CRED: Inaugurate COW credentials

Inaugurate copy-on-write credentials management.  This uses RCU to manage the
credentials pointer in the task_struct with respect to accesses by other tasks.
A process may only modify its own credentials, and so does not need locking to
access or modify its own credentials.

A mutex (cred_replace_mutex) is added to the task_struct to control the effect
of PTRACE_ATTACHED on credential calculations, particularly with respect to
execve().

With this patch, the contents of an active credentials struct may not be
changed directly; rather a new set of credentials must be prepared, modified
and committed using something like the following sequence of events:

	struct cred *new = prepare_creds();
	int ret = blah(new);
	if (ret < 0) {
		abort_creds(new);
		return ret;
	}
	return commit_creds(new);

There are some exceptions to this rule: the keyrings pointed to by the active
credentials may be instantiated - keyrings violate the COW rule as managing
COW keyrings is tricky, given that it is possible for a task to directly alter
the keys in a keyring in use by another task.

To help enforce this, various pointers to sets of credentials, such as those in
the task_struct, are declared const.  The purpose of this is compile-time
discouragement of altering credentials through those pointers.  Once a set of
credentials has been made public through one of these pointers, it may not be
modified, except under special circumstances:

  (1) Its reference count may incremented and decremented.

  (2) The keyrings to which it points may be modified, but not replaced.

The only safe way to modify anything else is to create a replacement and commit
using the functions described in Documentation/credentials.txt (which will be
added by a later patch).

This patch and the preceding patches have been tested with the LTP SELinux
testsuite.

This patch makes several logical sets of alteration:

 (1) execve().

     This now prepares and commits credentials in various places in the
     security code rather than altering the current creds directly.

 (2) Temporary credential overrides.

     do_coredump() and sys_faccessat() now prepare their own credentials and
     temporarily override the ones currently on the acting thread, whilst
     preventing interference from other threads by holding cred_replace_mutex
     on the thread being dumped.

     This will be replaced in a future patch by something that hands down the
     credentials directly to the functions being called, rather than altering
     the task's objective credentials.

 (3) LSM interface.

     A number of functions have been changed, added or removed:

     (*) security_capset_check(), ->capset_check()
     (*) security_capset_set(), ->capset_set()

     	 Removed in favour of security_capset().

     (*) security_capset(), ->capset()

     	 New.  This is passed a pointer to the new creds, a pointer to the old
     	 creds and the proposed capability sets.  It should fill in the new
     	 creds or return an error.  All pointers, barring the pointer to the
     	 new creds, are now const.

     (*) security_bprm_apply_creds(), ->bprm_apply_creds()

     	 Changed; now returns a value, which will cause the process to be
     	 killed if it's an error.

     (*) security_task_alloc(), ->task_alloc_security()

     	 Removed in favour of security_prepare_creds().

     (*) security_cred_free(), ->cred_free()

     	 New.  Free security data attached to cred->security.

     (*) security_prepare_creds(), ->cred_prepare()

     	 New. Duplicate any security data attached to cred->security.

     (*) security_commit_creds(), ->cred_commit()

     	 New. Apply any security effects for the upcoming installation of new
     	 security by commit_creds().

     (*) security_task_post_setuid(), ->task_post_setuid()

     	 Removed in favour of security_task_fix_setuid().

     (*) security_task_fix_setuid(), ->task_fix_setuid()

     	 Fix up the proposed new credentials for setuid().  This is used by
     	 cap_set_fix_setuid() to implicitly adjust capabilities in line with
     	 setuid() changes.  Changes are made to the new credentials, rather
     	 than the task itself as in security_task_post_setuid().

     (*) security_task_reparent_to_init(), ->task_reparent_to_init()

     	 Removed.  Instead the task being reparented to init is referred
     	 directly to init's credentials.

	 NOTE!  This results in the loss of some state: SELinux's osid no
	 longer records the sid of the thread that forked it.

     (*) security_key_alloc(), ->key_alloc()
     (*) security_key_permission(), ->key_permission()

     	 Changed.  These now take cred pointers rather than task pointers to
     	 refer to the security context.

 (4) sys_capset().

     This has been simplified and uses less locking.  The LSM functions it
     calls have been merged.

 (5) reparent_to_kthreadd().

     This gives the current thread the same credentials as init by simply using
     commit_thread() to point that way.

 (6) __sigqueue_alloc() and switch_uid()

     __sigqueue_alloc() can't stop the target task from changing its creds
     beneath it, so this function gets a reference to the currently applicable
     user_struct which it then passes into the sigqueue struct it returns if
     successful.

     switch_uid() is now called from commit_creds(), and possibly should be
     folded into that.  commit_creds() should take care of protecting
     __sigqueue_alloc().

 (7) [sg]et[ug]id() and co and [sg]et_current_groups.

     The set functions now all use prepare_creds(), commit_creds() and
     abort_creds() to build and check a new set of credentials before applying
     it.

     security_task_set[ug]id() is called inside the prepared section.  This
     guarantees that nothing else will affect the creds until we've finished.

     The calling of set_dumpable() has been moved into commit_creds().

     Much of the functionality of set_user() has been moved into
     commit_creds().

     The get functions all simply access the data directly.

 (8) security_task_prctl() and cap_task_prctl().

     security_task_prctl() has been modified to return -ENOSYS if it doesn't
     want to handle a function, or otherwise return the return value directly
     rather than through an argument.

     Additionally, cap_task_prctl() now prepares a new set of credentials, even
     if it doesn't end up using it.

 (9) Keyrings.

     A number of changes have been made to the keyrings code:

     (a) switch_uid_keyring(), copy_keys(), exit_keys() and suid_keys() have
     	 all been dropped and built in to the credentials functions directly.
     	 They may want separating out again later.

     (b) key_alloc() and search_process_keyrings() now take a cred pointer
     	 rather than a task pointer to specify the security context.

     (c) copy_creds() gives a new thread within the same thread group a new
     	 thread keyring if its parent had one, otherwise it discards the thread
     	 keyring.

     (d) The authorisation key now points directly to the credentials to extend
     	 the search into rather pointing to the task that carries them.

     (e) Installing thread, process or session keyrings causes a new set of
     	 credentials to be created, even though it's not strictly necessary for
     	 process or session keyrings (they're shared).

(10) Usermode helper.

     The usermode helper code now carries a cred struct pointer in its
     subprocess_info struct instead of a new session keyring pointer.  This set
     of credentials is derived from init_cred and installed on the new process
     after it has been cloned.

     call_usermodehelper_setup() allocates the new credentials and
     call_usermodehelper_freeinfo() discards them if they haven't been used.  A
     special cred function (prepare_usermodeinfo_creds()) is provided
     specifically for call_usermodehelper_setup() to call.

     call_usermodehelper_setkeys() adjusts the credentials to sport the
     supplied keyring as the new session keyring.

(11) SELinux.

     SELinux has a number of changes, in addition to those to support the LSM
     interface changes mentioned above:

     (a) selinux_setprocattr() no longer does its check for whether the
     	 current ptracer can access processes with the new SID inside the lock
     	 that covers getting the ptracer's SID.  Whilst this lock ensures that
     	 the check is done with the ptracer pinned, the result is only valid
     	 until the lock is released, so there's no point doing it inside the
     	 lock.

(12) is_single_threaded().

     This function has been extracted from selinux_setprocattr() and put into
     a file of its own in the lib/ directory as join_session_keyring() now
     wants to use it too.

     The code in SELinux just checked to see whether a task shared mm_structs
     with other tasks (CLONE_VM), but that isn't good enough.  We really want
     to know if they're part of the same thread group (CLONE_THREAD).

(13) nfsd.

     The NFS server daemon now has to use the COW credentials to set the
     credentials it is going to use.  It really needs to pass the credentials
     down to the functions it calls, but it can't do that until other patches
     in this series have been applied.

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

1 files changed, 47 insertions, 35 deletions

diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
index 11167fd567b..e952b397153 100644
--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -104,8 +104,7 @@ static int smack_ptrace_may_access(struct task_struct *ctp, unsigned int mode)
         if (rc != 0)
                 return rc;
 
-        rc = smk_access(current->cred->security, ctp->cred->security,
-                        MAY_READWRITE);
+        rc = smk_access(current_security(), task_security(ctp), MAY_READWRITE);
         if (rc != 0 && capable(CAP_MAC_OVERRIDE))
                 return 0;
         return rc;
@@ -127,8 +126,7 @@ static int smack_ptrace_traceme(struct task_struct *ptp)
         if (rc != 0)
                 return rc;
 
-        rc = smk_access(ptp->cred->security, current->cred->security,
-                        MAY_READWRITE);
+        rc = smk_access(task_security(ptp), current_security(), MAY_READWRITE);
         if (rc != 0 && has_capability(ptp, CAP_MAC_OVERRIDE))
                 return 0;
         return rc;
@@ -977,22 +975,6 @@ static int smack_file_receive(struct file *file)
  */
 
 /**
- * smack_cred_alloc_security - "allocate" a task cred blob
- * @cred: the task creds in need of a blob
- *
- * Smack isn't using copies of blobs. Everyone
- * points to an immutable list. No alloc required.
- * No data copy required.
- *
- * Always returns 0
- */
-static int smack_cred_alloc_security(struct cred *cred)
-{
-        cred->security = current_security();
-        return 0;
-}
-
-/**
  * smack_cred_free - "free" task-level security credentials
  * @cred: the credentials in question
  *
@@ -1006,6 +988,30 @@ static void smack_cred_free(struct cred *cred)
 }
 
 /**
+ * smack_cred_prepare - prepare new set of credentials for modification
+ * @new: the new credentials
+ * @old: the original credentials
+ * @gfp: the atomicity of any memory allocations
+ *
+ * Prepare a new set of credentials for modification.
+ */
+static int smack_cred_prepare(struct cred *new, const struct cred *old,
+                              gfp_t gfp)
+{
+        new->security = old->security;
+        return 0;
+}
+
+/*
+ * commit new credentials
+ * @new: the new credentials
+ * @old: the original credentials
+ */
+static void smack_cred_commit(struct cred *new, const struct cred *old)
+{
+}
+
+/**
  * smack_task_setpgid - Smack check on setting pgid
  * @p: the task object
  * @pgid: unused
@@ -2036,6 +2042,7 @@ static int smack_getprocattr(struct task_struct *p, char *name, char **value)
 static int smack_setprocattr(struct task_struct *p, char *name,
                              void *value, size_t size)
 {
+        struct cred *new;
         char *newsmack;
 
         /*
@@ -2058,7 +2065,11 @@ static int smack_setprocattr(struct task_struct *p, char *name,
         if (newsmack == NULL)
                 return -EINVAL;
 
-        p->cred->security = newsmack;
+        new = prepare_creds();
+        if (!new)
+                return -ENOMEM;
+        new->security = newsmack;
+        commit_creds(new);
         return size;
 }
 
@@ -2354,17 +2365,17 @@ static int smack_inet_conn_request(struct sock *sk, struct sk_buff *skb,
 /**
  * smack_key_alloc - Set the key security blob
  * @key: object
- * @tsk: the task associated with the key
+ * @cred: the credentials to use
  * @flags: unused
  *
  * No allocation required
  *
  * Returns 0
  */
-static int smack_key_alloc(struct key *key, struct task_struct *tsk,
+static int smack_key_alloc(struct key *key, const struct cred *cred,
                            unsigned long flags)
 {
-        key->security = tsk->cred->security;
+        key->security = cred->security;
         return 0;
 }
 
@@ -2382,14 +2393,14 @@ static void smack_key_free(struct key *key)
 /*
  * smack_key_permission - Smack access on a key
  * @key_ref: gets to the object
- * @context: task involved
+ * @cred: the credentials to use
  * @perm: unused
  *
  * Return 0 if the task has read and write to the object,
  * an error code otherwise
  */
 static int smack_key_permission(key_ref_t key_ref,
-                                struct task_struct *context, key_perm_t perm)
+                                const struct cred *cred, key_perm_t perm)
 {
         struct key *keyp;
 
@@ -2405,11 +2416,10 @@ static int smack_key_permission(key_ref_t key_ref,
         /*
          * This should not occur
          */
-        if (context->cred->security == NULL)
+        if (cred->security == NULL)
                 return -EACCES;
 
-        return smk_access(context->cred->security, keyp->security,
-                          MAY_READWRITE);
+        return smk_access(cred->security, keyp->security, MAY_READWRITE);
 }
 #endif /* CONFIG_KEYS */
 
@@ -2580,8 +2590,7 @@ struct security_operations smack_ops = {
         .ptrace_may_access =            smack_ptrace_may_access,
         .ptrace_traceme =               smack_ptrace_traceme,
         .capget =                       cap_capget,
-        .capset_check =                 cap_capset_check,
-        .capset_set =                   cap_capset_set,
+        .capset =                       cap_capset,
         .capable =                      cap_capable,
         .syslog =                       smack_syslog,
         .settime =                      cap_settime,
@@ -2630,9 +2639,10 @@ struct security_operations smack_ops = {
         .file_send_sigiotask =          smack_file_send_sigiotask,
         .file_receive =                 smack_file_receive,
 
-        .cred_alloc_security =          smack_cred_alloc_security,
         .cred_free =                    smack_cred_free,
-        .task_post_setuid =             cap_task_post_setuid,
+        .cred_prepare =                 smack_cred_prepare,
+        .cred_commit =                  smack_cred_commit,
+        .task_fix_setuid =              cap_task_fix_setuid,
         .task_setpgid =                 smack_task_setpgid,
         .task_getpgid =                 smack_task_getpgid,
         .task_getsid =                  smack_task_getsid,
@@ -2645,7 +2655,6 @@ struct security_operations smack_ops = {
         .task_movememory =              smack_task_movememory,
         .task_kill =                    smack_task_kill,
         .task_wait =                    smack_task_wait,
-        .task_reparent_to_init =        cap_task_reparent_to_init,
         .task_to_inode =                smack_task_to_inode,
         .task_prctl =                   cap_task_prctl,
 
@@ -2721,6 +2730,8 @@ struct security_operations smack_ops = {
  */
 static __init int smack_init(void)
 {
+        struct cred *cred;
+
         if (!security_module_enable(&smack_ops))
                 return 0;
 
@@ -2729,7 +2740,8 @@ static __init int smack_init(void)
         /*
          * Set the security state for the initial task.
          */
-        current->cred->security = &smack_known_floor.smk_known;
+        cred = (struct cred *) current->cred;
+        cred->security = &smack_known_floor.smk_known;
 
         /*
          * Initialize locks