[PATCH] bacct_add_tsk: fix unsafe and wrong parent/group_leader dereference

1. ts = timespec_sub(uptime, current->group_leader->start_time);

   It is possible that current != tsk. Probably it was supposed
   to be 'tsk->group_leader->start_time. But why we are reading
   group_leader's start_time ? This accounting is per thread,
   not per procees, I changed this to 'tsk->start_time.
   Please corect me.

2. stats->ac_ppid = (tsk->parent) ? tsk->parent->pid : 0;

   tsk->parent never == NULL, and it is unsafe to dereference it.
   Both the task and it's parent may exit after the caller unlocks
   tasklist_lock, the memory could be unmapped (DEBUG_SLAB).
   (And we should use ->real_parent->tgid in fact).

Q: I don't understand the 'if (thread_group_leader(tsk))' check.
Why it is needed ?

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Shailabh Nagar <nagar@watson.ibm.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Acked-by: Jay Lan <jlan@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 5 insertions, 2 deletions

diff --git a/kernel/tsacct.c b/kernel/tsacct.c
index db443221ba5b..65a5036a3d95 100644
--- a/kernel/tsacct.c
+++ b/kernel/tsacct.c
@@ -36,7 +36,7 @@ void bacct_add_tsk(struct taskstats *stats, struct task_struct *tsk)
 
         /* calculate task elapsed time in timespec */
         do_posix_clock_monotonic_gettime(&uptime);
-        ts = timespec_sub(uptime, current->group_leader->start_time);
+        ts = timespec_sub(uptime, tsk->start_time);
         /* rebase elapsed time to usec */
         ac_etime = timespec_to_ns(&ts);
         do_div(ac_etime, NSEC_PER_USEC);
@@ -58,7 +58,10 @@ void bacct_add_tsk(struct taskstats *stats, struct task_struct *tsk)
         stats->ac_uid    = tsk->uid;
         stats->ac_gid    = tsk->gid;
         stats->ac_pid    = tsk->pid;
-        stats->ac_ppid   = (tsk->parent) ? tsk->parent->pid : 0;
+        rcu_read_lock();
+        stats->ac_ppid   = pid_alive(tsk) ?
+                                rcu_dereference(tsk->real_parent)->tgid : 0;
+        rcu_read_unlock();
         stats->ac_utime  = cputime_to_msecs(tsk->utime) * USEC_PER_MSEC;
         stats->ac_stime  = cputime_to_msecs(tsk->stime) * USEC_PER_MSEC;
         stats->ac_minflt = tsk->min_flt;