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Subject[PATCH 2/2] pids: sys_getpgid: fix unsafe *pid usage, s/tasklist/rcu/
1. sys_getpgid() needs rcu_read_lock() to derive the pgrp _nr, even if
the task is current, otherwise we can race with another thread which
does sys_setpgid().

2. Use rcu_read_lock() instead of tasklist_lock when pid != 0, make sure
that we don't use the NULL pid if the task exits right after successful
find_task_by_vpid().

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

--- 25/kernel/sys.c~2_GETPGID 2008-03-24 21:09:19.000000000 +0300
+++ 25/kernel/sys.c 2008-03-24 21:14:43.000000000 +0300
@@ -985,31 +985,37 @@ out:

asmlinkage long sys_getpgid(pid_t pid)
{
+ struct task_struct *p;
+ struct pid *grp;
+ int retval;
+
+ rcu_read_lock();
if (!pid)
- return task_pgrp_vnr(current);
+ grp = task_pgrp(current);
else {
- int retval;
- struct task_struct *p;
-
- read_lock(&tasklist_lock);
- p = find_task_by_vpid(pid);
retval = -ESRCH;
- if (p) {
- retval = security_task_getpgid(p);
- if (!retval)
- retval = task_pgrp_vnr(p);
- }
- read_unlock(&tasklist_lock);
- return retval;
+ p = find_task_by_vpid(pid);
+ if (!p)
+ goto out;
+ grp = task_pgrp(p);
+ if (!grp)
+ goto out;
+
+ retval = security_task_getpgid(p);
+ if (retval)
+ goto out;
}
+ retval = pid_vnr(grp);
+out:
+ rcu_read_unlock();
+ return retval;
}

#ifdef __ARCH_WANT_SYS_GETPGRP

asmlinkage long sys_getpgrp(void)
{
- /* SMP - assuming writes are word atomic this is fine */
- return task_pgrp_vnr(current);
+ return sys_getpgid(0);
}

#endif


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