1. 24 Nov, 2008 1 commit
    • Serge Hallyn's avatar
      User namespaces: set of cleanups (v2) · 18b6e041
      Serge Hallyn authored
      The user_ns is moved from nsproxy to user_struct, so that a struct
      cred by itself is sufficient to determine access (which it otherwise
      would not be).  Corresponding ecryptfs fixes (by David Howells) are
      here as well.
      
      Fix refcounting.  The following rules now apply:
              1. The task pins the user struct.
              2. The user struct pins its user namespace.
              3. The user namespace pins the struct user which created it.
      
      User namespaces are cloned during copy_creds().  Unsharing a new user_ns
      is no longer possible.  (We could re-add that, but it'll cause code
      duplication and doesn't seem useful if PAM doesn't need to clone user
      namespaces).
      
      When a user namespace is created, its first user (uid 0) gets empty
      keyrings and a clean group_info.
      
      This incorporates a previous patch by David Howells.  Here
      is his original patch description:
      
      >I suggest adding the attached incremental patch.  It makes the following
      >changes:
      >
      > (1) Provides a current_user_ns() macro to wrap accesses to current's user
      >     namespace.
      >
      > (2) Fixes eCryptFS.
      >
      > (3) Renames create_new_userns() to create_user_ns() to be more consistent
      >     with the other associated functions and because the 'new' in the name is
      >     superfluous.
      >
      > (4) Moves the argument and permission checks made for CLONE_NEWUSER to the
      >     beginning of do_fork() so that they're done prior to making any attempts
      >     at allocation.
      >
      > (5) Calls create_user_ns() after prepare_creds(), and gives it the new creds
      >     to fill in rather than have it return the new root user.  I don't imagine
      >     the new root user being used for anything other than filling in a cred
      >     struct.
      >
      >     This also permits me to get rid of a get_uid() and a free_uid(), as the
      >     reference the creds were holding on the old user_struct can just be
      >     transferred to the new namespace's creator pointer.
      >
      > (6) Makes create_user_ns() reset the UIDs and GIDs of the creds under
      >     preparation rather than doing it in copy_creds().
      >
      >David
      
      >Signed-off-by: David Howells <dhowells@redhat.com>
      
      Changelog:
      	Oct 20: integrate dhowells comments
      		1. leave thread_keyring alone
      		2. use current_user_ns() in set_user()
      Signed-off-by: default avatarSerge Hallyn <serue@us.ibm.com>
      18b6e041
  2. 13 Nov, 2008 10 commits
    • David Howells's avatar
      CRED: Allow kernel services to override LSM settings for task actions · 3a3b7ce9
      David Howells authored
      Allow kernel services to override LSM settings appropriate to the actions
      performed by a task by duplicating a set of credentials, modifying it and then
      using task_struct::cred to point to it when performing operations on behalf of
      a task.
      
      This is used, for example, by CacheFiles which has to transparently access the
      cache on behalf of a process that thinks it is doing, say, NFS accesses with a
      potentially inappropriate (with respect to accessing the cache) set of
      credentials.
      
      This patch provides two LSM hooks for modifying a task security record:
      
       (*) security_kernel_act_as() which allows modification of the security datum
           with which a task acts on other objects (most notably files).
      
       (*) security_kernel_create_files_as() which allows modification of the
           security datum that is used to initialise the security data on a file that
           a task creates.
      
      The patch also provides four new credentials handling functions, which wrap the
      LSM functions:
      
       (1) prepare_kernel_cred()
      
           Prepare a set of credentials for a kernel service to use, based either on
           a daemon's credentials or on init_cred.  All the keyrings are cleared.
      
       (2) set_security_override()
      
           Set the LSM security ID in a set of credentials to a specific security
           context, assuming permission from the LSM policy.
      
       (3) set_security_override_from_ctx()
      
           As (2), but takes the security context as a string.
      
       (4) set_create_files_as()
      
           Set the file creation LSM security ID in a set of credentials to be the
           same as that on a particular inode.
      
      Signed-off-by: Casey Schaufler <casey@schaufler-ca.com> [Smack changes]
      Signed-off-by: default avatarDavid Howells <dhowells@redhat.com>
      Signed-off-by: default avatarJames Morris <jmorris@namei.org>
      3a3b7ce9
    • David Howells's avatar
      CRED: Differentiate objective and effective subjective credentials on a task · 3b11a1de
      David Howells authored
      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: default avatarDavid Howells <dhowells@redhat.com>
      Signed-off-by: default avatarJames Morris <jmorris@namei.org>
      3b11a1de
    • David Howells's avatar
      CRED: Documentation · 98870ab0
      David Howells authored
      Document credentials and the new credentials API.
      Signed-off-by: default avatarDavid Howells <dhowells@redhat.com>
      Signed-off-by: default avatarJames Morris <jmorris@namei.org>
      98870ab0
    • David Howells's avatar
      CRED: Make execve() take advantage of copy-on-write credentials · a6f76f23
      David Howells authored
      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: default avatarDavid Howells <dhowells@redhat.com>
      Acked-by: default avatarJames Morris <jmorris@namei.org>
      Acked-by: default avatarSerge Hallyn <serue@us.ibm.com>
      Signed-off-by: default avatarJames Morris <jmorris@namei.org>
      a6f76f23
    • David Howells's avatar
      CRED: Inaugurate COW credentials · d84f4f99
      David Howells authored
      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: default avatarDavid Howells <dhowells@redhat.com>
      Acked-by: default avatarJames Morris <jmorris@namei.org>
      Signed-off-by: default avatarJames Morris <jmorris@namei.org>
      d84f4f99
    • David Howells's avatar
      CRED: Separate per-task-group keyrings from signal_struct · bb952bb9
      David Howells authored
      Separate per-task-group keyrings from signal_struct and dangle their anchor
      from the cred struct rather than the signal_struct.
      Signed-off-by: default avatarDavid Howells <dhowells@redhat.com>
      Reviewed-by: default avatarJames Morris <jmorris@namei.org>
      Signed-off-by: default avatarJames Morris <jmorris@namei.org>
      bb952bb9
    • David Howells's avatar
      CRED: Use RCU to access another task's creds and to release a task's own creds · c69e8d9c
      David Howells authored
      Use RCU to access another task's creds and to release a task's own creds.
      This means that it will be possible for the credentials of a task to be
      replaced without another task (a) requiring a full lock to read them, and (b)
      seeing deallocated memory.
      Signed-off-by: default avatarDavid Howells <dhowells@redhat.com>
      Acked-by: default avatarJames Morris <jmorris@namei.org>
      Acked-by: default avatarSerge Hallyn <serue@us.ibm.com>
      Signed-off-by: default avatarJames Morris <jmorris@namei.org>
      c69e8d9c
    • David Howells's avatar
      CRED: Wrap current->cred and a few other accessors · 86a264ab
      David Howells authored
      Wrap current->cred and a few other accessors to hide their actual
      implementation.
      Signed-off-by: default avatarDavid Howells <dhowells@redhat.com>
      Acked-by: default avatarJames Morris <jmorris@namei.org>
      Acked-by: default avatarSerge Hallyn <serue@us.ibm.com>
      Signed-off-by: default avatarJames Morris <jmorris@namei.org>
      86a264ab
    • David Howells's avatar
      CRED: Detach the credentials from task_struct · f1752eec
      David Howells authored
      Detach the credentials from task_struct, duplicating them in copy_process()
      and releasing them in __put_task_struct().
      Signed-off-by: default avatarDavid Howells <dhowells@redhat.com>
      Acked-by: default avatarJames Morris <jmorris@namei.org>
      Acked-by: default avatarSerge Hallyn <serue@us.ibm.com>
      Signed-off-by: default avatarJames Morris <jmorris@namei.org>
      f1752eec
    • David Howells's avatar
      CRED: Separate task security context from task_struct · b6dff3ec
      David Howells authored
      Separate the task security context from task_struct.  At this point, the
      security data is temporarily embedded in the task_struct with two pointers
      pointing to it.
      
      Note that the Alpha arch is altered as it refers to (E)UID and (E)GID in
      entry.S via asm-offsets.
      
      With comment fixes Signed-off-by: Marc Dionne <marc.c.dionne@gmail.com>
      Signed-off-by: default avatarDavid Howells <dhowells@redhat.com>
      Acked-by: default avatarJames Morris <jmorris@namei.org>
      Acked-by: default avatarSerge Hallyn <serue@us.ibm.com>
      Signed-off-by: default avatarJames Morris <jmorris@namei.org>
      b6dff3ec
  3. 13 Aug, 2008 1 commit
    • David Howells's avatar
      CRED: Introduce credential access wrappers · 9e2b2dc4
      David Howells authored
      The patches that are intended to introduce copy-on-write credentials for 2.6.28
      require abstraction of access to some fields of the task structure,
      particularly for the case of one task accessing another's credentials where RCU
      will have to be observed.
      
      Introduced here are trivial no-op versions of the desired accessors for current
      and other tasks so that other subsystems can start to be converted over more
      easily.
      
      Wrappers are introduced into a new header (linux/cred.h) for UID/GID,
      EUID/EGID, SUID/SGID, FSUID/FSGID, cap_effective and current's subscribed
      user_struct.  These wrappers are macros because the ordering between header
      files mitigates against making them inline functions.
      
      linux/cred.h is #included from linux/sched.h.
      
      Further, XFS is modified such that it no longer defines and uses parameterised
      versions of current_fs[ug]id(), thus getting rid of the namespace collision
      otherwise incurred.
      Signed-off-by: default avatarDavid Howells <dhowells@redhat.com>
      Signed-off-by: default avatarJames Morris <jmorris@namei.org>
      9e2b2dc4