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sched, signals: fix the racy usage of ->signal in account_group_xxx/run_posix_cpu_timers

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Impact: fix potential NULL dereference

Contrary to ad474caca3 changelog, other
acct_group_xxx() helpers can be called after exit_notify() by timer tick.
Thanks to Roland for pointing out this. Somehow I missed this simple fact
when I read the original patch, and I am afraid I confused Frank during
the discussion. Sorry.

Fortunately, these helpers work with current, we can check ->exit_state
to ensure that ->signal can't go away under us.

Also, add the comment and compiler barrier to account_group_exec_runtime(),
to make sure we load ->signal only once.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Oleg Nesterov 2008-11-17 15:39:47 +01:00 committed by Ingo Molnar
parent 29d7b90c15
commit ad133ba3dc
2 changed files with 16 additions and 6 deletions
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7
kernel/posix-cpu-timers.c
View file

@ -1308,9 +1308,10 @@ static inline int task_cputime_expired(const struct task_cputime *sample,
*/
static inline int fastpath_timer_check(struct task_struct *tsk)
{
struct signal_struct *sig = tsk->signal;
struct signal_struct *sig;
if (unlikely(!sig))
/* tsk == current, ensure it is safe to use ->signal/sighand */
if (unlikely(tsk->exit_state))
return 0;
if (!task_cputime_zero(&tsk->cputime_expires)) {
@ -1323,6 +1324,8 @@ static inline int fastpath_timer_check(struct task_struct *tsk)
if (task_cputime_expired(&task_sample, &tsk->cputime_expires))
return 1;
}
sig = tsk->signal;
if (!task_cputime_zero(&sig->cputime_expires)) {
struct task_cputime group_sample;

15
kernel/sched_stats.h
View file

@ -298,9 +298,11 @@ static inline void account_group_user_time(struct task_struct *tsk,
{
struct signal_struct *sig;
sig = tsk->signal;
if (unlikely(!sig))
/* tsk == current, ensure it is safe to use ->signal */
if (unlikely(tsk->exit_state))
return;
sig = tsk->signal;
if (sig->cputime.totals) {
struct task_cputime *times;
@ -325,9 +327,11 @@ static inline void account_group_system_time(struct task_struct *tsk,
{
struct signal_struct *sig;
sig = tsk->signal;
if (unlikely(!sig))
/* tsk == current, ensure it is safe to use ->signal */
if (unlikely(tsk->exit_state))
return;
sig = tsk->signal;
if (sig->cputime.totals) {
struct task_cputime *times;
@ -353,8 +357,11 @@ static inline void account_group_exec_runtime(struct task_struct *tsk,
struct signal_struct *sig;
sig = tsk->signal;
/* see __exit_signal()->task_rq_unlock_wait() */
barrier();
if (unlikely(!sig))
return;
if (sig->cputime.totals) {
struct task_cputime *times;