CRED: Documentation

Document credentials and the new credentials API.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>

1 files changed, 582 insertions, 0 deletions

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+			     ====================
+			     CREDENTIALS IN LINUX
+			     ====================
+
+By: David Howells <dhowells@redhat.com>
+
+Contents:
+
+ (*) 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.
+
+     Objects are things in the system that may be acted upon directly by
+     userspace programs.  Linux has a variety of actionable objects, including:
+
+	- Tasks
+	- Files/inodes
+	- Sockets
+	- Message queues
+	- Shared memory segments
+	- Semaphores
+	- Keys
+
+     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.
+
+     Amongst the credentials of most objects, there will be a subset that
+     indicates the ownership of that object.  This is used for resource
+     accounting and limitation (disk quotas and task rlimits for example).
+
+     In a standard UNIX filesystem, for instance, this will be defined by the
+     UID marked on the inode.
+
+ (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
+     the same set as in (2) - in standard UNIX files, for instance, this is the
+     defined by the UID and the GID marked on the inode.
+
+     The objective context is used as part of the security calculation that is
+     carried out when an object is acted upon.
+
+ (4) Subjects.
+
+     A subject is an object that is acting upon another object.
+
+     Most of the objects in the system are inactive: they don't act on other
+     objects within the system.  Processes/tasks are the obvious exception:
+     they do stuff; they access and manipulate things.
+
+     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,
+     the file struct will have a subjective context too.
+
+ (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
+     is used as part of the security calculation that is carried out when a
+     subject acts.
+
+     A Linux task, for example, has the FSUID, FSGID and the supplementary
+     group list for when it is acting upon a file - which are quite separate
+     from the real UID and GID that normally form the objective context of the
+     task.
+
+ (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
+     and the object.
+
+     Actions include reading, writing, creating and deleting files; forking or
+     signalling and tracing tasks.
+
+ (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
+     action, and searching one or more sets of rules to see whether the subject
+     is granted or denied permission to act in the desired manner on the
+     object, given those contexts.
+
+     There are two main sources of rules:
+
+     (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
+	 file may supply more than one ACL.
+
+	 A traditional UNIX file, for example, includes a permissions mask that
+	 is an abbreviated ACL with three fixed classes of subject ('user',
+	 'group' and 'other'), each of which may be granted certain privileges
+	 ('read', 'write' and 'execute' - whatever those map to for the object
+	 in question).  UNIX file permissions do not allow the arbitrary
+	 specification of subjects, however, and so are of limited use.
+
+	 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):
+
+	 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.
+	 SELinux and Smack are examples of this.
+
+	 In the case of SELinux and Smack, each object is given a label as part
+	 of its credentials.  When an action is requested, they take the
+	 subject label, the object label and the action and look for a rule
+	 that says that this action is either granted or denied.
+
+
+====================
+TYPES OF CREDENTIALS
+====================
+
+The Linux kernel supports the following types of credentials:
+
+ (1) Traditional UNIX credentials.
+
+	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
+
+     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.
+
+	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.
+
+     The permitted capabilities are those caps that the process might grant
+     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().
+
+     The bounding set limits the capabilities that may be inherited across
+     execve(), especially when a binary is executed that will execute as UID 0.
+
+ (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.
+
+     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
+     making such things as network filesystem keys available to the file
+     accesses performed by processes, without the necessity of ordinary
+     programs having to know about security details involved.
+
+     Keyrings are a special type of key.  They carry sets of other keys and can
+     be searched for the desired key.  Each process may subscribe to a number
+     of keyrings:
+
+	Per-thread keying
+	Per-process keyring
+	Per-session keyring
+
+     When a process accesses a key, if not already present, it will normally be
+     cached on one of these keyrings for future accesses to find.
+
+     For more information on using keys, see Documentation/keys.txt.
+
+ (5) LSM
+
+     The Linux Security Module allows extra controls to be placed over the
+     operations that a task may do.  Currently Linux supports two main
+     alternate LSM options: SELinux and Smack.
+
+     Both work by labelling the objects in a system and then applying sets of
+     rules (policies) that say what operations a task with one label may do to
+     an object with another label.
+
+ (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
+     interact directly with task and file credentials; rather it keeps system
+     level credentials.
+
+
+When a file is opened, part of the opening task's subjective context is
+recorded in the file struct created.  This allows operations using that file
+struct to use those credentials instead of the subjective context of the task
+that issued the operation.  An example of this would be a file opened on a
+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
+=============
+
+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.
+
+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
+privilege escalation bits come into play, and may allow the resulting process
+extra privileges, based on the annotations on the executable file.
+
+
+================
+TASK CREDENTIALS
+================
+
+In Linux, all of a task's credentials are held in (uid, gid) or through
+(groups, keys, LSM security) a refcounted structure of type 'struct cred'.
+Each task points to its credentials by a pointer called 'cred' in its
+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;
+
+ (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;
+
+ (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
+
+ (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
+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.
+
+
+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:
+
+ (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.
+
+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.
+
+
+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:
+
+	const struct cred *current_cred()
+
+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):
+
+	uid_t current_uid(void)		Current's real UID
+	gid_t current_gid(void)		Current's real GID
+	uid_t current_euid(void)	Current's effective UID
+	gid_t current_egid(void)	Current's effective GID
+	uid_t current_fsuid(void)	Current's file access UID
+	gid_t current_fsgid(void)	Current's file access GID
+	kernel_cap_t current_cap(void)	Current's effective capabilities
+	void *current_security(void)	Current's LSM security pointer
+	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:
+
+	void current_uid_gid(uid_t *, gid_t *);
+	void current_euid_egid(uid_t *, gid_t *);
+	void current_fsuid_fsgid(uid_t *, gid_t *);
+
+which return these pairs of values through their arguments after retrieving
+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:
+
+	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:
+
+	struct user_struct *get_current_user(void);
+	struct group_info *get_current_groups(void);
+
+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.
+
+
+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().
+
+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:
+
+	void foo(struct task_struct *t, struct foo_data *f)
+	{
+		const struct cred *tcred;
+		...
+		rcu_read_lock();
+		tcred = __task_cred(t);
+		f->uid = tcred->uid;
+		f->gid = tcred->gid;
+		f->groups = get_group_info(tcred->groups);
+		rcu_read_unlock();
+		...
+	}
+
+A function need not get RCU read lock to use __task_cred() if it is holding a
+spinlock at the time as this implicitly holds the RCU read lock.
+
+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:
+
+	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.
+
+There are a couple of convenience functions to access bits of another task's
+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 a spinlock or 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 or a spinlock 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:
+
+	task_cred_xxx(task, member)
+
+where 'member' is a non-pointer member of the cred struct.  For instance:
+
+	uid_t task_cred_xxx(task, suid);
+
+will retrieve 'struct cred::suid' from the task, doing the appropriate RCU
+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
+--------------------
+
+As previously mentioned, a task may only alter its own credentials, and may not
+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:
+
+	struct cred *prepare_creds(void);
+
+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 new credentials set should be altered appropriately, and any security
+checks and hooks done.  Both the current and the proposed sets of credentials
+are available for this purpose as current_cred() will return the current set
+still at this point.
+
+
+When the credential set is ready, it should be committed to the current process
+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.
+
+This function is guaranteed to return 0, so that it can be tail-called at the
+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.
+
+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:
+
+	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.
+
+
+A typical credentials alteration function would look something like this:
+
+	int alter_suid(uid_t suid)
+	{
+		struct cred *new;
+		int ret;
+
+		new = prepare_creds();
+		if (!new)
+			return -ENOMEM;
+
+		new->suid = suid;
+		ret = security_alter_suid(new);
+		if (ret < 0) {
+			abort_creds(new);
+			return ret;
+		}
+
+		return commit_creds(new);
+	}
+
+
+MANAGING CREDENTIALS
+--------------------
+
+There are some functions to help manage credentials:
+
+ (*) 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);
+
+     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);
+
+     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
+=====================
+
+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.
+
+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
+=======================================
+
+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
+different set of credentials.  This is done in the following places:
+
+ (*) sys_faccessat().
+
+ (*) do_coredump().
+
+ (*) nfs4recover.c.