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David Howells [Thu, 13 Nov 2008 23:39:23 +0000 (10:39 +1100)]

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>