[PATCH] move cred_guard_mutex from task_struct to signal_struct

From: KOSAKI Motohiro
Date: Fri Sep 10 2010 - 05:57:33 EST

> 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@xxxxxxxxxx>
Cc: Roland McGrath <roland@xxxxxxxxxx>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
---
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



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