Dr Andrew Scott G7VAV

MBCS, MIET, Member ACM, Member IEEE

Senior Lecturer
School of Computing
and Communications

D35 InfoLab21
Lancaster University
Lancaster, LA1 4WA
United Kingdom

April 2024
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APPARMOR.D

Section: AppArmor (5)
Updated: 2012-10-07
Index Return to Main Contents

NAME

apparmor.d - syntax of security profiles for AppArmor.

DESCRIPTION

AppArmor profiles describe mandatory access rights granted to given programs and are fed to the AppArmor policy enforcement module using apparmor_parser(8). This man page describes the format of the AppArmor configuration files; see apparmor(7) for an overview of AppArmor.

FORMAT

The following is a BNF-style description of AppArmor policy configuration files; see below for an example AppArmor policy file. AppArmor configuration files are line-oriented; # introduces a comment, similar to shell scripting languages. The exception to this rule is that #include will include the contents of a file inline to the policy; this behaviour is modelled after cpp(1).

INCLUDE = '#include' ( ABS PATH | MAGIC PATH )

ABS PATH = '``' path '''' (the path is passed to open(2))

MAGIC PATH = '<' relative path '>' (the path is relative to /etc/apparmor.d/)

COMMENT = '#' TEXT

TEXT = any characters

PROFILE = [ COMMENT ... ] [ VARIABLE ASSIGNMENT ... ] ( '"' PROGRAM '"' | PROGRAM ) [ 'flags=(complain)' ]'{' [ ( RESOURCE RULE | COMMENT | INCLUDE | SUBPROFILE | 'capability ' CAPABILITY | NETWORK RULE | MOUNT RULE | FILE RULE | 'change_profile -> ' PROGRAMCHILD ) ... ] '}'

SUBPROFILE = [ COMMENT ... ] ( PROGRAMHAT | 'profile ' PROGRAMCHILD ) '{' [ ( FILE RULE | COMMENT | INCLUDE ) ... ] '}'

CAPABILITY = (lowercase capability name without 'CAP_' prefix; see capabilities(7))

NETWORK RULE = 'network' [ [ DOMAIN ] [ TYPE ] [ I <PROTOCOL> ] ] ','

PROTOCOL = ( 'tcp' | 'udp' | 'icmp' )

PROGRAM = (non-whitespace characters except for '^', must start with '/'. Embedded spaces or tabs must be quoted.)

PROGRAMHAT = '^' (non-whitespace characters; see aa_change_hat(2) for a description of how this ``hat'' is used.)

PROGRAMCHILD = SUBPROFILE name

MOUNT RULE = ( MOUNT | REMOUNT | UMOUNT | PIVOT ROOT )

MOUNT = [ 'audit' ] [ 'deny' ] 'mount' [ MOUNT CONDITIONS ] [ SOURCE FILEGLOB ] [ -> [ MOUNTPOINT FILEGLOB ]

REMOUNT = [ 'audit' ] [ 'deny' ] 'remount' [ MOUNT CONDITIONS ] MOUNTPOINT FILEGLOB

UMOUNT = [ 'audit' ] [ 'deny' ] 'umount' [ MOUNT CONDITIONS ] MOUNTPOINT FILEGLOB

PIVOT ROOT = [ 'audit' ] [ 'deny' ] pivot_root [ OLD ABS PATH ] [ MOUNTPOINT ABS PATH ] [ -> PROGRAMCHILD ]

MOUNT CONDITIONS = [ ( 'fstype' | 'vfstype' ) ( '=' | 'in' ) MOUNT FSTYPE EXPRESSION ] [ 'options' ( '=' | 'in' ) MOUNT FLAGS EXPRESSION ]

MOUNT FSTYPE EXPRESSION = ( MOUNT FSTYPE LIST | MOUNT EXPRESSION )

MOUNT FSTYPE LIST = Comma separated list of valid filesystem and virtual filesystem types (eg ext4, debugfs, devfs, etc)

MOUNT FLAGS EXPRESSION = ( MOUNT FLAGS LIST | MOUNT EXPRESSION )

MOUNT FLAGS LIST = Comma separated list of MOUNT FLAGS.

MOUNT EXPRESSION = ( ALPHANUMERIC | AARE ) ...

AARE = ?*[]{}^ (see below for meanings)

FILE RULE = RULE QUALIFIER ( '"' FILEGLOB '"' | FILEGLOB ) ACCESS ','

RULE QUALIFIER = [ 'audit' ] [ 'deny' ] [ 'owner' ]

FILEGLOB = (must start with '/' (after variable expansion), AARE have special meanings; see below. May include VARIABLE. Rules with embedded spaces or tabs must be quoted. Rules must end with '/' to apply to directories.)

ACCESS = ( 'r' | 'w' | 'l' | 'ix' | 'ux' | 'Ux' | 'px' | 'Px' | 'cx -> ' PROGRAMCHILD | 'Cx -> ' PROGRAMCHILD | 'm' ) [ ACCESS ... ] (not all combinations are allowed; see below.)

VARIABLE = '@{' ALPHA [ ( ALPHANUMERIC | '_' ) ... ] '}'

VARIABLE ASSIGNMENT = VARIABLE ('=' | '+=') (space separated values)

ALIAS RULE = ABS PATH '->' REWRITTEN ABS PATH ','

ALPHA = ('a', 'b', 'c', ... 'z', 'A', 'B', ... 'Z')

ALPHANUMERIC = ('0', '1', '2', ... '9', 'a', 'b', 'c', ... 'z', 'A', 'B', ... 'Z')

All resources and programs need a full path. There may be any number of subprofiles (aka child profiles) in a profile, limited only by kernel memory. Subprofile names are limited to 974 characters. Child profiles can be used to confine an application in a special way, or when you want the child to be unconfined on the system, but confined when called from the parent. Hats are a special child profile that can be used with the aa_change_hat(2) API call. Applications written or modified to use aa_change_hat(2) can take advantage of subprofiles to run under different confinements, dependent on program logic. Several aa_change_hat(2)-aware applications exist, including an Apache module, mod_apparmor(5); a PAM module, pam_apparmor; and a Tomcat valve, tomcat_apparmor. Applications written or modified to use change_profile(2) transition permanently to the specified profile. libvirt is one such application.

Access Modes

File permission access modes consists of combinations of the following modes:

r - read
w - write --- conflicts with append
a - append --- conflicts with write
ux - unconfined execute
Ux - unconfined execute --- scrub the environment
px - discrete profile execute
Px - discrete profile execute --- scrub the environment
cx - transition to subprofile on execute
Cx - transition to subprofile on execute --- scrub the environment
ix - inherit execute
m - allow PROT_EXEC with mmap(2) calls
l - link
k - lock

Access Modes Details

r - Read mode

Allows the program to have read access to the file or directory listing. Read access is required for shell scripts and other interpreted content.

w - Write mode

Allows the program to have write access to the file. Files and directories must have this permission if they are to be unlinked (removed.) Write mode is not required on a directory to rename or create files within the directory.

This mode conflicts with append mode.

a - Append mode

Allows the program to have a limited appending only write access to the file. Append mode will prevent an application from opening the file for write unless it passes the O_APPEND parameter flag on open.

The mode conflicts with Write mode.

ux - Unconfined execute mode

Allows the program to execute the program without any AppArmor profile being applied to the program.

This mode is useful when a confined program needs to be able to perform a privileged operation, such as rebooting the machine. By placing the privileged section in another executable and granting unconfined execution rights, it is possible to bypass the mandatory constraints imposed on all confined processes. For more information on what is constrained, see the apparmor(7) man page.

WARNING 'ux' should only be used in very special cases. It enables the designated child processes to be run without any AppArmor protection. 'ux' does not scrub the environment of variables such as LD_PRELOAD; as a result, the calling domain may have an undue amount of influence over the callee. Use this mode only if the child absolutely must be run unconfined and LD_PRELOAD must be used. Any profile using this mode provides negligible security. Use at your own risk.

Incompatible with 'Ux', 'px', 'Px', 'cx', 'Cx', 'ix'.

Ux - unconfined execute --- scrub the environment

'Ux' allows the named program to run in 'ux' mode, but AppArmor will invoke the Linux Kernel's unsafe_exec routines to scrub the environment, similar to setuid programs. (See ld.so(8) for some information on setuid/setgid environment scrubbing.)

WARNING 'Ux' should only be used in very special cases. It enables the designated child processes to be run without any AppArmor protection. Use this mode only if the child absolutely must be run unconfined. Use at your own risk.

Incompatible with 'ux', 'px', 'Px', 'cx', 'Cx', 'ix'.

px - Discrete Profile execute mode

This mode requires that a discrete security profile is defined for a program executed and forces an AppArmor domain transition. If there is no profile defined then the access will be denied.

WARNING 'px' does not scrub the environment of variables such as LD_PRELOAD; as a result, the calling domain may have an undue amount of influence over the callee.

Incompatible with 'Ux', 'ux', 'Px', 'cx', 'Cx', 'ix'.

Px - Discrete Profile execute mode --- scrub the environment

'Px' allows the named program to run in 'px' mode, but AppArmor will invoke the Linux Kernel's unsafe_exec routines to scrub the environment, similar to setuid programs. (See ld.so(8) for some information on setuid/setgid environment scrubbing.)

Incompatible with 'Ux', 'ux', 'px', 'cx', 'Cx', 'ix'.

cx - Transition to Subprofile execute mode

This mode requires that a local security profile is defined and forces an AppArmor domain transition to the named profile. If there is no profile defined then the access will be denied.

WARNING 'cx' does not scrub the environment of variables such as LD_PRELOAD; as a result, the calling domain may have an undue amount of influence over the callee.

Incompatible with 'Ux', 'ux', 'px', 'Px', 'Cx', 'ix'.

Cx - Transition to Subprofile execute mode --- scrub the environment

'Cx' allows the named program to run in 'cx' mode, but AppArmor will invoke the Linux Kernel's unsafe_exec routines to scrub the environment, similar to setuid programs. (See ld.so(8) for some information on setuid/setgid environment scrubbing.)

Incompatible with 'Ux', 'ux', 'px', 'Px', 'cx', 'ix'.

ix - Inherit execute mode

Prevent the normal AppArmor domain transition on execve(2) when the profiled program executes the named program. Instead, the executed resource will inherit the current profile.

This mode is useful when a confined program needs to call another confined program without gaining the permissions of the target's profile, or losing the permissions of the current profile. There is no version to scrub the environment because 'ix' executions don't change privileges.

Incompatible with 'Ux', 'ux', 'Px', 'px', 'cx', 'Cx'. Implies 'm'.

m - Allow executable mapping

This mode allows a file to be mapped into memory using mmap(2)'s PROT_EXEC flag. This flag marks the pages executable; it is used on some architectures to provide non-executable data pages, which can complicate exploit attempts. AppArmor uses this mode to limit which files a well-behaved program (or all programs on architectures that enforce non-executable memory access controls) may use as libraries, to limit the effect of invalid -L flags given to ld(1) and LD_PRELOAD, LD_LIBRARY_PATH, given to ld.so(8).

l - Link mode

Allows the program to be able to create a link with this name. When a link is created, the new link MUST have a subset of permissions as the original file (with the exception that the destination does not have to have link access.) If there is an 'x' rule on the new link, it must match the original file exactly.

k - lock mode

Allows the program to be able lock a file with this name. This permission covers both advisory and mandatory locking.

Comments

Comments start with # and may begin at any place within a line. The comment ends when the line ends. This is the same comment style as shell scripts.

Capabilities

The only capabilities a confined process may use may be enumerated; for the complete list, please refer to capabilities(7). Note that granting some capabilities renders AppArmor confinement for that domain advisory; while open(2), read(2), write(2), etc., will still return error when access is not granted, some capabilities allow loading kernel modules, arbitrary access to IPC, ability to bypass discretionary access controls, and other operations that are typically reserved for the root user.

Network Rules

AppArmor supports simple coarse grained network mediation. The network rule restrict all socket(2) based operations. The mediation done is a course grained check on whether a socket of a given type and family can be created, read, or written. There is no mediation based of port number or protocol beyond tcp, udp, and raw.

AppArmor network rules are accumulated so that the granted network permissions are the union of all the listed network rule permissions.

AppArmor network rules are broad and general and become more restrictive as further information is specified.

eg.

 network,               #allow access to all networking
 network tcp,           #allow access to tcp
 network inet tcp,      #allow access to tcp only for inet4 addresses
 network inet6 tcp,     #allow access to tcp only for inet6 addresses

Mount Rules

AppArmor supports mount mediation and allows specifying filesystem types and mount flags. The syntax of mount rules in AppArmor is based on the mount(8) command syntax. Mount rules must contain one of the mount, remount, umount or pivot_root keywords, but all mount conditions are optional. Unspecified optional conditionals are assumed to match all entries (eg, not specifying fstype means all fstypes are matched). Due to the complexity of the mount command and how options may be specified, AppArmor allows specifying conditionals three different ways:

1.

If a conditional is specified using '=', then the rule only grants permission for mounts matching the exactly specified options. For example, an AppArmor policy with the following rule:

: mount options=ro /dev/foo -> /mnt/,

Would match:

: $ mount -o ro /dev/foo /mnt

but not either of these:

: $ mount -o ro,atime /dev/foo /mnt
$ mount -o rw /dev/foo /mnt

2.

If a conditional is specified using 'in', then the rule grants permission for mounts matching any combination of the specified options. For example, if an AppArmor policy has the following rule:

: mount options in (ro,atime) /dev/foo -> /mnt/,

all of these mount commands will match:

$ mount -o ro /dev/foo /mnt

$ mount -o ro,atime /dev/foo /mnt

$ mount -o atime /dev/foo /mnt

but none of these will:

$ mount -o ro,sync /dev/foo /mnt

$ mount -o ro,atime,sync /dev/foo /mnt

$ mount -o rw /dev/foo /mnt

$ mount -o rw,noatime /dev/foo /mnt

$ mount /dev/foo /mnt

3.

If multiple conditionals are specified in a single mount rule, then the rule grants permission for each set of options. This provides a shorthand when writing mount rules which might help to logically break up a conditional. For example, if an AppArmor policy has the following rule:

: mount options=ro options=atime

both of these mount commands will match:

: $ mount -o ro /dev/foo /mnt
$ mount -o atime /dev/foo /mnt

but this one will not:

: $ mount -o ro,atime /dev/foo /mnt

Note that separate mount rules are distinct and the options do not accumulate. For example, these AppArmor mount rules:

: mount options=ro, mount options=atime,

are not equivalent to either of these mount rules:

: mount options=(ro,atime),
mount options in (ro,atime),

To help clarify the flexibility and complexity of mount rules, here are some example rules with accompanying matching commands:

mount,

the 'mount' rule without any conditionals is the most generic and allows any mount. Equivalent to 'mount fstype=** options=** ** -> /**'.

mount /dev/foo,

allow mounting of /dev/foo anywhere with any options. Some matching mount commands:

$ mount /dev/foo /mnt

$ mount -t ext3 /dev/foo /mnt

$ mount -t vfat /dev/foo /mnt

$ mount -o ro,atime,noexec,nodiratime /dev/foo /srv/some/mountpoint

mount options=ro /dev/foo,

allow mounting of /dev/foo anywhere, as read only. Some matching mount commands:

: $ mount -o ro /dev/foo /mnt
$ mount -o ro /dev/foo /some/where/else

mount options=(ro,atime) /dev/foo,

allow mount of /dev/foo anywhere, as read only and using inode access times. Some matching mount commands:

: $ mount -o ro,atime /dev/foo /mnt
$ mount -o ro,atime /dev/foo /some/where/else

mount options in (ro,atime) /dev/foo,

allow mount of /dev/foo anywhere using some combination of 'ro' and 'atime' (see above). Some matching mount commands:

$ mount -o ro /dev/foo /mnt

$ mount -o atime /dev/foo /some/where/else

$ mount -o ro,atime /dev/foo /some/other/place

mount options=ro /dev/foo, mount options=atime /dev/foo,

allow mount of /dev/foo anywhere as read only, and allow mount of /dev/foo anywhere using inode access times. Note this is expressed as two different rules. Matches:

: $ mount -o ro /dev/foo /mnt/1
$ mount -o atime /dev/foo /mnt/2

mount -> /mnt/**,

allow mounting anything under a directory in /mnt/**. Some matching mount commands:

: $ mount /dev/foo1 /mnt/1
$ mount -o ro,atime,noexec,nodiratime /dev/foo2 /mnt/deep/path/foo2

mount options=ro -> /mnt/**,

allow mounting anything under /mnt/**, as read only. Some matching mount commands:

: $ mount -o ro /dev/foo1 /mnt/1
$ mount -o ro /dev/foo2 /mnt/deep/path/foo2

mount fstype=ext3 options=(rw,atime) /dev/sdb1 -> /mnt/stick/,

allow mounting an ext3 filesystem in /dev/sdb1 on /mnt/stick as read/write and using inode access times. Matches only:

: $ mount -o rw,atime /dev/sdb1 /mnt/stick

mount options=(ro, atime) options in (nodev, user) /dev/foo -> /mnt/,

allow mounting /dev/foo on /mmt/ read only and using inode access times or allow mounting /dev/foo on /mnt/ with some combination of 'nodev' and 'user'. Matches only:

$ mount -o ro,atime /dev/foo /mnt

$ mount -o nodev /dev/foo /mnt

$ mount -o user /dev/foo /mnt

$ mount -o nodev,user /dev/foo /mnt

Variables

AppArmor's policy language allows embedding variables into file rules to enable easier configuration for some common (and pervasive) setups. Variables may have multiple values assigned, but any variable assignments must be made before the start of the profile.

The parser will automatically expand variables to include all values that they have been assigned; it is an error to reference a variable without setting at least one value.

At the time of this writing, only @{HOME} and @{HOMEDIRS} are defined in the AppArmor policy provided, in the /etc/apparmor.d/tunables/home file; these variables are used in many of the abstractions described later. You may also add files in /etc/apparmor.d/tunables/home.d for site-specific customization of @{HOMEDIRS}.

Alias rules

AppArmor also provides alias rules for remapping paths for site-specific layouts. They are an alternative form of path rewriting to using variables, and are done after variable resolution. Alias rules must occur within the preamble of the profile. System-wide aliases are found in /etc/apparmor.d/tunables/alias, which is included by /etc/apparmor.d/tunables/global. /etc/apparmor.d/tunables/global is typically included at the beginning of an AppArmor profile.

Globbing

File resources may be specified with a globbing syntax similar to that used by popular shells, such as csh(1), bash(1), zsh(1).

*: can substitute for any number of characters, excepting '/'
**: can substitute for any number of characters, including '/'
?: can substitute for any single character excepting '/'
[abc]: will substitute for the single character a, b, or c
[a-c]: will substitute for the single character a, b, or c
[^a-c]: will substitute for any single character not matching a, b or c
{ab,cd}: will expand to one rule to match ab, one rule to match cd

When AppArmor looks up a directory the pathname being looked up will end with a slash (e.g., /var/tmp/); otherwise it will not end with a slash. Only rules that match a trailing slash will match directories. Some examples, none matching the /tmp/ directory itself, are:

/tmp/*: Files directly in /tmp.
/tmp/*/: Directories directly in /tmp.
/tmp/**: Files and directories anywhere underneath /tmp.
/tmp/**/: Directories anywhere underneath /tmp.

Rule Qualifiers

There are several rule qualifiers that can be applied to permission rules. Rule qualifiers can modify the rule and/or permissions within the rule.

audit: Specifies that permissions requests that match the rule should be recorded to the audit log.
deny: Specifies that permissions requests that match the rule should be denied without logging. Can be combined with 'audit' to enable logging.
owner: Specifies that the task must have the same euid/fsuid as the object being referenced by the permission check.

#include mechanism

AppArmor provides an easy abstraction mechanism to group common file access requirements; this abstraction is an extremely flexible way to grant site-specific rights and makes writing new AppArmor profiles very simple by assembling the needed building blocks for any given program.

The use of '#include' is modelled directly after cpp(1); its use will replace the '#include' statement with the specified file's contents. #include ``/absolute/path'' specifies that /absolute/path should be used. #include ``relative/path'' specifies that relative/path should be used, where the path is relative to the current working directory. #include <magic/path> is the most common usage; it will load magic/path relative to a directory specified to apparmor_parser(8). /etc/apparmor.d/ is the AppArmor default.

The supplied AppArmor profiles follow several conventions; the abstractions stored in /etc/apparmor.d/abstractions/ are some large clusters that are used in most profiles. What follows are short descriptions of how some of the abstractions are used.

abstractions/audio: Includes accesses to device files used for audio applications.
abstractions/authentication: Includes access to files and services typically necessary for services that perform user authentication.
abstractions/base: Includes files that should be readable and writable in all profiles.
abstractions/bash: Includes many files used by bash; useful for interactive shells and programs that call system(3).
abstractions/consoles: Includes read and write access to the device files controlling the virtual console, sshd(8), xterm(1), etc. This abstraction is needed for many programs that interact with users.
abstractions/fonts: Includes access to fonts and the font libraries.
abstractions/gnome: Includes read and write access to GNOME configuration files, as well as read access to GNOME libraries.
abstractions/kde: Includes read and write access to KDE configuration files, as well as read access to KDE libraries.
abstractions/kerberosclient: Includes file access rules needed for common kerberos clients.
abstractions/nameservice: Includes file rules to allow DNS, LDAP, NIS, SMB, user and group password databases, services, and protocols lookups.
abstractions/perl: Includes read access to perl modules.
abstractions/user-download
abstractions/user-mail
abstractions/user-manpages
abstractions/user-tmp
abstractions/user-write: Some profiles for typical ``user'' programs will use these include files to describe rights that users have in the system.
abstractions/wutmp: Includes write access to files used to maintain wtmp(5) and utmp(5) databases, used with the w(1) and associated commands.
abstractions/X: Includes read access to libraries, configuration files, X authentication files, and the X socket.

The abstractions stored in /etc/apparmor.d/program-chunks/ are intended for use by specific program suites, and are not generally useful.

Some of the abstractions rely on variables that are set in files in the /etc/apparmor.d/tunables/ directory. These variables are currently @{HOME} and @{HOMEDIRS}. Variables cannot be set in profile scope; they can only be set before the profile. Therefore, any profiles that use abstractions should either #include <tunables/global> or otherwise ensure that @{HOME} and @{HOMEDIRS} are set before starting the profile definition. The aa-autodep(8) and aa-genprof(8) utilities will automatically emit #include <tunables/global> in generated profiles.

EXAMPLE

An example AppArmor profile:

        # a variable definition in the preamble
        @{HOME} = /home/*/ /root/

        # a comment about foo.
        /usr/bin/foo {
          /bin/mount          ux,
          /dev/{,u}random     r,
          /etc/ld.so.cache    r,
          /etc/foo.conf       r,
          /etc/foo/*          r,
          /lib/ld-*.so*       rmix,
          /lib/lib*.so*       r,
          /proc/[0-9]**       r,
          /usr/lib/**         r,
          /tmp/foo.pid        wr,
          /tmp/foo.*          lrw,
          /@{HOME}/.foo_file  rw,
          /usr/bin/baz        Cx -> baz,

          # a comment about foo's hat (subprofile), bar.
          ^bar {
            /lib/ld-*.so*       rmix,
            /usr/bin/bar        rmix,
            /var/spool/*        rwl,
          }

          # a comment about foo's subprofile, baz.
          profile baz {
            #include <abstractions/bash>
            owner /proc/[0-9]*/stat r,
            /bin/bash ixr,
            /var/lib/baz/ r,
            owner /var/lib/baz/* rw,
          }
        }

FILES

/etc/init.d/boot.apparmor
/etc/apparmor.d/

KNOWN BUGS

Mount options support the use of pattern matching but mount flags are not correctly intersected against specified patterns. Eg, 'mount options=**,' should be equivalent to 'mount,', but it is not. (LP: #965690)

The fstype may not be matched against when certain mount command flags are used. Specifically fstype matching currently only works when creating a new mount and not remount, bind, etc.

Mount rules with multiple 'options' conditionals are not applied as documented but instead merged such that 'options in (ro,nodev) options in (atime)' is equivalent to 'options in (ro,nodev,atime)'.

When specifying mount options with the 'in' conditional, both the positive and negative values match when specifying one or the other. Eg, 'rw' matches when 'ro' is specified and 'dev' matches when 'nodev' is specified such that 'options in (ro,nodev)' is equivalent to 'options in (rw,dev)'.

Index

NAME

DESCRIPTION

FORMAT

Access Modes
Access Modes Details
Comments
Capabilities
Network Rules
Mount Rules
Variables
Alias rules
Globbing
Rule Qualifiers
#include mechanism

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http://www.andrew-scott.uk/

Andrew Scott

http://www.andrew-scott.co.uk/