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>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 21 insertions, 15 deletions

diff --git a/kernel/sys.c b/kernel/sys.c
index ddd28e261f3a..895d2d4c9493 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -991,31 +991,37 @@ 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