signals: move cred_guard_mutex from task_struct to signal_struct

Oleg Nesterov pointed out we have to prevent multiple-threads-inside-exec
itself and we can reuse ->cred_guard_mutex for it.  Yes, concurrent
execve() has no worth.

Let's move ->cred_guard_mutex from task_struct to signal_struct.  It
naturally prevent multiple-threads-inside-exec.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Roland McGrath <roland@redhat.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 7 insertions, 6 deletions

diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index 2fea6c8ef6ba..1f8c06ce0fa6 100644
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -29,6 +29,8 @@ extern struct fs_struct init_fs;
                 .running = 0,                                           \
                 .lock = __SPIN_LOCK_UNLOCKED(sig.cputimer.lock),        \
         },                                                              \
+        .cred_guard_mutex =                                             \
+                 __MUTEX_INITIALIZER(sig.cred_guard_mutex),             \
 }
 
 extern struct nsproxy init_nsproxy;
@@ -145,8 +147,6 @@ extern struct cred init_cred;
         .group_leader   = &tsk,                                         \
         RCU_INIT_POINTER(.real_cred, &init_cred),                       \
         RCU_INIT_POINTER(.cred, &init_cred),                            \
-        .cred_guard_mutex =                                             \
-                 __MUTEX_INITIALIZER(tsk.cred_guard_mutex),             \
         .comm           = "swapper",                                    \
         .thread         = INIT_THREAD,                                  \
         .fs             = &init_fs,                                     \
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 3ff5c8519abd..be7adb7588e5 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -626,6 +626,10 @@ struct signal_struct {
 
         int oom_adj;            /* OOM kill score adjustment (bit shift) */
         int oom_score_adj;      /* OOM kill score adjustment */
+
+        struct mutex cred_guard_mutex;  /* guard against foreign influences on
+                                         * credential calculations
+                                         * (notably. ptrace) */
 };
 
 /* Context switch must be unlocked if interrupts are to be enabled */
@@ -1305,9 +1309,6 @@ struct task_struct {
                                          * credentials (COW) */
         const struct cred __rcu *cred;  /* effective (overridable) subjective task
                                          * credentials (COW) */
-        struct mutex cred_guard_mutex;  /* guard against foreign influences on
-                                         * credential calculations
-                                         * (notably. ptrace) */
         struct cred *replacement_session_keyring; /* for KEYCTL_SESSION_TO_PARENT */
 
         char comm[TASK_COMM_LEN]; /* executable name excluding path
diff --git a/include/linux/tracehook.h b/include/linux/tracehook.h
index 10db0102a890..3a2e66d88a32 100644
--- a/include/linux/tracehook.h
+++ b/include/linux/tracehook.h
@@ -150,7 +150,7 @@ static inline void tracehook_report_syscall_exit(struct pt_regs *regs, int step)
  *
  * Return %LSM_UNSAFE_* bits applied to an exec because of tracing.
  *
- * @task->cred_guard_mutex is held by the caller through the do_execve().
+ * @task->signal->cred_guard_mutex is held by the caller through the do_execve().
  */
 static inline int tracehook_unsafe_exec(struct task_struct *task)
 {