[PATCH] move cred_guard_mutex from task_struct to signal_struct

[KOSAKI Motohiro <kosaki.motohiro () jp fujitsu com>]

> On 09/09, KOSAKI Motohiro wrote:
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -1226,6 +1226,7 @@ struct task_struct {
> >     int pdeath_signal;  /*  The signal sent when the parent dies  */
> >     /* ??? */
> >     unsigned int personality;
> > +   struct mm_struct *in_exec_mm;
>
> Oh, yes, this has to be per-thread (unlike ->mm, btw).
>
> I wonder if it makes sense to move ->cred_guard_mutex from task_struct
> to signal_struct and thus make multiple-threads-inside-exec impossible.
> Only one thread can win anyway.

Interesting idea, really.
I've thought

1) moving cread_guard_mutex itself
   - no increase execve overhead
        -> very good
   - it also prevent parallel ptrace
        -> probably no problem. nobody want parallel debugging
2) move in_exec_mm to signal_struct too
   -> very hard. oom-killer can use very few lock because it's called
      from various place. now both ->mm and ->in_exec_mm are protected
      task_lock() and it help to avoid messy.


So, I've done only (1). I think this restriction effectivly prevent 
some theorical execve() resouce smashing attack.


======================================================================
Subject: [PATCH] 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.

Cc: Oleg Nesterov <oleg () redhat com>
Cc: Roland McGrath <roland () redhat com>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro () jp fujitsu com>
---
 fs/exec.c                 |    8 ++++----
 fs/proc/base.c            |    8 ++++----
 include/linux/init_task.h |    4 ++--
 include/linux/sched.h     |    7 ++++---
 kernel/cred.c             |    2 --
 kernel/fork.c             |    2 ++
 kernel/ptrace.c           |    4 ++--
 7 files changed, 18 insertions(+), 17 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 81d0d06..052ed54 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1091,14 +1091,14 @@ EXPORT_SYMBOL(setup_new_exec);
  */
 int prepare_bprm_creds(struct linux_binprm *bprm)
 {
-       if (mutex_lock_interruptible(&current->cred_guard_mutex))
+       if (mutex_lock_interruptible(&current->signal->cred_guard_mutex))
                return -ERESTARTNOINTR;
 
        bprm->cred = prepare_exec_creds();
        if (likely(bprm->cred))
                return 0;
 
-       mutex_unlock(&current->cred_guard_mutex);
+       mutex_unlock(&current->signal->cred_guard_mutex);
        return -ENOMEM;
 }
 
@@ -1106,7 +1106,7 @@ void free_bprm(struct linux_binprm *bprm)
 {
        free_arg_pages(bprm);
        if (bprm->cred) {
-               mutex_unlock(&current->cred_guard_mutex);
+               mutex_unlock(&current->signal->cred_guard_mutex);
                abort_creds(bprm->cred);
        }
        kfree(bprm);
@@ -1127,7 +1127,7 @@ void install_exec_creds(struct linux_binprm *bprm)
         * credentials; any time after this it may be unlocked.
         */
        security_bprm_committed_creds(bprm);
-       mutex_unlock(&current->cred_guard_mutex);
+       mutex_unlock(&current->signal->cred_guard_mutex);
 }
 EXPORT_SYMBOL(install_exec_creds);
 
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 55a16f2..fd97c8f 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -226,7 +226,7 @@ struct mm_struct *mm_for_maps(struct task_struct *task)
 {
        struct mm_struct *mm;
 
-       if (mutex_lock_killable(&task->cred_guard_mutex))
+       if (mutex_lock_killable(&task->signal->cred_guard_mutex))
                return NULL;
 
        mm = get_task_mm(task);
@@ -235,7 +235,7 @@ struct mm_struct *mm_for_maps(struct task_struct *task)
                mmput(mm);
                mm = NULL;
        }
-       mutex_unlock(&task->cred_guard_mutex);
+       mutex_unlock(&task->signal->cred_guard_mutex);
 
        return mm;
 }
@@ -2273,14 +2273,14 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf,
                goto out_free;
 
        /* Guard against adverse ptrace interaction */
-       length = mutex_lock_interruptible(&task->cred_guard_mutex);
+       length = mutex_lock_interruptible(&task->signal->cred_guard_mutex);
        if (length < 0)
                goto out_free;
 
        length = security_setprocattr(task,
                                      (char*)file->f_path.dentry->d_name.name,
                                      (void*)page, count);
-       mutex_unlock(&task->cred_guard_mutex);
+       mutex_unlock(&task->signal->cred_guard_mutex);
 out_free:
        free_page((unsigned long) page);
 out:
diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index 1f43fa5..ff3cc33 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;
@@ -139,8 +141,6 @@ extern struct cred init_cred;
        .group_leader   = &tsk,                                         \
        .real_cred      = &init_cred,                                   \
        .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 bb5bf3d..a7e7c2a 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -623,6 +623,10 @@ struct signal_struct {
 
        int oom_adj;            /* OOM kill score adjustment (bit shift) */
        long 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 */
@@ -1293,9 +1297,6 @@ struct task_struct {
                                         * credentials (COW) */
        const struct cred *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/kernel/cred.c b/kernel/cred.c
index 9a3e226..a81833d 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -384,8 +384,6 @@ int copy_creds(struct task_struct *p, unsigned long clone_flags)
        struct cred *new;
        int ret;
 
-       mutex_init(&p->cred_guard_mutex);
-
        if (
 #ifdef CONFIG_KEYS
                !p->cred->thread_keyring &&
diff --git a/kernel/fork.c b/kernel/fork.c
index b7e9d60..4c0d3ea 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -904,6 +904,8 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
        sig->oom_adj = current->signal->oom_adj;
        sig->oom_score_adj = current->signal->oom_score_adj;
 
+       mutex_init(&sig->cred_guard_mutex);
+
        return 0;
 }
 
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index f34d798..ac5013a 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -181,7 +181,7 @@ int ptrace_attach(struct task_struct *task)
         * under ptrace.
         */
        retval = -ERESTARTNOINTR;
-       if (mutex_lock_interruptible(&task->cred_guard_mutex))
+       if (mutex_lock_interruptible(&task->signal->cred_guard_mutex))
                goto out;
 
        task_lock(task);
@@ -208,7 +208,7 @@ int ptrace_attach(struct task_struct *task)
 unlock_tasklist:
        write_unlock_irq(&tasklist_lock);
 unlock_creds:
-       mutex_unlock(&task->cred_guard_mutex);
+       mutex_unlock(&task->signal->cred_guard_mutex);
 out:
        return retval;
 }
-- 
1.6.5.2