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doc: ReSTify credentials.txt

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This updates the credentials API documentation to ReST markup and moves
it under the security subsection of kernel API documentation.

Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Kees Cook 2017-05-13 04:51:40 -07:00 committed by Jonathan Corbet
parent 7b05b11684
commit af777cd1b8
5 changed files with 127 additions and 155 deletions
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2
Documentation/security/00-INDEX
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@ -10,8 +10,6 @@ Yama.txt
- documentation on the Yama Linux Security Module.
apparmor.txt
- documentation on the AppArmor security extension.
credentials.txt
- documentation about credentials in Linux.
keys-ecryptfs.txt
- description of the encryption keys for the ecryptfs filesystem.
keys-request-key.txt

275
Documentation/security/credentials.txt → Documentation/security/credentials.rst
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@ -1,38 +1,18 @@
====================
CREDENTIALS IN LINUX
====================
====================
Credentials in Linux
====================
By: David Howells <dhowells@redhat.com>
Contents:
.. contents:: :local:
(*) Overview.
(*) Types of credentials.
(*) File markings.
(*) Task credentials.
- Immutable credentials.
- Accessing task credentials.
- Accessing another task's credentials.
- Altering credentials.
- Managing credentials.
(*) Open file credentials.
(*) Overriding the VFS's use of credentials.
========
OVERVIEW
Overview
========
There are several parts to the security check performed by Linux when one
object acts upon another:
(1) Objects.
1. Objects.
Objects are things in the system that may be acted upon directly by
userspace programs. Linux has a variety of actionable objects, including:
@ -48,7 +28,7 @@ object acts upon another:
As a part of the description of all these objects there is a set of
credentials. What's in the set depends on the type of object.
(2) Object ownership.
2. Object ownership.
Amongst the credentials of most objects, there will be a subset that
indicates the ownership of that object. This is used for resource
@ -57,7 +37,7 @@ object acts upon another:
In a standard UNIX filesystem, for instance, this will be defined by the
UID marked on the inode.
(3) The objective context.
3. The objective context.
Also amongst the credentials of those objects, there will be a subset that
indicates the 'objective context' of that object. This may or may not be
@ -67,7 +47,7 @@ object acts upon another:
The objective context is used as part of the security calculation that is
carried out when an object is acted upon.
(4) Subjects.
4. Subjects.
A subject is an object that is acting upon another object.
@ -77,10 +57,10 @@ object acts upon another:
Objects other than tasks may under some circumstances also be subjects.
For instance an open file may send SIGIO to a task using the UID and EUID
given to it by a task that called fcntl(F_SETOWN) upon it. In this case,
given to it by a task that called ``fcntl(F_SETOWN)`` upon it. In this case,
the file struct will have a subjective context too.
(5) The subjective context.
5. The subjective context.
A subject has an additional interpretation of its credentials. A subset
of its credentials forms the 'subjective context'. The subjective context
@ -92,7 +72,7 @@ object acts upon another:
from the real UID and GID that normally form the objective context of the
task.
(6) Actions.
6. Actions.
Linux has a number of actions available that a subject may perform upon an
object. The set of actions available depends on the nature of the subject
@ -101,7 +81,7 @@ object acts upon another:
Actions include reading, writing, creating and deleting files; forking or
signalling and tracing tasks.
(7) Rules, access control lists and security calculations.
7. Rules, access control lists and security calculations.
When a subject acts upon an object, a security calculation is made. This
involves taking the subjective context, the objective context and the
@ -111,7 +91,7 @@ object acts upon another:
There are two main sources of rules:
(a) Discretionary access control (DAC):
a. Discretionary access control (DAC):
Sometimes the object will include sets of rules as part of its
description. This is an 'Access Control List' or 'ACL'. A Linux
@ -127,7 +107,7 @@ object acts upon another:
A Linux file might also sport a POSIX ACL. This is a list of rules
that grants various permissions to arbitrary subjects.
(b) Mandatory access control (MAC):
b. Mandatory access control (MAC):
The system as a whole may have one or more sets of rules that get
applied to all subjects and objects, regardless of their source.
@ -139,65 +119,65 @@ object acts upon another:
that says that this action is either granted or denied.
====================
TYPES OF CREDENTIALS
Types of Credentials
====================
The Linux kernel supports the following types of credentials:
(1) Traditional UNIX credentials.
1. Traditional UNIX credentials.
Real User ID
Real Group ID
- Real User ID
- Real Group ID
The UID and GID are carried by most, if not all, Linux objects, even if in
some cases it has to be invented (FAT or CIFS files for example, which are
derived from Windows). These (mostly) define the objective context of
that object, with tasks being slightly different in some cases.
Effective, Saved and FS User ID
Effective, Saved and FS Group ID
Supplementary groups
- Effective, Saved and FS User ID
- Effective, Saved and FS Group ID
- Supplementary groups
These are additional credentials used by tasks only. Usually, an
EUID/EGID/GROUPS will be used as the subjective context, and real UID/GID
will be used as the objective. For tasks, it should be noted that this is
not always true.
(2) Capabilities.
2. Capabilities.
Set of permitted capabilities
Set of inheritable capabilities
Set of effective capabilities
Capability bounding set
- Set of permitted capabilities
- Set of inheritable capabilities
- Set of effective capabilities
- Capability bounding set
These are only carried by tasks. They indicate superior capabilities
granted piecemeal to a task that an ordinary task wouldn't otherwise have.
These are manipulated implicitly by changes to the traditional UNIX
credentials, but can also be manipulated directly by the capset() system
call.
credentials, but can also be manipulated directly by the ``capset()``
system call.
The permitted capabilities are those caps that the process might grant
itself to its effective or permitted sets through capset(). This
itself to its effective or permitted sets through ``capset()``. This
inheritable set might also be so constrained.
The effective capabilities are the ones that a task is actually allowed to
make use of itself.
The inheritable capabilities are the ones that may get passed across
execve().
``execve()``.
The bounding set limits the capabilities that may be inherited across
execve(), especially when a binary is executed that will execute as UID 0.
``execve()``, especially when a binary is executed that will execute as
UID 0.
(3) Secure management flags (securebits).
3. Secure management flags (securebits).
These are only carried by tasks. These govern the way the above
credentials are manipulated and inherited over certain operations such as
execve(). They aren't used directly as objective or subjective
credentials.
(4) Keys and keyrings.
4. Keys and keyrings.
These are only carried by tasks. They carry and cache security tokens
that don't fit into the other standard UNIX credentials. They are for
@ -218,7 +198,7 @@ The Linux kernel supports the following types of credentials:
For more information on using keys, see Documentation/security/keys.txt.
(5) LSM
5. LSM
The Linux Security Module allows extra controls to be placed over the
operations that a task may do. Currently Linux supports several LSM
@ -228,7 +208,7 @@ The Linux kernel supports the following types of credentials:
rules (policies) that say what operations a task with one label may do to
an object with another label.
(6) AF_KEY
6. AF_KEY
This is a socket-based approach to credential management for networking
stacks [RFC 2367]. It isn't discussed by this document as it doesn't
@ -244,25 +224,19 @@ network filesystem where the credentials of the opened file should be presented
to the server, regardless of who is actually doing a read or a write upon it.
=============
FILE MARKINGS
File Markings
=============
Files on disk or obtained over the network may have annotations that form the
objective security context of that file. Depending on the type of filesystem,
this may include one or more of the following:
(*) UNIX UID, GID, mode;
(*) Windows user ID;
(*) Access control list;
(*) LSM security label;
(*) UNIX exec privilege escalation bits (SUID/SGID);
(*) File capabilities exec privilege escalation bits.
* UNIX UID, GID, mode;
* Windows user ID;
* Access control list;
* LSM security label;
* UNIX exec privilege escalation bits (SUID/SGID);
* File capabilities exec privilege escalation bits.
These are compared to the task's subjective security context, and certain
operations allowed or disallowed as a result. In the case of execve(), the
@ -270,8 +244,7 @@ privilege escalation bits come into play, and may allow the resulting process
extra privileges, based on the annotations on the executable file.
================
TASK CREDENTIALS
Task Credentials
================
In Linux, all of a task's credentials are held in (uid, gid) or through
@ -282,20 +255,20 @@ task_struct.
Once a set of credentials has been prepared and committed, it may not be
changed, barring the following exceptions:
(1) its reference count may be changed;
1. its reference count may be changed;
(2) the reference count on the group_info struct it points to may be changed;
2. the reference count on the group_info struct it points to may be changed;
(3) the reference count on the security data it points to may be changed;
3. the reference count on the security data it points to may be changed;
(4) the reference count on any keyrings it points to may be changed;
4. the reference count on any keyrings it points to may be changed;
(5) any keyrings it points to may be revoked, expired or have their security
attributes changed; and
5. any keyrings it points to may be revoked, expired or have their security
attributes changed; and
(6) the contents of any keyrings to which it points may be changed (the whole
point of keyrings being a shared set of credentials, modifiable by anyone
with appropriate access).
6. the contents of any keyrings to which it points may be changed (the whole
point of keyrings being a shared set of credentials, modifiable by anyone
with appropriate access).
To alter anything in the cred struct, the copy-and-replace principle must be
adhered to. First take a copy, then alter the copy and then use RCU to change
@ -303,37 +276,37 @@ the task pointer to make it point to the new copy. There are wrappers to aid
with this (see below).
A task may only alter its _own_ credentials; it is no longer permitted for a
task to alter another's credentials. This means the capset() system call is no
longer permitted to take any PID other than the one of the current process.
Also keyctl_instantiate() and keyctl_negate() functions no longer permit
attachment to process-specific keyrings in the requesting process as the
instantiating process may need to create them.
task to alter another's credentials. This means the ``capset()`` system call
is no longer permitted to take any PID other than the one of the current
process. Also ``keyctl_instantiate()`` and ``keyctl_negate()`` functions no
longer permit attachment to process-specific keyrings in the requesting
process as the instantiating process may need to create them.
IMMUTABLE CREDENTIALS
Immutable Credentials
---------------------
Once a set of credentials has been made public (by calling commit_creds() for
example), it must be considered immutable, barring two exceptions:
Once a set of credentials has been made public (by calling ``commit_creds()``
for example), it must be considered immutable, barring two exceptions:
(1) The reference count may be altered.
1. The reference count may be altered.
(2) Whilst the keyring subscriptions of a set of credentials may not be
changed, the keyrings subscribed to may have their contents altered.
2. Whilst the keyring subscriptions of a set of credentials may not be
changed, the keyrings subscribed to may have their contents altered.
To catch accidental credential alteration at compile time, struct task_struct
has _const_ pointers to its credential sets, as does struct file. Furthermore,
certain functions such as get_cred() and put_cred() operate on const pointers,
thus rendering casts unnecessary, but require to temporarily ditch the const
qualification to be able to alter the reference count.
certain functions such as ``get_cred()`` and ``put_cred()`` operate on const
pointers, thus rendering casts unnecessary, but require to temporarily ditch
the const qualification to be able to alter the reference count.
ACCESSING TASK CREDENTIALS
Accessing Task Credentials
--------------------------
A task being able to alter only its own credentials permits the current process
to read or replace its own credentials without the need for any form of locking
- which simplifies things greatly. It can just call:
-- which simplifies things greatly. It can just call::
const struct cred *current_cred()
@ -341,7 +314,7 @@ to get a pointer to its credentials structure, and it doesn't have to release
it afterwards.
There are convenience wrappers for retrieving specific aspects of a task's
credentials (the value is simply returned in each case):
credentials (the value is simply returned in each case)::
uid_t current_uid(void) Current's real UID
gid_t current_gid(void) Current's real GID
@ -354,7 +327,7 @@ credentials (the value is simply returned in each case):
struct user_struct *current_user(void) Current's user account
There are also convenience wrappers for retrieving specific associated pairs of
a task's credentials:
a task's credentials::
void current_uid_gid(uid_t *, gid_t *);
void current_euid_egid(uid_t *, gid_t *);
@ -365,12 +338,12 @@ them from the current task's credentials.
In addition, there is a function for obtaining a reference on the current
process's current set of credentials:
process's current set of credentials::
const struct cred *get_current_cred(void);
and functions for getting references to one of the credentials that don't
actually live in struct cred:
actually live in struct cred::
struct user_struct *get_current_user(void);
struct group_info *get_current_groups(void);
@ -378,22 +351,22 @@ actually live in struct cred:
which get references to the current process's user accounting structure and
supplementary groups list respectively.
Once a reference has been obtained, it must be released with put_cred(),
free_uid() or put_group_info() as appropriate.
Once a reference has been obtained, it must be released with ``put_cred()``,
``free_uid()`` or ``put_group_info()`` as appropriate.
ACCESSING ANOTHER TASK'S CREDENTIALS
Accessing Another Task's Credentials
------------------------------------
Whilst a task may access its own credentials without the need for locking, the
same is not true of a task wanting to access another task's credentials. It
must use the RCU read lock and rcu_dereference().
must use the RCU read lock and ``rcu_dereference()``.
The rcu_dereference() is wrapped by:
The ``rcu_dereference()`` is wrapped by::
const struct cred *__task_cred(struct task_struct *task);
This should be used inside the RCU read lock, as in the following example:
This should be used inside the RCU read lock, as in the following example::
void foo(struct task_struct *t, struct foo_data *f)
{
@ -410,39 +383,40 @@ This should be used inside the RCU read lock, as in the following example:
Should it be necessary to hold another task's credentials for a long period of
time, and possibly to sleep whilst doing so, then the caller should get a
reference on them using:
reference on them using::
const struct cred *get_task_cred(struct task_struct *task);
This does all the RCU magic inside of it. The caller must call put_cred() on
the credentials so obtained when they're finished with.
[*] Note: The result of __task_cred() should not be passed directly to
get_cred() as this may race with commit_cred().
.. note::
The result of ``__task_cred()`` should not be passed directly to
``get_cred()`` as this may race with ``commit_cred()``.
There are a couple of convenience functions to access bits of another task's
credentials, hiding the RCU magic from the caller:
credentials, hiding the RCU magic from the caller::
uid_t task_uid(task) Task's real UID
uid_t task_euid(task) Task's effective UID
If the caller is holding the RCU read lock at the time anyway, then:
If the caller is holding the RCU read lock at the time anyway, then::
__task_cred(task)->uid
__task_cred(task)->euid
should be used instead. Similarly, if multiple aspects of a task's credentials
need to be accessed, RCU read lock should be used, __task_cred() called, the
result stored in a temporary pointer and then the credential aspects called
need to be accessed, RCU read lock should be used, ``__task_cred()`` called,
the result stored in a temporary pointer and then the credential aspects called
from that before dropping the lock. This prevents the potentially expensive
RCU magic from being invoked multiple times.
Should some other single aspect of another task's credentials need to be
accessed, then this can be used:
accessed, then this can be used::
task_cred_xxx(task, member)
where 'member' is a non-pointer member of the cred struct. For instance:
where 'member' is a non-pointer member of the cred struct. For instance::
uid_t task_cred_xxx(task, suid);
@ -451,7 +425,7 @@ magic. This may not be used for pointer members as what they point to may
disappear the moment the RCU read lock is dropped.
ALTERING CREDENTIALS
Altering Credentials
--------------------
As previously mentioned, a task may only alter its own credentials, and may not
@ -459,7 +433,7 @@ alter those of another task. This means that it doesn't need to use any
locking to alter its own credentials.
To alter the current process's credentials, a function should first prepare a
new set of credentials by calling:
new set of credentials by calling::
struct cred *prepare_creds(void);
@ -467,9 +441,10 @@ this locks current->cred_replace_mutex and then allocates and constructs a
duplicate of the current process's credentials, returning with the mutex still
held if successful. It returns NULL if not successful (out of memory).
The mutex prevents ptrace() from altering the ptrace state of a process whilst
security checks on credentials construction and changing is taking place as
the ptrace state may alter the outcome, particularly in the case of execve().
The mutex prevents ``ptrace()`` from altering the ptrace state of a process
whilst security checks on credentials construction and changing is taking place
as the ptrace state may alter the outcome, particularly in the case of
``execve()``.
The new credentials set should be altered appropriately, and any security
checks and hooks done. Both the current and the proposed sets of credentials
@ -478,36 +453,37 @@ still at this point.
When the credential set is ready, it should be committed to the current process
by calling:
by calling::
int commit_creds(struct cred *new);
This will alter various aspects of the credentials and the process, giving the
LSM a chance to do likewise, then it will use rcu_assign_pointer() to actually
commit the new credentials to current->cred, it will release
current->cred_replace_mutex to allow ptrace() to take place, and it will notify
the scheduler and others of the changes.
LSM a chance to do likewise, then it will use ``rcu_assign_pointer()`` to
actually commit the new credentials to ``current->cred``, it will release
``current->cred_replace_mutex`` to allow ``ptrace()`` to take place, and it
will notify the scheduler and others of the changes.
This function is guaranteed to return 0, so that it can be tail-called at the
end of such functions as sys_setresuid().
end of such functions as ``sys_setresuid()``.
Note that this function consumes the caller's reference to the new credentials.
The caller should _not_ call put_cred() on the new credentials afterwards.
The caller should _not_ call ``put_cred()`` on the new credentials afterwards.
Furthermore, once this function has been called on a new set of credentials,
those credentials may _not_ be changed further.
Should the security checks fail or some other error occur after prepare_creds()
has been called, then the following function should be invoked:
Should the security checks fail or some other error occur after
``prepare_creds()`` has been called, then the following function should be
invoked::
void abort_creds(struct cred *new);
This releases the lock on current->cred_replace_mutex that prepare_creds() got
and then releases the new credentials.
This releases the lock on ``current->cred_replace_mutex`` that
``prepare_creds()`` got and then releases the new credentials.
A typical credentials alteration function would look something like this:
A typical credentials alteration function would look something like this::
int alter_suid(uid_t suid)
{
@ -529,53 +505,50 @@ A typical credentials alteration function would look something like this:
}
MANAGING CREDENTIALS
Managing Credentials
--------------------
There are some functions to help manage credentials:
(*) void put_cred(const struct cred *cred);
- ``void put_cred(const struct cred *cred);``
This releases a reference to the given set of credentials. If the
reference count reaches zero, the credentials will be scheduled for
destruction by the RCU system.
(*) const struct cred *get_cred(const struct cred *cred);
- ``const struct cred *get_cred(const struct cred *cred);``
This gets a reference on a live set of credentials, returning a pointer to
that set of credentials.
(*) struct cred *get_new_cred(struct cred *cred);
- ``struct cred *get_new_cred(struct cred *cred);``
This gets a reference on a set of credentials that is under construction
and is thus still mutable, returning a pointer to that set of credentials.
=====================
OPEN FILE CREDENTIALS
Open File Credentials
=====================
When a new file is opened, a reference is obtained on the opening task's
credentials and this is attached to the file struct as 'f_cred' in place of
'f_uid' and 'f_gid'. Code that used to access file->f_uid and file->f_gid
should now access file->f_cred->fsuid and file->f_cred->fsgid.
credentials and this is attached to the file struct as ``f_cred`` in place of
``f_uid`` and ``f_gid``. Code that used to access ``file->f_uid`` and
``file->f_gid`` should now access ``file->f_cred->fsuid`` and
``file->f_cred->fsgid``.
It is safe to access f_cred without the use of RCU or locking because the
It is safe to access ``f_cred`` without the use of RCU or locking because the
pointer will not change over the lifetime of the file struct, and nor will the
contents of the cred struct pointed to, barring the exceptions listed above
(see the Task Credentials section).
=======================================
OVERRIDING THE VFS'S USE OF CREDENTIALS
Overriding the VFS's Use of Credentials
=======================================
Under some circumstances it is desirable to override the credentials used by
the VFS, and that can be done by calling into such as vfs_mkdir() with a
the VFS, and that can be done by calling into such as ``vfs_mkdir()`` with a
different set of credentials. This is done in the following places:
(*) sys_faccessat().
(*) do_coredump().
(*) nfs4recover.c.
* ``sys_faccessat()``.
* ``do_coredump()``.
* nfs4recover.c.

1
Documentation/security/index.rst
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@ -5,5 +5,6 @@ Security Documentation
.. toctree::
:maxdepth: 1
credentials
IMA-templates
tpm/index

2
include/linux/cred.h
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@ -1,4 +1,4 @@
/* Credentials management - see Documentation/security/credentials.txt
/* Credentials management - see Documentation/security/credentials.rst
*
* Copyright (C) 2008 Red Hat, Inc. All Rights Reserved.
* Written by David Howells (dhowells@redhat.com)

2
kernel/cred.c
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@ -1,4 +1,4 @@
/* Task credentials management - see Documentation/security/credentials.txt
/* Task credentials management - see Documentation/security/credentials.rst
*
* Copyright (C) 2008 Red Hat, Inc. All Rights Reserved.
* Written by David Howells (dhowells@redhat.com)