SecurityPolicySection: File Formats (5)
Updated: xorg-server 220.127.116.11
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NAMESecurityPolicy - X Window System SECURITY Extension Policy file format
DESCRIPTIONThe SECURITY extension to the X Window System uses a policy file to determine which operations should be allowed or denied. The default location for this file is /usr/lib/xserver/SecurityPolicy.
The syntax of the security policy file is as follows. Notation: "*" means zero or more occurrences of the preceding element, and "+" means one or more occurrences. To interpret <foo/bar>, ignore the text after the /; it is used to distinguish between instances of <foo> in the next section.
<policy file> ::= <version line> <other line>* <version line> ::= <string/v> '\n' <other line > ::= <comment> | <access rule> | <site policy> | <blank line> <comment> ::= # <not newline>* '\n' <blank line> ::= <space> '\n' <site policy> ::= sitepolicy <string/sp> '\n' <access rule> ::= property <property/ar> <window> <perms> '\n' <property> ::= <string> <window> ::= any | root | <required property> <required property> ::= <property/rp> | <property with value> <property with value> ::= <property/rpv> = <string/rv> <perms> ::= [ <operation> | <action> | <space> ]* <operation> ::= r | w | d <action> ::= a | i | e <string> ::= <dbl quoted string> | <single quoted string> | <unquoted string> <dbl quoted string> ::= <space> " <not dquote>* " <space> <single quoted string> ::= <space> ' <not squote>* ' <space> <unquoted string> ::= <space> <not space>+ <space> <space> ::= [ ' ' | '\t' ]* Character sets: <not newline> ::= any character except '\n' <not dquote> ::= any character except " <not squote> ::= any character except ' <not space> ::= any character except those in <space>
The semantics associated with the above syntax are as follows.
<version line>, the first line in the file, specifies the file format version. If the server does not recognize the version <string/v>, it ignores the rest of the file. The version string for the file format described here is "version-1" .
Once past the <version line>, lines that do not match the above syntax are ignored.
<comment> lines are ignored.
<sitepolicy> lines are currently ignored. They are intended to specify the site policies used by the XC-QUERY-SECURITY-1 authorization method.
<access rule> lines specify how the server should react to untrusted client requests that affect the X Window property named <property/ar>. The rest of this section describes the interpretation of an <access rule>.
For an <access rule> to apply to a given instance of <property/ar>, <property/ar> must be on a window that is in the set of windows specified by <window>. If <window> is any, the rule applies to <property/ar> on any window. If <window> is root, the rule applies to <property/ar> only on root windows.
If <window> is <required property>, the following apply. If <required property> is a <property/rp>, the rule applies when the window also has that <property/rp>, regardless of its value. If <required property> is a <property with value>, <property/rpv> must also have the value specified by <string/rv>. In this case, the property must have type STRING and format 8, and should contain one or more null-terminated strings. If any of the strings match <string/rv>, the rule applies.
The definition of string matching is simple case-sensitive string comparison with one elaboration: the occurrence of the character '*' in <string/rv> is a wildcard meaning "any string." A <string/rv> can contain multiple wildcards anywhere in the string. For example, "x*" matches strings that begin with x, "*x" matches strings that end with x, "*x*" matches strings containing x, and "x*y*" matches strings that start with x and subsequently contain y.
There may be multiple <access rule> lines for a given <property/ar>. The rules are tested in the order that they appear in the file. The first rule that applies is used.
<perms> specify operations that untrusted clients may attempt, and the actions that the server should take in response to those operations.
<operation> can be r (read), w (write), or d (delete). The following table shows how X Protocol property requests map to these operations in the X.Org server implementation.
GetProperty r, or r and d if delete = True ChangeProperty w RotateProperties r and w DeleteProperty d ListProperties none, untrusted clients can always list all properties
<action> can be a (allow), i (ignore), or e (error). Allow means execute the request as if it had been issued by a trusted client. Ignore means treat the request as a no-op. In the case of GetProperty, ignore means return an empty property value if the property exists, regardless of its actual value. Error means do not execute the request and return a BadAtom error with the atom set to the property name. Error is the default action for all properties, including those not listed in the security policy file.
An <action> applies to all <operation>s that follow it, until the next <action> is encountered. Thus, irwad means ignore read and write, allow delete.
GetProperty and RotateProperties may do multiple operations (r and d, or r and w). If different actions apply to the operations, the most severe action is applied to the whole request; there is no partial request execution. The severity ordering is: allow < ignore < error. Thus, if the <perms> for a property are ired (ignore read, error delete), and an untrusted client attempts GetProperty on that property with delete = True, an error is returned, but the property value is not. Similarly, if any of the properties in a RotateProperties do not allow both read and write, an error is returned without changing any property values.
Here is an example security policy file.
version-1 # Allow reading of application resources, but not writing. property RESOURCE_MANAGER root ar iw property SCREEN_RESOURCES root ar iw # Ignore attempts to use cut buffers. Giving errors causes apps to crash, # and allowing access may give away too much information. property CUT_BUFFER0 root irw property CUT_BUFFER1 root irw property CUT_BUFFER2 root irw property CUT_BUFFER3 root irw property CUT_BUFFER4 root irw property CUT_BUFFER5 root irw property CUT_BUFFER6 root irw property CUT_BUFFER7 root irw # If you are using Motif, you probably want these. property _MOTIF_DEFAULT_BINDINGS rootar iw property _MOTIF_DRAG_WINDOW root ar iw property _MOTIF_DRAG_TARGETS any ar iw property _MOTIF_DRAG_ATOMS any ar iw property _MOTIF_DRAG_ATOM_PAIRS any ar iw # The next two rules let xwininfo -tree work when untrusted. property WM_NAME any ar # Allow read of WM_CLASS, but only for windows with WM_NAME. # This might be more restrictive than necessary, but demonstrates # the <required property> facility, and is also an attempt to # say "top level windows only." property WM_CLASS WM_NAME ar # These next three let xlsclients work untrusted. Think carefully # before including these; giving away the client machine name and command # may be exposing too much. property WM_STATE WM_NAME ar property WM_CLIENT_MACHINE WM_NAME ar property WM_COMMAND WM_NAME ar # To let untrusted clients use the standard colormaps created by # xstdcmap, include these lines. property RGB_DEFAULT_MAP root ar property RGB_BEST_MAP root ar property RGB_RED_MAP root ar property RGB_GREEN_MAP root ar property RGB_BLUE_MAP root ar property RGB_GRAY_MAP root ar # To let untrusted clients use the color management database created # by xcmsdb, include these lines. property XDCCC_LINEAR_RGB_CORRECTION rootar property XDCCC_LINEAR_RGB_MATRICES rootar property XDCCC_GRAY_SCREENWHITEPOINT rootar property XDCCC_GRAY_CORRECTION rootar # To let untrusted clients use the overlay visuals that many vendors # support, include this line. property SERVER_OVERLAY_VISUALS rootar # Dumb examples to show other capabilities. # oddball property names and explicit specification of error conditions property "property with spaces" 'property with "'aw er ed # Allow deletion of Woo-Hoo if window also has property OhBoy with value # ending in "son". Reads and writes will cause an error. property Woo-Hoo OhBoy = "*son"ad
- Default X server security policy
Xserver(1), Security Extension Specification