CRED: Make execve() take advantage of copy-on-write credentials

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>

1 files changed, 37 insertions, 66 deletions

diff --git a/include/linux/security.h b/include/linux/security.h
index 68be11251447..56a0eed65673 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -57,8 +57,7 @@ extern int cap_capset(struct cred *new, const struct cred *old,
                       const kernel_cap_t *effective,
                       const kernel_cap_t *inheritable,
                       const kernel_cap_t *permitted);
-extern int cap_bprm_set_security(struct linux_binprm *bprm);
-extern int cap_bprm_apply_creds(struct linux_binprm *bprm, int unsafe);
+extern int cap_bprm_set_creds(struct linux_binprm *bprm);
 extern int cap_bprm_secureexec(struct linux_binprm *bprm);
 extern int cap_inode_setxattr(struct dentry *dentry, const char *name,
                               const void *value, size_t size, int flags);
@@ -110,7 +109,7 @@ extern unsigned long mmap_min_addr;
 struct sched_param;
 struct request_sock;
 
-/* bprm_apply_creds unsafe reasons */
+/* bprm->unsafe reasons */
 #define LSM_UNSAFE_SHARE        1
 #define LSM_UNSAFE_PTRACE       2
 #define LSM_UNSAFE_PTRACE_CAP   4
@@ -154,36 +153,7 @@ static inline void security_free_mnt_opts(struct security_mnt_opts *opts)
  *
  * Security hooks for program execution operations.
  *
- * @bprm_alloc_security:
- *      Allocate and attach a security structure to the @bprm->security field.
- *      The security field is initialized to NULL when the bprm structure is
- *      allocated.
- *      @bprm contains the linux_binprm structure to be modified.
- *      Return 0 if operation was successful.
- * @bprm_free_security:
- *      @bprm contains the linux_binprm structure to be modified.
- *      Deallocate and clear the @bprm->security field.
- * @bprm_apply_creds:
- *      Compute and set the security attributes of a process being transformed
- *      by an execve operation based on the old attributes (current->security)
- *      and the information saved in @bprm->security by the set_security hook.
- *      Since this function may return an error, in which case the process will
- *      be killed.  However, it can leave the security attributes of the
- *      process unchanged if an access failure occurs at this point.
- *      bprm_apply_creds is called under task_lock.  @unsafe indicates various
- *      reasons why it may be unsafe to change security state.
- *      @bprm contains the linux_binprm structure.
- * @bprm_post_apply_creds:
- *      Runs after bprm_apply_creds with the task_lock dropped, so that
- *      functions which cannot be called safely under the task_lock can
- *      be used.  This hook is a good place to perform state changes on
- *      the process such as closing open file descriptors to which access
- *      is no longer granted if the attributes were changed.
- *      Note that a security module might need to save state between
- *      bprm_apply_creds and bprm_post_apply_creds to store the decision
- *      on whether the process may proceed.
- *      @bprm contains the linux_binprm structure.
- * @bprm_set_security:
+ * @bprm_set_creds:
  *      Save security information in the bprm->security field, typically based
  *      on information about the bprm->file, for later use by the apply_creds
  *      hook.  This hook may also optionally check permissions (e.g. for
@@ -196,15 +166,30 @@ static inline void security_free_mnt_opts(struct security_mnt_opts *opts)
  *      @bprm contains the linux_binprm structure.
  *      Return 0 if the hook is successful and permission is granted.
  * @bprm_check_security:
- *      This hook mediates the point when a search for a binary handler will
- *      begin.  It allows a check the @bprm->security value which is set in
- *      the preceding set_security call.  The primary difference from
- *      set_security is that the argv list and envp list are reliably
- *      available in @bprm.  This hook may be called multiple times
- *      during a single execve; and in each pass set_security is called
- *      first.
+ *      This hook mediates the point when a search for a binary handler will
+ *      begin.  It allows a check the @bprm->security value which is set in the
+ *      preceding set_creds call.  The primary difference from set_creds is
+ *      that the argv list and envp list are reliably available in @bprm.  This
+ *      hook may be called multiple times during a single execve; and in each
+ *      pass set_creds is called first.
  *      @bprm contains the linux_binprm structure.
  *      Return 0 if the hook is successful and permission is granted.
+ * @bprm_committing_creds:
+ *      Prepare to install the new security attributes of a process being
+ *      transformed by an execve operation, based on the old credentials
+ *      pointed to by @current->cred and the information set in @bprm->cred by
+ *      the bprm_set_creds hook.  @bprm points to the linux_binprm structure.
+ *      This hook is a good place to perform state changes on the process such
+ *      as closing open file descriptors to which access will no longer be
+ *      granted when the attributes are changed.  This is called immediately
+ *      before commit_creds().
+ * @bprm_committed_creds:
+ *      Tidy up after the installation of the new security attributes of a
+ *      process being transformed by an execve operation.  The new credentials
+ *      have, by this point, been set to @current->cred.  @bprm points to the
+ *      linux_binprm structure.  This hook is a good place to perform state
+ *      changes on the process such as clearing out non-inheritable signal
+ *      state.  This is called immediately after commit_creds().
  * @bprm_secureexec:
  *      Return a boolean value (0 or 1) indicating whether a "secure exec"
  *      is required.  The flag is passed in the auxiliary table
@@ -1301,13 +1286,11 @@ struct security_operations {
         int (*settime) (struct timespec *ts, struct timezone *tz);
         int (*vm_enough_memory) (struct mm_struct *mm, long pages);
 
-        int (*bprm_alloc_security) (struct linux_binprm *bprm);
-        void (*bprm_free_security) (struct linux_binprm *bprm);
-        int (*bprm_apply_creds) (struct linux_binprm *bprm, int unsafe);
-        void (*bprm_post_apply_creds) (struct linux_binprm *bprm);
-        int (*bprm_set_security) (struct linux_binprm *bprm);
+        int (*bprm_set_creds) (struct linux_binprm *bprm);
         int (*bprm_check_security) (struct linux_binprm *bprm);
         int (*bprm_secureexec) (struct linux_binprm *bprm);
+        void (*bprm_committing_creds) (struct linux_binprm *bprm);
+        void (*bprm_committed_creds) (struct linux_binprm *bprm);
 
         int (*sb_alloc_security) (struct super_block *sb);
         void (*sb_free_security) (struct super_block *sb);
@@ -1569,12 +1552,10 @@ int security_settime(struct timespec *ts, struct timezone *tz);
 int security_vm_enough_memory(long pages);
 int security_vm_enough_memory_mm(struct mm_struct *mm, long pages);
 int security_vm_enough_memory_kern(long pages);
-int security_bprm_alloc(struct linux_binprm *bprm);
-void security_bprm_free(struct linux_binprm *bprm);
-int security_bprm_apply_creds(struct linux_binprm *bprm, int unsafe);
-void security_bprm_post_apply_creds(struct linux_binprm *bprm);
-int security_bprm_set(struct linux_binprm *bprm);
+int security_bprm_set_creds(struct linux_binprm *bprm);
 int security_bprm_check(struct linux_binprm *bprm);
+void security_bprm_committing_creds(struct linux_binprm *bprm);
+void security_bprm_committed_creds(struct linux_binprm *bprm);
 int security_bprm_secureexec(struct linux_binprm *bprm);
 int security_sb_alloc(struct super_block *sb);
 void security_sb_free(struct super_block *sb);
@@ -1812,32 +1793,22 @@ static inline int security_vm_enough_memory_mm(struct mm_struct *mm, long pages)
         return cap_vm_enough_memory(mm, pages);
 }
 
-static inline int security_bprm_alloc(struct linux_binprm *bprm)
-{
-        return 0;
-}
-
-static inline void security_bprm_free(struct linux_binprm *bprm)
-{ }
-
-static inline int security_bprm_apply_creds(struct linux_binprm *bprm, int unsafe)
+static inline int security_bprm_set_creds(struct linux_binprm *bprm)
 {
-        return cap_bprm_apply_creds(bprm, unsafe);
+        return cap_bprm_set_creds(bprm);
 }
 
-static inline void security_bprm_post_apply_creds(struct linux_binprm *bprm)
+static inline int security_bprm_check(struct linux_binprm *bprm)
 {
-        return;
+        return 0;
 }
 
-static inline int security_bprm_set(struct linux_binprm *bprm)
+static inline void security_bprm_committing_creds(struct linux_binprm *bprm)
 {
-        return cap_bprm_set_security(bprm);
 }
 
-static inline int security_bprm_check(struct linux_binprm *bprm)
+static inline void security_bprm_committed_creds(struct linux_binprm *bprm)
 {
-        return 0;
 }
 
 static inline int security_bprm_secureexec(struct linux_binprm *bprm)