ubuntu sudoers改坏了。

pkexec bash

通过如上命令，可以进入root模式，然后恢复sudoers。前提是root账户没被禁用。

precise (1) pkexec.1.gz

Provided by: policykit-1_0.104-1_i386

NAME

       pkexec - Execute a command as another user

SYNOPSIS

       pkexec [--version] [--disable-internal-agent] [--help]

       pkexec [--user username] PROGRAM [ARGUMENTS...]

DESCRIPTION

       pkexec allows an authorized user to execute PROGRAM as another user. If
       username is not specified, then the program will be executed as the
       administrative super user, root.

RETURN VALUE

       Upon successful completion, the return value is the return value of
       PROGRAM. If the calling process is not authorized or an authorization
       could not be obtained through authentication or an error occured,
       pkexec exits with a return value of 127. If the authorization could not
       be obtained because the user dismissed the authentication dialog,
       pkexec exits with a return value of 126.

AUTHENTICATION AGENT

       pkexec, like any other PolicyKit application, will use the
       authentication agent registered for the calling process. However, if no
       authentication agent is available, then pkexec will register its own
       textual authentication agent. This behavior can be turned off by
       passing the --disable-internal-agent option.

SECURITY NOTES

       Executing a program as another user is a privileged operation. By
       default the required authorization (See the section called "REQUIRED
       AUTHORIZATIONS") requires administrator authentication. In addition,
       the authentication dialog presented to the user will display the full
       path to the program to be executed so the user is aware of what will
       happen:

           [IMAGE][1]

               +----------------------------------------------------------+
               |                     Authenticate                     [X] |
               +----------------------------------------------------------+
               |                                                          |
               |  [Icon]  Authentication is needed to run `/bin/bash'     |
               |          as the super user                               |
               |                                                          |
               |          An application is attempting to perform an      |
               |          action that requires privileges. Authentication |
               |          as the super user is required to perform this   |
               |          action.                                         |
               |                                                          |
               |          Password for root: [_________________________]  |
               |                                                          |
               | [V] Details:                                             |
               |  Command: /bin/bash                                      |
               |  Run As:  Super User (root)                              |
               |  Action:  org.freedesktop.policykit.exec                 |
               |  Vendor:  The PolicyKit Project                          |
               |                                                          |
               |                                  [Cancel] [Authenticate] |
               +----------------------------------------------------------+

       The environment that PROGRAM will run it, will be set to a minimal
       known and safe environment in order to avoid injecting code through
       LD_LIBRARY_PATH or similar mechanisms. In addition the PKEXEC_UID
       environment variable is set to the user id of the process invoking
       pkexec. As a result, pkexec will not allow you to run X11 applications
       as another user since the $DISPLAY and $XAUTHORITY environment
       variables are not set. These two variables will be retained if the
       org.freedesktop.policykit.exec.allow_gui annotation on an action is set
       to a nonempty value; this is discouraged, though, and should only be
       used for legacy programs.

REQUIRED AUTHORIZATIONS

       By default, the org.freedesktop.policykit.exec authorization is
       required unless an action definition file is present for the program in
       question. To require another authorization, it can be specified using
       the org.freedesktop.policykit.exec.path annotation on an action (See
       the section called "EXAMPLE" for details).

EXAMPLE

       To specify what kind of authorization is needed to execute the program
       /usr/bin/pk-example-frobnicate as another user, simply write an action
       definition file like this

           <?xml version="1.0" encoding="UTF-8"?>
           <!DOCTYPE policyconfig PUBLIC
            "-//freedesktop//DTD PolicyKit Policy Configuration 1.0//EN"
            "http://www.freedesktop.org/standards/PolicyKit/1/policyconfig.dtd">
           <policyconfig>

             <vendor>Examples for the PolicyKit Project</vendor>
             <vendor_url>http://hal.freedesktop.org/docs/PolicyKit/</vendor_url>

             <action id="org.freedesktop.policykit.example.pkexec.run-frobnicate">
               <description>Run the PolicyKit example program Frobnicate</description>
               <description xml:lang="da">Kor PolicyKit eksemplet Frobnicate</description>
               <message>Authentication is required to run the PolicyKit example program Frobnicate (user=$(user), program=$(program), command_line=$(command_line))</message>
               <message xml:lang="da">Autorisering er pakraevet for at afvikle PolicyKit eksemplet Frobnicate (user=$(user), program=$(program), command_line=$(command_line))</message>
               <icon_name>audio-x-generic</icon_name>
               <defaults>
                 <allow_any>no</allow_any>
                 <allow_inactive>no</allow_inactive>
                 <allow_active>auth_self_keep</allow_active>
               </defaults>
               <annotate key="org.freedesktop.policykit.exec.path">/usr/bin/pk-example-frobnicate</annotate>
             </action>

           </policyconfig>

       and drop it in the /usr/share/polkit-1/actions directory under a
       suitable name (e.g. matching the namespace of the action). Note that in
       addition to specifying the program, the authentication message,
       description, icon and defaults can be specified. Note that occurences
       of the strings $(user), $(program) and $(command_line) in the message
       will be replaced with respectively the user (of the form "Real Name
       (username)" or just "username" if there is no real name for the
       username), the binary to execute (a fully-qualified path, e.g.
       "/usr/bin/pk-example-frobnicate") and the command-line, e.g.
       "pk-example-frobnicate foo bar". For example, for the action defined
       above, the following authentication dialog will be shown:

           [IMAGE][2]

               +----------------------------------------------------------+
               |                     Authenticate                     [X] |
               +----------------------------------------------------------+
               |                                                          |
               |  [Icon]  Authentication is required to run the PolicyKit |
               |          example program Frobnicate                      |
               |                                                          |
               |          An application is attempting to perform an      |
               |          action that requires privileges. Authentication |
               |          is required to perform this action.             |
               |                                                          |
               |          Password: [__________________________________]  |
               |                                                          |
               | [V] Details:                                             |
               |  Command: /usr/bin/pk-example-frobnicate                 |
               |  Run As:  Super User (root)                              |
               |  Action:  org.fd.pk.example.pkexec.run-frobnicate        |
               |  Vendor:  Examples for the PolicyKit Project             |
               |                                                          |
               |                                  [Cancel] [Authenticate] |
               +----------------------------------------------------------+

       If the user is using the da_DK locale, the dialog looks like this:

           [IMAGE][3]

               +----------------------------------------------------------+
               |                     Autorisering                     [X] |
               +----------------------------------------------------------+
               |                                                          |
               |  [Icon]  Autorisering er pakraevet for at afvikle         |
               |          PolicyKit eksemplet Frobnicate                  |
               |                                                          |
               |          Et program forsoger at udfore en handling der   |
               |          kraever privilegier. Autorisering er pakraevet.   |
               |                                                          |
               |          Kodeord: [___________________________________]  |
               |                                                          |
               | [V] Detaljer:                                            |
               |  Bruger:   Super User (root)                             |
               |  Program:  /usr/bin/pk-example-frobnicate                |
               |  Handling: org.fd.pk.example.pkexec.run-frobnicate       |
               |  Vendor:   Examples for the PolicyKit Project            |
               |                                                          |
               |                                [Annuller] [Autorisering] |
               +----------------------------------------------------------+

       Note that pkexec does no validation of the ARGUMENTS passed to PROGRAM.
       In the normal case (where administrator authentication is required
       every time pkexec is used), this is not a problem since if the user is
       an administrator he might as well just run pkexec bash to get root.

       However, if an action is used for which the user can retain
       authorization (or if the user is implicitly authorized), such as with
       pk-example-frobnicate above, this could be a security hole. Therefore,
       as a rule of thumb, programs for which the default required
       authorization is changed, should never implicitly trust user input
       (e.g. like any other well-written suid program).

AUTHOR

       Written by David Zeuthen davidz@redhat.com with a lot of help from many
       others.

BUGS

       Please send bug reports to either the distribution or the polkit-devel
       mailing list, see the link
       http://lists.freedesktop.org/mailman/listinfo/polkit-devel on how to
       subscribe.

SEE ALSO

       polkit(8), pkaction(1), pkcheck(1)

NOTES

        1. /usr/share/gtk-doc/html/polkit-1/pkexec-bash.png

        2. /usr/share/gtk-doc/html/polkit-1/pkexec-frobnicate.png

        3. /usr/share/gtk-doc/html/polkit-1/pkexec-frobnicate-da.png