a6f76f23d2
Make execve() take advantage of copy-on-write credentials, allowing it to set
up the credentials in advance, and then commit the whole lot after the point
of no return.
This patch and the preceding patches have been tested with the LTP SELinux
testsuite.
This patch makes several logical sets of alteration:
(1) execve().
The credential bits from struct linux_binprm are, for the most part,
replaced with a single credentials pointer (bprm->cred). This means that
all the creds can be calculated in advance and then applied at the point
of no return with no possibility of failure.
I would like to replace bprm->cap_effective with:
cap_isclear(bprm->cap_effective)
but this seems impossible due to special behaviour for processes of pid 1
(they always retain their parent's capability masks where normally they'd
be changed - see cap_bprm_set_creds()).
The following sequence of events now happens:
(a) At the start of do_execve, the current task's cred_exec_mutex is
locked to prevent PTRACE_ATTACH from obsoleting the calculation of
creds that we make.
(a) prepare_exec_creds() is then called to make a copy of the current
task's credentials and prepare it. This copy is then assigned to
bprm->cred.
This renders security_bprm_alloc() and security_bprm_free()
unnecessary, and so they've been removed.
(b) The determination of unsafe execution is now performed immediately
after (a) rather than later on in the code. The result is stored in
bprm->unsafe for future reference.
(c) prepare_binprm() is called, possibly multiple times.
(i) This applies the result of set[ug]id binaries to the new creds
attached to bprm->cred. Personality bit clearance is recorded,
but now deferred on the basis that the exec procedure may yet
fail.
(ii) This then calls the new security_bprm_set_creds(). This should
calculate the new LSM and capability credentials into *bprm->cred.
This folds together security_bprm_set() and parts of
security_bprm_apply_creds() (these two have been removed).
Anything that might fail must be done at this point.
(iii) bprm->cred_prepared is set to 1.
bprm->cred_prepared is 0 on the first pass of the security
calculations, and 1 on all subsequent passes. This allows SELinux
in (ii) to base its calculations only on the initial script and
not on the interpreter.
(d) flush_old_exec() is called to commit the task to execution. This
performs the following steps with regard to credentials:
(i) Clear pdeath_signal and set dumpable on certain circumstances that
may not be covered by commit_creds().
(ii) Clear any bits in current->personality that were deferred from
(c.i).
(e) install_exec_creds() [compute_creds() as was] is called to install the
new credentials. This performs the following steps with regard to
credentials:
(i) Calls security_bprm_committing_creds() to apply any security
requirements, such as flushing unauthorised files in SELinux, that
must be done before the credentials are changed.
This is made up of bits of security_bprm_apply_creds() and
security_bprm_post_apply_creds(), both of which have been removed.
This function is not allowed to fail; anything that might fail
must have been done in (c.ii).
(ii) Calls commit_creds() to apply the new credentials in a single
assignment (more or less). Possibly pdeath_signal and dumpable
should be part of struct creds.
(iii) Unlocks the task's cred_replace_mutex, thus allowing
PTRACE_ATTACH to take place.
(iv) Clears The bprm->cred pointer as the credentials it was holding
are now immutable.
(v) Calls security_bprm_committed_creds() to apply any security
alterations that must be done after the creds have been changed.
SELinux uses this to flush signals and signal handlers.
(f) If an error occurs before (d.i), bprm_free() will call abort_creds()
to destroy the proposed new credentials and will then unlock
cred_replace_mutex. No changes to the credentials will have been
made.
(2) LSM interface.
A number of functions have been changed, added or removed:
(*) security_bprm_alloc(), ->bprm_alloc_security()
(*) security_bprm_free(), ->bprm_free_security()
Removed in favour of preparing new credentials and modifying those.
(*) security_bprm_apply_creds(), ->bprm_apply_creds()
(*) security_bprm_post_apply_creds(), ->bprm_post_apply_creds()
Removed; split between security_bprm_set_creds(),
security_bprm_committing_creds() and security_bprm_committed_creds().
(*) security_bprm_set(), ->bprm_set_security()
Removed; folded into security_bprm_set_creds().
(*) security_bprm_set_creds(), ->bprm_set_creds()
New. The new credentials in bprm->creds should be checked and set up
as appropriate. bprm->cred_prepared is 0 on the first call, 1 on the
second and subsequent calls.
(*) security_bprm_committing_creds(), ->bprm_committing_creds()
(*) security_bprm_committed_creds(), ->bprm_committed_creds()
New. Apply the security effects of the new credentials. This
includes closing unauthorised files in SELinux. This function may not
fail. When the former is called, the creds haven't yet been applied
to the process; when the latter is called, they have.
The former may access bprm->cred, the latter may not.
(3) SELinux.
SELinux has a number of changes, in addition to those to support the LSM
interface changes mentioned above:
(a) The bprm_security_struct struct has been removed in favour of using
the credentials-under-construction approach.
(c) flush_unauthorized_files() now takes a cred pointer and passes it on
to inode_has_perm(), file_has_perm() and dentry_open().
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: James Morris <jmorris@namei.org>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Signed-off-by: James Morris <jmorris@namei.org>
328 lines
9.6 KiB
C
328 lines
9.6 KiB
C
/* Credentials management
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*
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* Copyright (C) 2008 Red Hat, Inc. All Rights Reserved.
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* Written by David Howells (dhowells@redhat.com)
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public Licence
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* as published by the Free Software Foundation; either version
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* 2 of the Licence, or (at your option) any later version.
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*/
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#ifndef _LINUX_CRED_H
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#define _LINUX_CRED_H
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#include <linux/capability.h>
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#include <linux/key.h>
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#include <asm/atomic.h>
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struct user_struct;
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struct cred;
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/*
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* COW Supplementary groups list
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*/
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#define NGROUPS_SMALL 32
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#define NGROUPS_PER_BLOCK ((unsigned int)(PAGE_SIZE / sizeof(gid_t)))
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struct group_info {
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atomic_t usage;
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int ngroups;
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int nblocks;
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gid_t small_block[NGROUPS_SMALL];
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gid_t *blocks[0];
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};
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/**
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* get_group_info - Get a reference to a group info structure
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* @group_info: The group info to reference
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*
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* This gets a reference to a set of supplementary groups.
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*
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* If the caller is accessing a task's credentials, they must hold the RCU read
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* lock when reading.
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*/
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static inline struct group_info *get_group_info(struct group_info *gi)
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{
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atomic_inc(&gi->usage);
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return gi;
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}
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/**
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* put_group_info - Release a reference to a group info structure
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* @group_info: The group info to release
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*/
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#define put_group_info(group_info) \
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do { \
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if (atomic_dec_and_test(&(group_info)->usage)) \
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groups_free(group_info); \
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} while (0)
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extern struct group_info *groups_alloc(int);
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extern void groups_free(struct group_info *);
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extern int set_current_groups(struct group_info *);
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extern int set_groups(struct cred *, struct group_info *);
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extern int groups_search(const struct group_info *, gid_t);
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/* access the groups "array" with this macro */
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#define GROUP_AT(gi, i) \
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((gi)->blocks[(i) / NGROUPS_PER_BLOCK][(i) % NGROUPS_PER_BLOCK])
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extern int in_group_p(gid_t);
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extern int in_egroup_p(gid_t);
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/*
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* The common credentials for a thread group
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* - shared by CLONE_THREAD
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*/
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#ifdef CONFIG_KEYS
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struct thread_group_cred {
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atomic_t usage;
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pid_t tgid; /* thread group process ID */
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spinlock_t lock;
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struct key *session_keyring; /* keyring inherited over fork */
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struct key *process_keyring; /* keyring private to this process */
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struct rcu_head rcu; /* RCU deletion hook */
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};
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#endif
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/*
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* The security context of a task
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*
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* The parts of the context break down into two categories:
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*
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* (1) The objective context of a task. These parts are used when some other
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* task is attempting to affect this one.
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*
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* (2) The subjective context. These details are used when the task is acting
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* upon another object, be that a file, a task, a key or whatever.
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*
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* Note that some members of this structure belong to both categories - the
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* LSM security pointer for instance.
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*
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* A task has two security pointers. task->real_cred points to the objective
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* context that defines that task's actual details. The objective part of this
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* context is used whenever that task is acted upon.
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*
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* task->cred points to the subjective context that defines the details of how
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* that task is going to act upon another object. This may be overridden
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* temporarily to point to another security context, but normally points to the
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* same context as task->real_cred.
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*/
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struct cred {
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atomic_t usage;
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uid_t uid; /* real UID of the task */
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gid_t gid; /* real GID of the task */
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uid_t suid; /* saved UID of the task */
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gid_t sgid; /* saved GID of the task */
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uid_t euid; /* effective UID of the task */
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gid_t egid; /* effective GID of the task */
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uid_t fsuid; /* UID for VFS ops */
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gid_t fsgid; /* GID for VFS ops */
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unsigned securebits; /* SUID-less security management */
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kernel_cap_t cap_inheritable; /* caps our children can inherit */
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kernel_cap_t cap_permitted; /* caps we're permitted */
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kernel_cap_t cap_effective; /* caps we can actually use */
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kernel_cap_t cap_bset; /* capability bounding set */
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#ifdef CONFIG_KEYS
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unsigned char jit_keyring; /* default keyring to attach requested
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* keys to */
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struct key *thread_keyring; /* keyring private to this thread */
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struct key *request_key_auth; /* assumed request_key authority */
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struct thread_group_cred *tgcred; /* thread-group shared credentials */
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#endif
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#ifdef CONFIG_SECURITY
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void *security; /* subjective LSM security */
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#endif
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struct user_struct *user; /* real user ID subscription */
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struct group_info *group_info; /* supplementary groups for euid/fsgid */
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struct rcu_head rcu; /* RCU deletion hook */
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};
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extern void __put_cred(struct cred *);
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extern int copy_creds(struct task_struct *, unsigned long);
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extern struct cred *prepare_creds(void);
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extern struct cred *prepare_exec_creds(void);
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extern struct cred *prepare_usermodehelper_creds(void);
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extern int commit_creds(struct cred *);
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extern void abort_creds(struct cred *);
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extern const struct cred *override_creds(const struct cred *) __deprecated;
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extern void revert_creds(const struct cred *) __deprecated;
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extern void __init cred_init(void);
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/**
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* get_new_cred - Get a reference on a new set of credentials
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* @cred: The new credentials to reference
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*
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* Get a reference on the specified set of new credentials. The caller must
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* release the reference.
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*/
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static inline struct cred *get_new_cred(struct cred *cred)
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{
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atomic_inc(&cred->usage);
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return cred;
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}
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/**
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* get_cred - Get a reference on a set of credentials
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* @cred: The credentials to reference
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*
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* Get a reference on the specified set of credentials. The caller must
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* release the reference.
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*/
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static inline const struct cred *get_cred(const struct cred *cred)
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{
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return get_new_cred((struct cred *) cred);
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}
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/**
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* put_cred - Release a reference to a set of credentials
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* @cred: The credentials to release
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*
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* Release a reference to a set of credentials, deleting them when the last ref
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* is released.
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*/
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static inline void put_cred(const struct cred *_cred)
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{
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struct cred *cred = (struct cred *) _cred;
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BUG_ON(atomic_read(&(cred)->usage) <= 0);
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if (atomic_dec_and_test(&(cred)->usage))
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__put_cred(cred);
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}
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/**
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* current_cred - Access the current task's credentials
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*
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* Access the credentials of the current task.
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*/
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#define current_cred() \
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(current->cred)
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/**
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* __task_cred - Access another task's credentials
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* @task: The task to query
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*
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* Access the credentials of another task. The caller must hold the
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* RCU readlock.
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*
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* The caller must make sure task doesn't go away, either by holding a ref on
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* task or by holding tasklist_lock to prevent it from being unlinked.
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*/
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#define __task_cred(task) \
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((const struct cred *)(rcu_dereference((task)->cred)))
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/**
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* get_task_cred - Get another task's credentials
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* @task: The task to query
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*
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* Get the credentials of a task, pinning them so that they can't go away.
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* Accessing a task's credentials directly is not permitted.
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*
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* The caller must make sure task doesn't go away, either by holding a ref on
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* task or by holding tasklist_lock to prevent it from being unlinked.
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*/
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#define get_task_cred(task) \
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({ \
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struct cred *__cred; \
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rcu_read_lock(); \
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__cred = (struct cred *) __task_cred((task)); \
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get_cred(__cred); \
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rcu_read_unlock(); \
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__cred; \
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})
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/**
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* get_current_cred - Get the current task's credentials
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*
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* Get the credentials of the current task, pinning them so that they can't go
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* away. Accessing the current task's credentials directly is not permitted.
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*/
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#define get_current_cred() \
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(get_cred(current_cred()))
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/**
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* get_current_user - Get the current task's user_struct
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*
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* Get the user record of the current task, pinning it so that it can't go
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* away.
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*/
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#define get_current_user() \
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({ \
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struct user_struct *__u; \
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struct cred *__cred; \
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__cred = (struct cred *) current_cred(); \
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__u = get_uid(__cred->user); \
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__u; \
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})
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/**
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* get_current_groups - Get the current task's supplementary group list
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*
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* Get the supplementary group list of the current task, pinning it so that it
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* can't go away.
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*/
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#define get_current_groups() \
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({ \
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struct group_info *__groups; \
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struct cred *__cred; \
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__cred = (struct cred *) current_cred(); \
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__groups = get_group_info(__cred->group_info); \
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__groups; \
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})
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#define task_cred_xxx(task, xxx) \
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({ \
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__typeof__(((struct cred *)NULL)->xxx) ___val; \
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rcu_read_lock(); \
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___val = __task_cred((task))->xxx; \
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rcu_read_unlock(); \
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___val; \
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})
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#define task_uid(task) (task_cred_xxx((task), uid))
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#define task_euid(task) (task_cred_xxx((task), euid))
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#define current_cred_xxx(xxx) \
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({ \
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current->cred->xxx; \
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})
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#define current_uid() (current_cred_xxx(uid))
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#define current_gid() (current_cred_xxx(gid))
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#define current_euid() (current_cred_xxx(euid))
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#define current_egid() (current_cred_xxx(egid))
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#define current_suid() (current_cred_xxx(suid))
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#define current_sgid() (current_cred_xxx(sgid))
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#define current_fsuid() (current_cred_xxx(fsuid))
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#define current_fsgid() (current_cred_xxx(fsgid))
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#define current_cap() (current_cred_xxx(cap_effective))
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#define current_user() (current_cred_xxx(user))
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#define current_security() (current_cred_xxx(security))
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#define current_uid_gid(_uid, _gid) \
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do { \
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const struct cred *__cred; \
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__cred = current_cred(); \
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*(_uid) = __cred->uid; \
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*(_gid) = __cred->gid; \
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} while(0)
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#define current_euid_egid(_euid, _egid) \
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do { \
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const struct cred *__cred; \
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__cred = current_cred(); \
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*(_euid) = __cred->euid; \
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*(_egid) = __cred->egid; \
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} while(0)
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#define current_fsuid_fsgid(_fsuid, _fsgid) \
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do { \
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const struct cred *__cred; \
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__cred = current_cred(); \
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*(_fsuid) = __cred->fsuid; \
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*(_fsgid) = __cred->fsgid; \
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} while(0)
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#endif /* _LINUX_CRED_H */
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