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CRED: Differentiate objective and effective subjective credentials on a task

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Differentiate the objective and real subjective credentials from the effective
subjective credentials on a task by introducing a second credentials pointer
into the task_struct.

task_struct::real_cred then refers to the objective and apparent real
subjective credentials of a task, as perceived by the other tasks in the
system.

task_struct::cred then refers to the effective subjective credentials of a
task, as used by that task when it's actually running.  These are not visible
to the other tasks in the system.

__task_cred(task) then refers to the objective/real credentials of the task in
question.

current_cred() refers to the effective subjective credentials of the current
task.

prepare_creds() uses the objective creds as a base and commit_creds() changes
both pointers in the task_struct (indeed commit_creds() requires them to be the
same).

override_creds() and revert_creds() change the subjective creds pointer only,
and the former returns the old subjective creds.  These are used by NFSD,
faccessat() and do_coredump(), and will by used by CacheFiles.

In SELinux, current_has_perm() is provided as an alternative to
task_has_perm().  This uses the effective subjective context of current,
whereas task_has_perm() uses the objective/real context of the subject.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>
This commit is contained in:
David Howells 2008-11-14 10:39:26 +11:00 • committed by James Morris
parent 98870ab0a5
commit 3b11a1dece
7 changed files with 95 additions and 54 deletions
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29
include/linux/cred.h
View file

@ -146,8 +146,8 @@ extern struct cred *prepare_exec_creds(void);
extern struct cred *prepare_usermodehelper_creds(void);
extern int commit_creds(struct cred *);
extern void abort_creds(struct cred *);
extern const struct cred *override_creds(const struct cred *) __deprecated;
extern void revert_creds(const struct cred *) __deprecated;
extern const struct cred *override_creds(const struct cred *);
extern void revert_creds(const struct cred *);
extern void __init cred_init(void);
/**
@ -202,32 +202,32 @@ static inline void put_cred(const struct cred *_cred)
}
/**
* current_cred - Access the current task's credentials
* current_cred - Access the current task's subjective credentials
*
* Access the credentials of the current task.
* Access the subjective credentials of the current task.
*/
#define current_cred() \
(current->cred)
/**
* __task_cred - Access another task's credentials
* __task_cred - Access a task's objective credentials
* @task: The task to query
*
* Access the credentials of another task. The caller must hold the
* RCU readlock.
* Access the objective credentials of a task. The caller must hold the RCU
* readlock.
*
* The caller must make sure task doesn't go away, either by holding a ref on
* task or by holding tasklist_lock to prevent it from being unlinked.
*/
#define __task_cred(task) \
((const struct cred *)(rcu_dereference((task)->cred)))
((const struct cred *)(rcu_dereference((task)->real_cred)))
/**
* get_task_cred - Get another task's credentials
* get_task_cred - Get another task's objective credentials
* @task: The task to query
*
* Get the credentials of a task, pinning them so that they can't go away.
* Accessing a task's credentials directly is not permitted.
* Get the objective credentials of a task, pinning them so that they can't go
* away. Accessing a task's credentials directly is not permitted.
*
* The caller must make sure task doesn't go away, either by holding a ref on
* task or by holding tasklist_lock to prevent it from being unlinked.
@ -243,10 +243,11 @@ static inline void put_cred(const struct cred *_cred)
})
/**
* get_current_cred - Get the current task's credentials
* get_current_cred - Get the current task's subjective credentials
*
* Get the credentials of the current task, pinning them so that they can't go
* away. Accessing the current task's credentials directly is not permitted.
* Get the subjective credentials of the current task, pinning them so that
* they can't go away. Accessing the current task's credentials directly is
* not permitted.
*/
#define get_current_cred() \
(get_cred(current_cred()))