KEYS: Make the session and process keyrings per-thread

Make the session keyring per-thread rather than per-process, but still
inherited from the parent thread to solve a problem with PAM and gdm.

The problem is that join_session_keyring() will reject attempts to change the
session keyring of a multithreaded program but gdm is now multithreaded before
it gets to the point of starting PAM and running pam_keyinit to create the
session keyring.  See:

	https://bugs.freedesktop.org/show_bug.cgi?id=49211

The reason that join_session_keyring() will only change the session keyring
under a single-threaded environment is that it's hard to alter the other
thread's credentials to effect the change in a multi-threaded program.  The
problems are such as:

 (1) How to prevent two threads both running join_session_keyring() from
     racing.

 (2) Another thread's credentials may not be modified directly by this process.

 (3) The number of threads is uncertain whilst we're not holding the
     appropriate spinlock, making preallocation slightly tricky.

 (4) We could use TIF_NOTIFY_RESUME and key_replace_session_keyring() to get
     another thread to replace its keyring, but that means preallocating for
     each thread.

A reasonable way around this is to make the session keyring per-thread rather
than per-process and just document that if you want a common session keyring,
you must get it before you spawn any threads - which is the current situation
anyway.

Whilst we're at it, we can the process keyring behave in the same way.  This
means we can clean up some of the ickyness in the creds code.

Basically, after this patch, the session, process and thread keyrings are about
inheritance rules only and not about sharing changes of keyring.

Reported-by: Mantas M. <grawity@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Ray Strode <rstrode@redhat.com>

1 files changed, 2 insertions, 15 deletions

diff --git a/include/linux/cred.h b/include/linux/cred.h
index ebbed2ce6637..0142aacb70b7 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -77,21 +77,6 @@ extern int in_group_p(kgid_t);
 extern int in_egroup_p(kgid_t);
 
 /*
- * The common credentials for a thread group
- * - shared by CLONE_THREAD
- */
-#ifdef CONFIG_KEYS
-struct thread_group_cred {
-        atomic_t        usage;
-        pid_t           tgid;                   /* thread group process ID */
-        spinlock_t      lock;
-        struct key __rcu *session_keyring;      /* keyring inherited over fork */
-        struct key      *process_keyring;       /* keyring private to this process */
-        struct rcu_head rcu;                    /* RCU deletion hook */
-};
-#endif
-
-/*
  * The security context of a task
  *
  * The parts of the context break down into two categories:
@@ -139,6 +124,8 @@ struct cred {
 #ifdef CONFIG_KEYS
         unsigned char   jit_keyring;    /* default keyring to attach requested
                                          * keys to */
+        struct key __rcu *session_keyring; /* keyring inherited over fork */
+        struct key      *process_keyring; /* keyring private to this process */
         struct key      *thread_keyring; /* keyring private to this thread */
         struct key      *request_key_auth; /* assumed request_key authority */
         struct thread_group_cred *tgcred; /* thread-group shared credentials */