doc: ReSTify credentials.txt

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>

Documentation/security/00-INDEX

@@ -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

Documentation/security/credentials.rst

@@ -1,38 +1,18 @@
-			     ====================
+====================
-			     CREDENTIALS IN LINUX
+Credentials in Linux
-			     ====================
+====================
 
 By: David Howells <dhowells@redhat.com>
 
-Contents:
+.. contents:: :local:
 
- (*) Overview.
+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
 ========
 
 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 User ID
-	Real Group 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 User ID
-	Effective, Saved and FS Group ID
+	- Effective, Saved and FS Group ID
-	Supplementary groups
+	- 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 permitted capabilities
-	Set of inheritable capabilities
+	- Set of inheritable capabilities
-	Set of effective capabilities
+	- Set of effective capabilities
-	Capability bounding set
+	- 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
+     credentials, but can also be manipulated directly by the ``capset()``
-     call.
+     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;
+ * UNIX UID, GID, mode;
-
+ * Windows user ID;
- (*) Windows user ID;
+ * Access control list;
-
+ * LSM security label;
- (*) Access control list;
+ * UNIX exec privilege escalation bits (SUID/SGID);
-
+ * File capabilities exec privilege escalation bits.
- (*) 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
+ 5. any keyrings it points to may be revoked, expired or have their security
-     attributes changed; and
+    attributes changed; and
 
- (6) the contents of any keyrings to which it points may be changed (the whole
+ 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
+    point of keyrings being a shared set of credentials, modifiable by anyone
-     with appropriate access).
+    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
+task to alter another's credentials.  This means the ``capset()`` system call
-longer permitted to take any PID other than the one of the current process.
+is no longer permitted to take any PID other than the one of the current
-Also keyctl_instantiate() and keyctl_negate() functions no longer permit
+process. Also ``keyctl_instantiate()`` and ``keyctl_negate()`` functions no
-attachment to process-specific keyrings in the requesting process as the
+longer permit attachment to process-specific keyrings in the requesting
-instantiating process may need to create them.
+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
+Once a set of credentials has been made public (by calling ``commit_creds()``
-example), it must be considered immutable, barring two exceptions:
+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
+ 2. Whilst the keyring subscriptions of a set of credentials may not be
-     changed, the keyrings subscribed to may have their contents altered.
+    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,
+certain functions such as ``get_cred()`` and ``put_cred()`` operate on const
-thus rendering casts unnecessary, but require to temporarily ditch the const
+pointers, thus rendering casts unnecessary, but require to temporarily ditch
-qualification to be able to alter the reference count.
+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(),
+Once a reference has been obtained, it must be released with ``put_cred()``,
-free_uid() or put_group_info() as appropriate.
+``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
+.. note::
-     get_cred() as this may race with commit_cred().
+   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
+need to be accessed, RCU read lock should be used, ``__task_cred()`` called,
-result stored in a temporary pointer and then the credential aspects 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
+The mutex prevents ``ptrace()`` from altering the ptrace state of a process
-security checks on credentials construction and changing is taking place as
+whilst security checks on credentials construction and changing is taking place
-the ptrace state may alter the outcome, particularly in the case of execve().
+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
+LSM a chance to do likewise, then it will use ``rcu_assign_pointer()`` to
-commit the new credentials to current->cred, it will release
+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
+``current->cred_replace_mutex`` to allow ``ptrace()`` to take place, and it
-the scheduler and others of the changes.
+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()
+Should the security checks fail or some other error occur after
-has been called, then the following function should be invoked:
+``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
+This releases the lock on ``current->cred_replace_mutex`` that
-and then releases the new credentials.
+``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
+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
+``f_uid`` and ``f_gid``.  Code that used to access ``file->f_uid`` and
-should now access file->f_cred->fsuid and file->f_cred->fsgid.
+``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().
+ * ``sys_faccessat()``.
-
+ * ``do_coredump()``.
- (*) do_coredump().
+ * nfs4recover.c.
-
- (*) nfs4recover.c.

Documentation/security/index.rst

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

include/linux/cred.h

@@ -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)

kernel/cred.c

@@ -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)