most of the other attacks are basically a matter of altering the program's
                     51: environment in unexpected ways and hoping it will fail in some
                     52: security-breaching manner.
                     53: There are generally three categories of environment manipulation:
                     54: supplying a legal but unexpected environment that may cause the
                     55: program to directly do something insecure,
                     56: arranging for error conditions that the program may not handle correctly,
                     57: and the specialized subcategory of giving the program inadequate
                     58: resources in hopes that it won't respond properly.
                     59: .Pp
                     60: The following are general considerations of security when writing
                     61: a setuid program.
                     62: .Bl -bullet
                     63: .It
                     64: The program should run with the weakest userid possible, preferably
                     65: one used only by itself.
                     66: A security hole in a setuid program running with a highly-privileged
                     67: userid can compromise an entire system.
                     68: Security-critical programs like
                     69: .Xr passwd 1
                     70: should always have private userids, to minimize possible damage
                     71: from penetrations elsewhere.
                     72: .It
                     73: The result of
                     74: .Xr getlogin 2
                     75: or
                     76: .Xr ttyname 3
                     77: may be wrong if the descriptors have been meddled with.
                     78: There is
                     79: .Em no
                     80: foolproof way to determine the controlling terminal
                     81: or the login name (as opposed to uid) on V7.
                     82: .It
                     83: On some systems, the setuid bit may not be honored if
                     84: the program is run by root,
                     85: so the program may find itself running as root.
                     86: .It
                     87: Programs that attempt to use
                     88: .Xr creat 3
                     89: for locking can foul up when run by root;
                     90: use of
                     91: .Xr link 2
                     92: is preferred when implementing locking.
                     93: Using
                     94: .Xr chmod 2
                     95: for locking is an obvious disaster.
                     96: .It
                     97: Breaking an existing lock is very dangerous; the breakdown of a locking
                     98: protocol may be symptomatic of far worse problems.
                     99: Doing so on the basis of the lock being
                    100: .Sq old
                    101: is sometimes necessary,
                    102: but programs can run for surprising lengths of time on heavily-loaded
                    103: systems.
                    104: .It
                    105: Care must be taken that user requests for I/O are checked for
                    106: permissions using the user's permissions, not the program's.
                    107: Use of
                    108: .Xr access 2
                    109: is recommended.
                    110: .It
                    111: Programs executed at user request (e.g. shell escapes) must
                    112: not receive the setuid program's permissions;
                    113: use of daughter processes and
                    114: .Dq setuid(getuid())
                    115: plus
                    116: .Dq setgid(getgid())
                    117: after
                    118: .Xr fork 2
                    119: but before
                    120: .Xr exec 3
                    121: is vital.
                    122: .It
                    123: Similarly, programs executed at user request must not receive other
                    124: sensitive resources, notably file descriptors.
1.3       christos  125: Use of
                    126: .Xr fcntl 2
                    127: .Dv F_CLOSEM ,
                    128: .Dv FILENO_STDERR + 1
                    129: (close all fd's greater than stderr)
                    130: and/or
                    131: .Xr fcntl 2
                    132: .Dv F_SETFD ,
                    133: .Dv FD_CLOEXEC
                    134: (close-on-exec) arrangements
                    135: on systems which have them
                    136: is recommended.
                    137: .Pp
                    138: Other resources should also be examined for sanity and possibly set to
                    139: desired settings, such as the current working directory, signal disposition,
                    140: resource limits, environment, umask, group membership, chroot.
1.1       wiz       141: .Pp
                    142: Programs activated by one user but handling traffic on behalf of
                    143: others (e.g. daemons) should avoid doing
                    144: .Dq setuid(getuid())
                    145: or
                    146: .Dq setgid(getgid()) ,
                    147: since the original invoker's identity is almost certainly inappropriate.
                    148: On systems which permit it, use of
                    149: .Dq setuid(geteuid())
                    150: and
                    151: .Dq setgid(getegid())
                    152: is recommended when performing work on behalf of the system as
                    153: opposed to a specific user.
                    154: .It
                    155: There are inherent permission problems when a setuid program executes
                    156: another setuid program,
                    157: since the permissions are not additive.
                    158: Care should be taken that created files are not owned by the wrong person.
                    159: Use of
                    160: .Dq setuid(geteuid())
                    161: and its gid counterpart can help, if the system allows them.
                    162: .It
                    163: Care should be taken that newly-created files do not have the wrong
                    164: permission or ownership even momentarily.
                    165: Permissions should be arranged by using
                    166: .Xr umask 2
                    167: in advance, rather than by creating the file wide-open and then using
                    168: .Xr chmod 2 .
                    169: Ownership can get sticky due to the limitations of the setuid concept,
                    170: although using a daughter process connected by a pipe can help.
                    171: .It
                    172: Setuid programs should be especially careful about error checking,
                    173: and the normal response to a strange situation should be termination,
                    174: rather than an attempt to carry on.
                    175: .El
                    176: .Pp
                    177: The following are ways in which the program may be induced to carelessly
                    178: give away its special privileges.
                    179: .Bl -bullet
                    180: .It
                    181: The directory the program is started in, or directories it may
                    182: plausibly
                    183: .Xr chdir 2
                    184: to, may contain programs with the same names as system programs,
                    185: placed there in hopes that the program will activate a shell with
                    186: a permissive
                    187: .Ev PATH
                    188: setting.
                    189: .Ev PATH
                    190: should
                    191: .Em always
                    192: be standardized before invoking a shell
                    193: (either directly or via
                    194: .Xr popen 3
                    195: or
                    196: .Xr execvp 3
                    197: or
                    198: .Xr execlp 3 ) .
                    199: .It
                    200: Similarly, a bizarre
                    201: .Ev IFS
                    202: setting may alter the interpretation of a shell command in really
                    203: strange ways, possibly causing a user-supplied program to be invoked.
                    204: .Ev IFS
                    205: too should always be standardized before invoking a shell.
                    206: .It
                    207: Environment variables in general cannot be trusted.
                    208: Their contents should never be taken for granted.
                    209: .It
                    210: Setuid shell files (on systems which implement such) simply cannot
                    211: cope adequately with some of these problems.
                    212: They also have some nasty problems like trying to run a
                    213: .Pa \&.profile
                    214: when run under a suitable name.
                    215: They are terminally insecure, and must be avoided.
                    216: .It
                    217: Relying on the contents of files placed in publically-writable
                    218: directories, such as
                    219: .Pa /tmp ,
                    220: is a nearly-incurable security problem.
                    221: Setuid programs should avoid using
                    222: .Pa /tmp
                    223: entirely, if humanly possible.
                    224: The sticky-directories modification (sticky bit on for a directory means
                    225: only owner of a file can remove it) helps,
                    226: but is not a complete solution.
                    227: .It
                    228: A related problem is that
                    229: spool directories, holding information that the program will trust
                    230: later, must never be publically writable even if the files in the
                    231: directory are protected.
                    232: Among other sinister manipulations that can be performed, note that
                    233: on many Unixes, a core dump of a setuid program is owned
                    234: by the program's owner and not by the user running it.
                    235: .El
                    236: .Pp
                    237: The following are unusual but possible error conditions that the
                    238: program should cope with properly (resource-exhaustion questions
                    239: are considered separately, see below).
                    240: .Bl -bullet
                    241: .It
                    242: The value of
                    243: .Ar argc
                    244: might be 0.
                    245: .It
                    246: The setting of the
                    247: .Xr umask 2
                    248: might not be sensible.
                    249: In any case, it should be standardized when creating files
                    250: not intended to be owned by the user.
                    251: .It
                    252: One or more of the standard descriptors might be closed, so that
                    253: an opened file might get (say) descriptor 1, causing chaos if the
                    254: program tries to do a
                    255: .Xr printf 3 .
                    256: .It
                    257: The current directory (or any of its parents)
                    258: may be unreadable and unsearchable.
                    259: On many systems
                    260: .Xr pwd 1
                    261: does not run setuid-root,
                    262: so it can fail under such conditions.
                    263: .It
                    264: Descriptors shared by other processes (i.e., any that are open
                    265: on startup) may be manipulated in strange ways by said processes.
                    266: .It
                    267: The standard descriptors may refer to a terminal which has a bizarre
                    268: mode setting, or which cannot be opened again,
                    269: or which gives end-of-file on any read attempt, or which cannot
                    270: be read or written successfully.
                    271: .It
                    272: The process may be hit by interrupt, quit, hangup, or broken-pipe signals,
                    273: singly or in fast succession.
                    274: The user may deliberately exploit the race conditions inherent
                    275: in catching signals;
                    276: ignoring signals is safe, but catching them is not.
                    277: .It
                    278: Although non-keyboard signals cannot be sent by ordinary users in V7,
                    279: they may perhaps be sent by the system authorities (e.g. to
                    280: indicate that the system is about to shut down),
                    281: so the possibility cannot be ignored.
                    282: .It
                    283: On some systems there may be an
                    284: .Xr alarm 3
                    285: signal pending on startup.
                    286: .It
                    287: The program may have children it did not create.
                    288: This is normal when the process is part of a pipeline.
                    289: .It
                    290: In some non-V7 systems, users can change the ownerships of their files.
                    291: Setuid programs should avoid trusting the owner identification of a file.
                    292: .It
                    293: User-supplied arguments and input data
                    294: .Em must
                    295: be checked meticulously.
                    296: Overly-long input stored in an array without proper bound checking
                    297: can easily breach security.
                    298: When software depends on a file being in a specific format, user-supplied
                    299: data should never be inserted into the file without being checked first.
                    300: Meticulous checking includes allowing for the possibility of non-ASCII
                    301: characters.
                    302: .It
                    303: Temporary files left in public directories like
                    304: .Pa /tmp
                    305: might vanish at inconvenient times.
                    306: .El
                    307: .Pp
                    308: The following are resource-exhaustion possibilities that the
                    309: program should respond properly to.
                    310: .Bl -bullet
                    311: .It
                    312: The user might have used up all of his allowed processes, so
                    313: any attempt to create a new one (via
                    314: .Xr fork 2
                    315: or
                    316: .Xr popen 3 )
                    317: will fail.
                    318: .It
                    319: There might be many files open, exhausting the supply of descriptors.
1.3       christos  320: Running
                    321: .Xr fcntl 2
1.4       wiz       322: .Dv F_CLOSEM
1.3       christos  323: on systems which have it,
                    324: is recommended.
1.1       wiz       325: .It
                    326: There might be many arguments.
                    327: .It
                    328: The arguments and the environment together might occupy a great deal
                    329: of space.
                    330: .El
                    331: .Pp
                    332: Systems which impose other resource limitations can open setuid
                    333: programs to similar resource-exhaustion attacks.
                    334: .Pp
                    335: Setuid programs which execute ordinary programs without reducing
                    336: authority pass all the above problems on to such unprepared children.
                    337: Standardizing the execution environment is only a partial solution.
1.3       christos  338: .Sh SEE ALSO
                    339: .Xr passwd 1 ,
                    340: .Xr pwd 1 ,
                    341: .Xr access 2 ,
                    342: .Xr chdir 2 ,
                    343: .Xr chroot 2 ,
                    344: .Xr execve 2 ,
                    345: .Xr fcntl 2 ,
                    346: .Xr fork 2 ,
                    347: .Xr getlogin 2 ,
                    348: .Xr link 2 ,
                    349: .Xr setegid 2 ,
                    350: .Xr seteuid 2 ,
                    351: .Xr setgid 2 ,
1.4       wiz       352: .Xr setgroups 2 ,
                    353: .Xr setrlimit 2 ,
1.3       christos  354: .Xr setuid 2 ,
                    355: .Xr sigaction 2 ,
                    356: .Xr umask 2 ,
                    357: .Xr alarm 3 ,
1.4       wiz       358: .Xr creat 3 ,
1.3       christos  359: .Xr execvp 3 ,
                    360: .Xr popen 3 ,
                    361: .Xr printf 3 ,
                    362: .Xr ttyname 3
1.1       wiz       363: .Sh HISTORY
                    364: Written by Henry Spencer, and based on additional outside contributions.
                    365: .Sh AUTHORS
1.5.12.1! tls       366: .An Henry Spencer Aq Mt henry@spsystems.net
1.1       wiz       367: .Sh BUGS
                    368: The list really is rather long...
                    369: and probably incomplete.

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